Aromatic Hydrocarbon

AROMATIC HYDROCARBON Main Source Bituminous coal when subjected to destructive distillation in the absence of air [1000°-1200°C] the chief constituent obtained is coal tar. Fractional distillation of coal tar : Diferent fractions are : Benzene Fraction Temperature Main constutuents (a) (b) Light oil Middle oil (carbolic oil) 80-170°C 170°-230°C (c) (d) (e) Heavy oil Green oil Pitch 230°-270°C 270°-360°C Residue Benzene, toluene, xylene etc. Nephthalene, Phenol etc. Cresols, naphthalene Anthracene Carbon (i) Discoverer : Michael Faraday (iii) Geometry – Hexagonal (v) C–C Bond length : 139Å (vii) Resonance in benzene : (ii) C-hybridisation : sp (iv) Bond angle : 120° (vi) C–H Bond length : 1.09Å 2 Kekule Dewar Resonance hybride (ix) Number of angles of 120° : 18 (x) Localized and delocalized -orbital Localized -orbital Localized -orbital 2 3 2 1 4 3 1 4 6 5 6 5 Partial overlapping of p-orbital C –C , C –C and C –C 1 3 4 5 2 6 Partial overlapping of p-orbital C –C , C –C and C –C 2 4 5 1 3 6 Obviously, each orbital formed in the partial overlapping is attracted by two carbon nucleus hence less stable. Delocalized p-orbital Methods of Preparation of Benzene Isolation From ‘Light Oil Fraction - Industrial method In ‘light oil’ at least 60-65% benzene is present. Alkaline impurities (aniline, pyridine, quinolene, etc.) are removed by washing ‘light oil’ with cold and conc. H SO . The acidic impurities are removed by washing with dilute NaOH solution. After that, NaOH is removed washing with water. Three fractions are obtained at different temperature ranges by fractional distillation of the neutral sample of oil. 2 4 [i] 90% Benzol (90’s benzol). 80-110° [ii] 50% Benzol (50’s benzol). 110-140° [iii] Solvent naphtha - 140°-170° The 90% benzol contains abount 70% benzene 24% toluene and remaining xylenes. Fractional distillation gives benzene at 80°, toluene at 110° and a mixture of all the three xylenes at 137-145°. Crystals are formed on colling benzene from which pure benzene can be obtained by melting the crystals. Electrophilic Aromatic Substitution Reaction Introduction Aromatic hydrocarbons are known generally as arenes. An aryl groups is one derived from an arene by removal of a hydrogen atom and its symbol is Ar–. Thus, arenes are designated ArH just as alkanes are desigenated RH. The most characteristic reactions of benzenoid arenes are the substitution reactions that occur when they react with electrophilic reagents. These reactions are of the general type shown below.  Ar – E + H – A Ar – H + E – A  E +E-A +H-A The electrophilic are either a positive ion (E ) or some other electron-deficient speices with a large partial positive charge. For exampole, benzene can be brominated when it reacts with bromine in the presence of FeBr . Bromine and FeBr react react to produce positive bromine ions, Br . These positive bromine ions act as electrophiles and attack the benzene ring, replacing one of the hydrogen atoms in a reaction that is called an electrophilic aromatic substitution (EAS). + + 3 3 Electrophilic aromatic substitutions allow the direct introduction of a wide variety of groups into an aromatic ring, and because of this they provide synthetic routes to many important compounds. The five electrophilic aromatic substitutions that we shall study in this chapter are outlined in figure. AROMATIC HYDROCARBON X X2, FeX3 (x=Cl, Br) + HX Halogenation NO2 HONO2 + H2O Nitration H2SO4 SO3H SO3 Sulfonation H2SO4 R (RCl, AlCl3) (R can rearrange) O O C R Cl R ,AlCl3 +HCl Friedel -Crafts Alkylation +HCl Friedel -Crafts Alkylation Electrophilic Aromatic Substitution Reactions (1) A General Mechanism For Electrophilic Aromatic Substitution Benzene is susceptible to electrophilic attack primarily because of its exposed -electrons. In this respect benzene resembles an alkene, for in the reaction of an alkene with an electrophilic the site of attack is the exposed -bond. We sae in however, that benzene differs from an alkene in a very significatnt way. Benzene’s closed shell of six six  electrons gives it a special stability. So although benzene is susceptible to electrophilic attack, it undergoes substitution reactions rather thatn addition reactions. Substitution reactions alow the aromatic sextet of  electrons to be regenerated after attack by the electrophile was occurred. We can see hwo this happens of we examine a general mechanism for electrophilic aromatic substitution. Once the electrophilic, E is generated in the reaction. it ethers into some kind of a weak interaction with the  cloud of benzene ring leading to the formation of a -complex. This -complex is a donor-acceptor type of a complex, benzene being the donor and electrophille, the acceptor. These adducts are known as charge transfer complex. In the complex that benzene forms with bromine, it has been shown that the halogen molecule is located centrally and at right angles to the plane of the benzene ring. + + E+ Step I E -A + (slow) (Fast) E E H E H E H + + In step 1 the electrophile takes two electrons of the six-electron  system to form a  bond to one carbon atom of the benzen ring. Formation of this bond interrupts the cyclic system of -electrons, because in the formation of the arenium ion the carbon that forms a bond to the electrophile becomes sp hybridized and therefore, no longer has an available -orbital. Now only five carbon atoms of the ring are still sp hybridized and still have p-orbitals. A calculated electrostatic potential map for the arenium ion formed by electrophilic addition of bromine to benzene indicatedthat positive charge is distributed in the arenium ion ring (figure) just as was shown in the contributing resonance structure. 3 2 In step 2 a proton is removed from the carbon atom of the arenium ion that bears the electrophile. The two electrons that bonded this proton to carbon becomes a part of the  system. The carbon atom that bears the electrophile becomes sp hybridized again and a benzene derivative with six fully delocalized  electrons is formed.We can represent step 2 with any one of the resonance structures for the arenium ion. 2 + E E H Step II :A +H-A (Fast )   (The proton is removed by any of the bases present, for example, by the anion derived from the electrophile). Kekul'e structure are more apropriate for writing mechanisms such as electrophilic aromatic substitution because they permit the use of resonance theory, it can be described however, using the modern formula for benzene in the following way. K1 +E-A Step I K1 E H + + :A (Slow step) Arenium ion Step II + E H E :A K2  + E - A (Fast step) Where K and K is the rate constant of the forward and backward reaction in step-I, K is the rate constant of the step-II. There is firm experimental evidence that the arenium ion is a true intermediate in electrophilic substitution reactions. It is not a transition state. This means that in a free energy (figure shown below) the arenium ion lies in an energy valley between two transition states. 1 -1 2 # The free energy of activation, G (1) , for the reaction leading from benzene and the electrophile, E , to the arenium + # ion has been shown to be much greater than the free energy of activation, G (2) , leading from the arenium ion to the final product. This is consistent with what we would expect. The reaction leading from benzen and an electrophile to the arenium ion is high endothermic, because the benzene ring loses its resonance energy. The reaction leading from the arenium ion to the substituted benzene, by contrast, is highly exothermic because in it the benzene ring regains its resonance energy. Of the above two steps, 1 the formation of the arenium ion - is usually the ratedetermining step in electrophilic aromatic substitution. Step 2, sthe removal of a proton, occurs rapidly relative to step 1 and has no effect on the overall rate of reaction. AROMATIC HYDROCARBON Isotope Effect The next question which comes up is hwo do we ascertain the presence of two discrete steps in the mechanism and also that the formation of -complex is the rate determining step. Answer to this question can be obtained from the study of kinetic isotope effect. If the rate of a reaction depends on a step whihc involves breaking of a C–H bond, then a kinetic isotope effect (K / K ) fo 6 to 7 is expected. Absence of any significant isotope effect in aromatic H D electrophilic substitution (except sulphonation) suggests that the proton is lost in the fast step subsequent to rds. We see that the isotope effect study has provided two pieces of important information regarding the mechanism. Firstly, it has shown that the reaction takes in two steps and secondly, that the first step is slower than the second step. (2) Nitration Nitration reaction is generally carried out with a mixture of concentrated nitric acid and sulphuric acid. The reagents which cause nitration are called nitrating agents. The various nitrating agents which are commonly employed are : (a) N O in CCl in the presence of P O is used when anhydrous conditions are required. 2 5 4 2 5 (b) Ethyl nitrate (C H ONO ) is used to carry out nitration in alkaline medium. 2 5 2 (c) In the case of polycyclic hydrocarbons N O and nitronium salts such as NO2 BF4 , NO2 PF6 , NO2SO3 , can be 2 4   used. The electrophile involved in nitration is nitronium ion NO 2 . Mechanism Generation of electrophile from nitrating agent. (a) In a mixture of nitric acid and sulphuric acid, an acid base reaction takes place in which nitric acid acts as the base.  H O NO2 + HSO4 HO–NO2 + H2SO4  H Base H O NO2   NO 2  H 2O H  H3O  HSO H2O + H2SO4  4    NO HNO3 + 2H2SO4  2  H3O  2HSO 4 (b) N2O5 in CCl4 when used, results in a spontaneous dissociation reaction. N2O5 (c) NO2 + NO3 With concentrated HNO3 alone 2HNO3 NO2 + NO3 + H2O The electrophile generated in this case is obtained by the behaviour of one nitric acid as the base and other molecules as the acid, but the equilibrium lies in the reactant side. (3) Sulphonation Sulphonation is another synthetically important reaction. It is often accomplish with concentrated sulphuric acid or fuming sulphuric acid containing excess of SO3 or chlorosulphonic acid, ClSO2OH. It is believed that the electrophile varies with the reagent, thought in all cases SO3, is involved either free or along with a carrier, like in H2SO4(SO3 + H3O+) or H2S2O7. Sulphur trioxide is generated form sulphuric acid as follows 2H2SO 4 HSO 4 + H 3O+ + SO3 . The mechanism of sulphonation of benzene is given below : + S O O O H K1(Slow) + K–1 SO3 SO3 II, Slow HSO4, H2SO4 III, (fast) SO3H + H3O , H2O Potential Energy Sulphonations is different from other aromatic electrophilic substitution reactions, Firstly, it is reversible and secondly, it shows some amount of isotope effect which is totally absent in other cases. Let us have a look at the potential energy diagram, Fig. of sulphonation reaction to understand these anomalies. H SO3 + SO3 + SO3 +H+ Reaction coordinate We see that once the  – complexed benzenium intermediate is formed ; the energy barriers on either side of the intermediate are roughly of the same magnitude. This means that the intermediate can cross over to the product and can also come back to the reactant. This accounts for a reversible nature. Now, if we have the deuterated substrate, then the potntial energy diagram gets slightly modified (dotted curve). The barrier to step II becomes higher as it now involves the cleavage of C–D bond. The barrier for step I. The barrier for step I, on the other hand, remains the same as it pertains to - complex foramtion. The rate of its reverting back to reactants is higher than its crossover to the product. Therefore, there is a net decrease in the overall rate of sulphonation for deuterated substrate-it shows a kinetic isotope effect. The loss of proton (Step II) is the slowest step (rds). The equilibrium in step III lies to the left as aryl sulphonic acids are strong acids. In the case of other electrophilic substitutions, in contrast, energy barrier for the first step is much higher than that for the second step even for a deuterated substrate. (4) Halogenation Halogenation of an aromatic ring is a synthetically important reaction. It takes place in the presence of varied reaction condition depending on the reactivity of the aromatic ring. For very reactive aromatic compounds in polar solvents, the molecular halogens themselves may acts as electrophiles. In the case of nonpolar solvents, halogenation is catalysed by a Lewis acid like AlCl3 or FeCl3, Reactivity of halogens has the following order, I2 < Br2 < Cl2 Let us take chlorination as a representative reaction to understant the mechanism of halogenation. Chlorine, in the presence of AlCl3 or FeCl3 forms a complex, Cl2–AlCl3. This complex can itself be the reactive electrophile or it may dissociate to give Cl+. AROMATIC HYDROCARBON Cl Cl:+ AlCl3 Step 1 :  AlCl3 (anhydrous)  Cl  Cl AlCl3 H Cl     AlCl4 slow  Cl  Cl AlCl3   Step 2 : Cl H Cl  Step 3 : + AlCl3 + HCl Fast  Al Cl4   However, there is no significant evidence for the involvement of Cl+ as an electrophile and it is likely that the complex itself attacks the substrate. In the Cl2 – AlCl3 complex, role of the Lewis acid is to polarize the halogen molecule and weaken the Cl–Cl bond. This lowers the activation energy for the formation of -complex. H Cl Cl-AlCl 3 H Cl Cl + + H Cl Potential Energy + intermediate H Cl + AlCl4 reactants Cl products + Cl 2 AlCl3 + HCl + AlCl3 Reaction coordinate Schematic potential energy diagram for chlorination of benzene with Cl2(-)and Cl2AlCl3 (--) as an electrophile Bromination follows a similar mechanism. As said above, lodine is weakest of the three halogens and even in the presence of a Lewis acid, It can halogenate reactive aromatics only. Therefore, in most other cases iodine-monochloride is used in the presence of Lewis acid, ZnCl2.   Halogenation may be effected by hypohalous acids, HO X , also. This is markedly slower than with molecular     Halogens as HO– is a poorer leaving group from HO X , than, X– is from X  X . The reaction is speeded in the presence of X–, however, as HO–X is then converted into the more reactive X2, e.g. :  OCl  Cl  2H    Cl 2  H 2O In the presence of strong acid, however, HO–Hal becomes a very powerful halogenating agent due to the formation of a highly polarized complex : HO X + H H2O X H 2O X The evidence is that this species is the effective electrophile under these conditions and does not support the further conversion of complex into Hal, i.e. unlike the case, with H2O–NO2. F2 reacts vigorously with benzene, but C–C bond breaking occurs and the reactions is of no preparative significance. (5) Fridel-Crafts Alkylation Alkyl halides react with benzene in presence of aluminium chloride to yield alkyl benzenes. Alkylation of benzenw with alkyl halides in the presence of aluminium chloride was discovered by Charles Friedel and James. M. Crafts in 1877. A general equation for a friedel-Crafts alkylation reaction is the following. R + R – X   + HX AlCl3 Alkyl halides by themselves are insufficiently electrophilic to react with benzene. AlCl 3 serves as a Lewis acid catalyst to enhance the electrophilicity of the alkylating agent. The mechanism for the reaction (shown in the following steps, with isopropyl chloride as R–X) starts with the formation of carbocation (step I). The carbocation then acts as an electrophile (step II) and attacks the benzene ring to form an arenium ion. The arenium ion (Step III) then loses a proton to generate isopropyl benzen. Some times carbocation rearrange to a more stable carbocation. Mechanism for the Reaction Step I Cl H3C H3C CH Cl: + Al H3C Cl Cl + HC Step II Step III H H3C Cl CH Cl + Cl H CH3 CH3 H3C H3C CH + AlCl4 CH3 CH3 CH3 + Al Cl Cl Cl Al Cl Cl CH3   other canonical forms CH3 + HCl + AlCl3 CH3 With R-X is a primary halide, a simple carbocation probably does not form. Instead, the AlCl3 forms a complex with the alkyl halide and this complex acts as the electrophile. In the complex the carbon halogen bond is nearly broken and one in which the carbon atom has a considerable +ve charge. These complexes are so carbocation like that they also undergo typical carbocation rearrangements. Friedel-Crafts alkylations are not restricted to the use of alkyl halides and AlCl3. Many other pairs of reagents that form carbocations (or carbocation like species) may be used as well. These possibilites include the use of a mixture of an alkene and an acid. CH(CH3)2 + CH3 CH= CH2 propene O°C HF Isopropyl benzene A mixture of an alcohol an acid may also be used. + HO 60°C BF3 AROMATIC HYDROCARBON (6) Friedel-crafts Acylation The group is called an acyl group and a reaction whereby an acyl group is introduced into a compound is called an acylation reaction. The Friedel-Crafts acylation reaction is an effective means of introducing an acyl group into an aromatic ring. The reaction is often carried out by treating the aromatic compound with an acyl halide. O O + H 3C CH2 CH3 AlCl3  Cl  + HCl Acly chlorides (also called acid chlorides) are early prepared by treating carboxylic acids with thionyl chloride (SOCl2) or phosphorous pentachloride (PCl5). H3C OH H3C Cl + SOCl2  + SO2 + HCl  O O Fridel-Crafts acylations can also be carried cut using carboxylic acid anhydrides. eg. 80 C O O + C CH3 H3 C AlCl3  O  O + H3C H3 C OH O Mechanism of the Reaction In most Fridel-Crafts acylation reactions the electrophile appears to be an acylium ion formed from an acyl halid ein the following way. Step I O R C Cl + AlCl3 Step II O H 3C C Cl AlCl3 O R C Cl AlCl3 R C O + AlCl4 R C O H R C O   Step III H Step IV O + R R O O  + AlCl4  + R + HCl + AlCl3 R O Step V R + AlCl3 Cl O AlCl3 In the last step, AlCl3 (a Lesis acid) forms a complex with the ketone (a Lewis base). After the reaction is over, treating the complex with water liberates the ketone. R R O AlCl3 + 2H2O   C6H5 O + Al(OH)3 + 3HCl C6H5 Limitations of Friedel - Crafts Reactions (a) Several restrictions limit the usefulness of Friedel-Crafts reactions. When the carbocation formed from an alkyl halide, alkene, or alcohol can rearrange to a more stable carbocation, it usually does so and the major product obtained from the reaction is usually the one from the more stable carbocation. When benzene is alkylated with butyl bromide, for example, some of the developing butyl cations rearrange by a hydride shift-some developing 1° carbocations (see following reactions) become more stable 2° carbocations. The benzene reacts with both kinds of carbocations to form bot butylbenzene and sec-butylbenzene. H 3C (b) AlCl3  Br  An aromatic ring less reactive than that of halobenzene don’t undergo Friedal Craft’s reaction. Aromatic ring containing –NH2, –NHR, –NR2, groups does not undergo friedal craf’s alkylation due to formation of anilinum complex which is meta direction and has more electron withdrawing power than halogen in benzene ring. H H H N: H N AlCl3  + AlCl3  Meta directing group and does not go Friedal Craft's reaction (c) Friedel-Crafts reactions do not occur when powerful electron-withdrawing groups are present on the aromatic ring or when the ring bears an –NH2, –NHR or –NR2 group. This applies to bot alkylations and acylations. NO2 O OH + C N(CH3)3 These to not undergo Friedel-Crafts reactions AROMATIC HYDROCARBON (d) Aryl and vinylic haldies cannot be used as the halide component because they do not florm carbocations readily. Cl No reaction AlCl3 No reaction Cl (e) Polyalkylations often occur - Alkyl groups are electron releasing groups, and once one is introduced into the benzene ring it activated the towards fruther substitution. CH(CH3)2 + H3C H3C CHOH CH(CH3)2 + BF3 60°C 24% Isopropyl benzene CH(CH3)2 14% disopropylbenzene Polyacylations are not a problem in Friedel-Crafts acylations however. The acyl group (RCO–) by itself is an electron-withdrawing group, and when it forms a complex with AlCl3 in the last step of the reaction, it is made even more electron withdrawing. This strongly inhibits further substitution and makes monoacylation easy. Aromatic Hydrocarbon CH 3 CH 3 CH 3 CH3 CH=CH2 CH(CH3)2 CH3 Benzene Toluene o-xylene (1,2-) m-xylene (1,3-) CH3 CH3 p-xylene (1,4-) styrene Cumene CH 3 CH2 biphenyl (phenylbenzene) biphenyl methane (benzyl benzene) naphthalene anthracene phenanthrene CH 3 H3C 1,3,5-trimethylbenzene (mesitylene) Source (a) Fractional distillation of Coaltar The dark-brown sticky liquid called coal-tar is a mixture of several aromatic hydrocarbons, phenols, base etc. The first step in the separation of coal-tar into its compounds is distillation in a fractionating column as given in table. (b) High temperature creacking Petroleum (kerosene and low boiling fractions) cyclisation/aromatisation cracking mixture of alkenes and alkanes. aromatic hydrocarbon HC HC CH2 CH2 + CH2 CH2    H2   Pt-Al2O3 -H2 Pt-Al2O3 -H2 (c) Polymerisation 3CHCH red-hot tube 1870-2070 K CH3 red-hot tube 2CH  CH + CH3CCH 1870-2070 K CH3 2CHC  CH + CH  CH red-hot tube + (o, p-also) CH3 CH3 3HC  CCH3 (d) red-hot tube CH3 H3C Alkyl benzene synthesis + CH2 = CH2 AlCl3, HCl 368 K, Pressure CH(CH3)2 + CH2 = CHCH3 (e) H3PO4 523 K, pressure Friedel-Crafts reaction CH2CH3 + CH3CH2Cl (i) AlCl3 (ii) H2O + HCl AROMATIC HYDROCARBON (f) Decarboxylation of Aromatic acids COONa + NaOH (g) Wurtz–Fitting reaction Br + 2Na + Br (h) + Na2CO3 R dry ether R + 2NaBr Deoxygenation of Phenols OH ; OH OH CH3 (i) Reduction of Diazonium salts + N2Cl + H3PO3 + HCl N2Cl + H3PO2 + H2O (j) Desulphonation of sulphonic acids SO3 + H2O 400-420 K + H2SO4 steam HCl, high pressure 1. 2. 3. Mechanisms of the above reactions have been given in further section. All meta-directing groups are deactivating groups; All ortho, para-direc ting groups except for the halogen are activating groups. The halogens are deactivating groups The activating substituents make the benzene ring more reactive towards electrophilic substitution, the deactivating substituents make the benzene ring less reactive towards electrophilic substitution. All the strong activating substituents donate electrons into the ring by resonance and withdraw electrons inductively. The fact that are strong activators indicate that electron donation into the ring by resonance (which increases electron-density hence, nucleophilicity) is much greater than electron withdrawl frm the ring by the inductive effect (which decreases electron-density hence nucleophilicity). Strongly activating substituents are : :NH2 :OH :OR The moderately activating substituents also donate into the ring by resonance and withdraw electrons from the ring inductively. However they donate electrons into the ring by resonance less effectively than do not strongly activating substituents. These substituents are less effective at resonance donatin since, they can donate electrons by resonance in two competing directions-into the ring and away from the ring and thus net resonance effect is diminished and still electron donation by resonance is more than the electron withdrawl by induction and thus, these substituents are activators though moderately. O HNCCH3 O O HNCCH3 HNCCH3 O O HNCCH3 HNCCH3 (substituents donates electrons by resonance into the benzene ring) O O HN CCH3 HN CCH3 (substituents donates electrons aways from the benzene ring) Alkyl, aryl and CH=CHR groups weakly activating substituents. Two of the three groups (aryl and CH = CHR) can donate electrons into the ring by resonance and can also withdraw from the ring by resonance. The fact that they are weak activators indicates that they are slightly more electron-donating than they are electron-withdrawing. An alkyl substituent is weak electron donor becuase of hyperconjugation. R CH=CH/R The halogen are weakly deactivating substituents. They donate electrons into the ring by resonance and withdraw electrons inductively. Since, they have been found deactivators, we can conclude that they withdraw electrons inductively more strongly than they donate electrons by resonance. Cl Cl : The moderately deactivating substituents all have carbonyl groups directly attached to the benzene ring. They withdraw electron both inductively and by resonance. O O O CH CR COR AROMATIC HYDROCARBON The strongly deactivating substituents are powerful electron withdrawers. With the exception of      NH 3 ,  NH 2 R,  NHR 2 and  NR 5 and they withdraw electrons both inductively and by resonance. The ammonium ions have no resonance effect, but the positive charge on the nitrogen atom causes them to strongly withdraw electron inductively. (a) Catalytic hydrogenation + 3H2 Pt CH3 CH3 + 3H2 Pt Treatment of areanes with group I metal (like sodium, lithium) and methanol (or ethanol) in liquid reduces arenes to non-conjugated dienes. H H H H Na, liquid NH3 CH3OH 1,4-cyclohexadienes Alkyl substituted arenes give, 1, 4-cyclohexadienes in which the alkyl group is a substituent on the double bond. CH3 CH3 Na, liquid NH3 CH3OH CH3 CH3 Na, liquid NH3 CH3OH CH3 rather than CH3 CH3 Metal ammonia-alcohol reduction of aromatic ring is known as Birch reductin and is one type of a more general class of react6ions called dissolving metal reduction. (b) Addition of Halogen (c) Ozonolysis + 3O3 Zn/H2O CHO CHO glyoxal (d) Oxidation + 3O3 Zn/H2O CHO CHO glyoxal When oxidised using KMnO4/OH–, Na2Cr2O7/H+,KMnO4/H+ the entire side chain, with at least one H at -C, regardless of length is oxidised to –COOH. AROMATIC HYDROCARBON The conversion of alkyl benzenes to benzoic acid with strong oxidations suggest that benzene ring is more stable than the side chains, this is correct so long as the side-chain contains at least one benzylic C–H bond. The first step in the mechanism is the removal of hydrogen from the benzylic carbon. If the side-chain has no C–H bond at the carbon (w.r.t. benzene nucleus), then benzene ring is cleaved. (e) (f) Combustion  2C6H 6  15O2  12CO 2  6H 2 Side-chain halogenation CH3 CH2Cl + Cl2 boiling CHCl2 CCl3 Cl2 benzyl chloride Cl2 benzal chloride benzotrichloride This provides a source of preparing other derivatives from benzene and toluene. CH3 CH3 CH3Cl, AlCl3 , Cl2, boiling 3Cl2 boiling 2Cl2, boiling CH2Cl CHCl2 aq. KOH CH2OH CH3 (NBS is N-bromo succinimide) NH3 CH2NH2 KCN, H3O+ CH2NH2 NBS, benzoyl peroxide CCl4, 350 K CCl3 aq. KOH CHO CH2Br + (i) KOH, (ii) H3O COOH Relative Reactivity of Substituted Benzene Ortho para directors Strongly activating Meta directors Moderately deactivating O O O O O O C H, C , C Cl, C NH2, C OH, C OR O, NH 2, NR2, OH, OR Moderately activating Strongly deactivating O O NH C CH 3, O C CH 3    NH 3 , NR 3 , NO 2 , C  N, CF3 , CCl3 Weakly activating –CH3, –CH2CH3, –R ,–CH=CH2 Weakly deactivating F , Cl , Br , I Directing Influence As Decided By Electron Density • –OH group is ortho and para-directing since it increase electron-density at ortho and para-positions for electrophilic attacks; electron-density at meta-position is decreased hence, electrophilic attack at this position is least. O H O H O H O H II III IV : : I O H V I to V are different resonance structures and VI resonance hybride structure. Electrophile can attack the sites with increased electron density which are ortho and para w.r.t. – OH group Naturally nucleophilic cannot attack ortho and para-sites but can attack at meta-site (but less reactive) AROMATIC HYDROCARBON • –NO2 group is electron- withdrawing group. It deactivates ortho and para-position for electrophilic attack and thus electrophile can ocupy meta-position. I to V are resonance and VI is resonance hybrid. O O O O O O O O O O N N N N N I II III IV V In this case nucleophilic attack at electron-rich ortho and para-sites is favoured. Anamalous Behaviour Of Halogen Substituent –X(F, Cl, Br, I) is o–, p-directing but is deactivating group. This deactivating nature is attributed to high electronegativity of the halogen atom due to which they withdraw electrons; resonance effect explains its electron-donating behavivour. Alkyl groups and Orientation Alkyl groups (–CH3, –CH2CH3, etc.) have complete octet and are without and are without one-pair of electrons; but they are ring activators and o–, p-directors. They not only show electron-pumping effect, but also when attached to conjugated system show hyperconjugated effect (no-bond resonance). This type of resonance requires the interaction of -electrons instead of usual lone-pair of electrons. It toluene, hyperconjugation is show below : Thus, hyperconjugation helps in increasing-density at o-and p-sites in case of alkyl groups. There must be at least one H-atom on the carbon of the alkyl joined to the ring carbon to take part in hyperconjugation and relative ability of alkyl groups for this effect is C(CH3)3 C(CH3)3 + NO2+ NO2 In this case, only p-isomer is obtained. It is due to steric influence of the substituents–C(CH 3)3 and due to its unability to show hyperconjugation. Orientation and Synthesis By using the nature of directing group (subtituent), it is possible to prepare variety of products. To prepare meta-bromonitrobenzene from benzene, we have to first carry out nitration and then bromination, and to prepare o-and p-dereivatives, sequence will be bromination and then nitration. Br NO2 Br2, Fe HNO3/H2SO4 NO2 Br2, Fe (-NO2 group is m-directing) (Br is o-, p-directing) Br Br NO2 HNO3 H2SO4 o- + NO2 p- Br m- AROMATIC HYDROCARBON Orientation in Disubstituted benzene When two substitution are present, orientatiion of the third coming SE is complicated. –CH3 group is o–, p-directing and –NO2 group is m-directing, thus in p-nitrotoluene, new electrophile can be in a position shown by arrow : CH3 meta w.r.t. -NO2 ortho w.r.t. -CH 3 NO2 Similarly we can decide the position of the new electrophile in other disubstituted compounds. SO3H NHCOCH3 CN NO2 NO2 both meta-directing - NHCOCH3 ortho, para and - CN meta-directing Problems may arise in the following cases : OH Strongly activating CH3 Weakly activating Both are ortho, para-directing and none one of the vacant positions are ortho-and para, w.r.t. each. In such cases we follows : (a) Strongly activating groups generally win over deactivating or weakly activating groups. OH OH HNO3/H2SO4 NO2 -NO2 is ortho w.r.t. - OH (Strongly activating) CH3 activating CH3 OH OH Br2, Fe CHO CHO deactivating weakly activating Br OH OH NO2 HNO3/H2SO4 deactivating Cl Cl CH3 + Cl NO2 (b) If the subtitution are meta to each for other, SE in between vacant sites takes place rarely. Cl Cl Cl Br Br 2, Fe Br Br + + Br Br 62% 1% Cl Br Br 37% Halogen Compound Aryl Halides Source/Preparation — Direct halogenation Cl + Cl2 Cl Fe/Cl2 Fe Cl Cl + Cl Low temperature and the presence of a halogen carrier favour nuclear substitution. The chlorides or bromides of Al, Fe, Sb may use used : Fe Cl2 FeCl2 H Cl FeCl4 + Cl+ H Cl H Cl H Cl + H Cl Cl + H+  FeCl3 + HCl FeCl4– + H+  Iron is most commonly used being converted to Lewis acid FeCl3 as shown above. It is agian SE reaction and without halogen-carrier (Lewis acid) Cl+ (halogena) is not formed and hence, reaction is not possible : CH3 CH3 + Br2 Fe CH3 Br + toluene o- bromotoluene Br p- AROMATIC HYDROCARBON A methoxy group is so strongly activating that anisole quickly bromines in water without a catalyst : OCH3 Br2 H2O Br OCH3 Br + HBr Br Direction iodination is not possible since, iodine is least reactive and HI formed makes reaction reversible. In presence of oxidising agensts like HNO3 or HIO3, iodination of benzene is possible and HI formed is converted to I2 [O] 1/2 I2 + HNO3 + I + H2O + NO2  3 I2 + 3 H2O 5 HI + HIO3  Iodination probably involves an electrophilic aromatic substitution with iodonium (I+) acting as the electrophile. I+ is formed from iodine by oxidation with HNO3.  I+ + NO2 + H2O H+ + HNO3 + 1/2 I2  OH Cl + PCl3 high temp. +POCl3 + HCl COOAg + Br2 Br + AgBr + CO2 (Hunsdiecker reaction) Diazonium salts are converted to halogen compounds Br N2 + N2 Cl Br + CuBr/HCl (Sandmeyer) CuBr/HCl (Sandmeyer) + N2 Diazonium salts are obtained by diazotisation of amino compounds and this provides a better route to convert amino compounds into halogen compounds : + HCl + 1/2 O2 CuCl Cl + H2O (Raschig process) Side -Chain Derivatives CH3 (boiling) CH2Cl Cl2 high temp; light CH2Cl CCl2 Cl2 Cl2 benzyl chloride benzal chloride benzotri chloride Side-chain halogenation involves free radical mechanism due to lower bond energy of the benzyl C–H bond: CH3 CH2 + Cl + HCl stable benzyl free radical Benzyl radical is stablised by resonance due to delocalisaton of odd electrons into ring : CH2 CH2 CH2 CH2 CH2 NBS can also be used for bromination of side-chain alkyl group : CH2CH3 NBS CH CH3 Br (by alkylic subtitution) In presence of an activating group –OH or –NH2, side chain halogenation is difficult : CH3 CH3 + Br2 OH Br OH Br Properties Physical   Less polar, insoluble in water but soluble in organic solvents like ethanol and ether. They show physiological activity and are used as insecticides. Examples are : Cl Cl (dichlorobenzene) CCl3 CH (DDT) AROMATIC HYDROCARBON Chemical Low reactivity of Aryl halide for SN reaction (a) C-X bond in aryl halide is stable due to delocalisation of electrons by resonance. Also (C-X) bond possesses a double bond character like vinyl chloride and is stronger than C-X bond in alkyl halide. while C X Nu weak C Nu + X weak Hence, SN reaction is not possible in benzene nucleus under ordinary conditions. However, under high temperature and pressure, SN reaction is made possible. Cl + OH 570 K, pressure OH + Cl + CN CuCN 470 K, pressure CN + Cl aq. NH3 470 K, pressure NH2 + CuCl + H2O + NH3 + Cu2O If the benzene ring has (1) one or more substituents that strongly withdraw electrons from the ring by resonance, and (Z) a good leaving group (such as halogens), nucleophilic aromatic substitution reactions can occur without using extreme conditions. These electron-withdrawing groups must be positioned ortho or para to the leaving group. The greater the number of electron-withdrawing substituents, the easier it will be to carryout the nucleophilic aromatic substitution reactions. Cl Z Cl +z: G G withdraws electron stabilies carbanian, activates, SN reaction is made favourable G  G   N(CH 3 )3 ,  NO 2 , CN, SO 3H, COOH, CHO O2N Cl + OH 430 K O2N OH O2N Cl + OH 370 K O2N OH NO2 NO2 NO2 O2N warm Cl + H2O NO2 Cl G O2N OH NO2 Z +z: NO2 Cl G G releases electrons destabilises carbanion, deactivates G = - NH2, -OH, -OR, -R Nucleophilic aromatic substitution takes place by a two-step reaction known as an SN Ar reaction byfollowing mechanism : X NO2 X +Y nucleophilic (slow) O X Y N O N O X Y Y O fast O X Y N O O Y N O +X NO2 (b) Elimination - Addition Through Benzyne Internate An aryl halide can undergo a nucleophilic substitution reaction in the presence of a very strong base such as NH 2 . When chlorobenzene-that has the carbon to which chlorine is attached isotropically labelled with 14C – is treated with amide ion in liquid ammonia, aniline is obtained as a product. Half of the product has the amino group attahced to the isotopically labelled carbon (*) as expected, but the other half has the amino group attached to the carbon adjacent to the labelled carbon. Cl * NH2 + NH2 * NH3(liquid) * NH2 + (approximately in equal amounts) The mechanisms that accounts for the experimental observations involves formation of a benzene intermediate which has two equivalent carbon atoms to which amino group can be attahced. Benzyne has an extra -bond between two adjacent carbon atoms of benzene and can be formed as : Step -I : Strong base NH 2 removes a proton from the position ortho to halogen : Cl * Cl H + :NH2 * + NH3 (amino) Step -II : Anion formed in step I eliminates the halide ion, thereby forming benzyne : Cl * * + Cl (benzyne) AROMATIC HYDROCARBON The incoming nucleophile can attack either of the carbons of the “triple bond” of benzyne. Protonation of the resulting anion form the substitutin product. The overall reaction is an elimination-addition reaction; benzyne is formed in an elimination reaction and immediately undergoes an addition reaction. NH2 NH2 attack at C* (labelled) * H NH2 + NH2 * :NH2 NH2 * attack at C* (unlabelled) H NH2 + NH2 * NH2 Substitution at the carbon (*) that was attached to the leaving group is called direct substitution. Substitution at the adjacent carbon is called cine substitution. CH3 Br + NaNH2 CH3 liquid NH3 o-bromotoluene (c) o-toluidine (by direct substitution) aryl halide + NH2 m-toluidine (by cin substitution) aryl halide (side chain) ether CH3 ether CH2 alkyl halide CH2Cl + Cl + 2Na alkyl halide If only aryl halide is involved, diphenyl is formed (Fitting reaction) Ullman synthesis 2 (e) CH3 Wurtz -Fitting Reaction Cl + CH3Cl + 2Na (d) Br I + 2Cu Reaction with Chloral DDT is formed when chloral react with chlorobenzene in presence of concentrated H2SO4. 2 Cl + H C = O CCl3 conc. H2SO4 Cl C C Cl H Cl 2, 2-di (p-chlorophenyl)-1,1 1-trichloroethane(DDT) DDT is causing ecological problems, its use as insecticides is being banned. Nitration Cl Cl conc. HNO3 conc. H2SO4 o- NO2 Cl + chloronitrobenzene NO2 p- Chlorination Cl Cl Cl2, Fe Cl Cl + dichlorobenzene Cl p- oSulphonation Cl Cl H2SO4 SO3H Cl + chlorosulphonic acid SO3H p- o- • Chlorine is not a good donor of electrons by resonance due to its high electronegativity. Thus, the inductive effect of the Cl atom overcomes the resonance effect, and thus, it deactivates the benzene ring. (g) Substitution on benzyl carbons (SN) : The greater reactivities of benzylic halides result from the stabilities of the carbocation intermediates that are formed when they react. CH2 Cl CH2 CH2 Cl CH2 CH2 CH2 CH2 Tertiary cumyl chloride ionises to a carbocation with four important resonacne structure : CH3 CH3 C Cl: C Cl: CH3 CH3 CH3 C C CH3 CH3 CH3 (resonance -stablised) Because of the possibilitie of resonance, ortho and para substituent group on the benzene ring that activate electrophilic aromatic substitution further accelerate SN 1 reaction at the benzylic position. Thus p-methoxy tertcumyl chloride (I) undergoes hydrolysis about 3400 times faster than tert-cumyl chloride II. CH3O I CH3 CH3 C Cl C Cl CH3 II CH3 AROMATIC HYDROCARBON Benzylic halides undergoes SN reactions like aliphatic halides. aq. KOH CH2OH KCN CH2CN NH3/NaNH2 CH3NH2 CH2 Cl This provides a path of converting toluence into so many other compounds benzyl chloride. Aromatic Nitro Compounds NO2 NO2 NO2 CH2NO2 NO2 nitrobenzene m-dinitrobenzene phenylnitromethane O2N O 2N CH3 NO2 NO2 1,3,5-trinitrobenzene 2,4,6-trinitrotoluene Source • Direct nitration Nitratig agent is (a) conc. HNO3 + conc. H2SO4 or (b) acetyl nitrate (N2O5 in acetic anhydride) –NO2 is deactivating group, hence, further nitration takes plac with furning HNO3 and H2SO4 mixture. Intermediate is o–complex (arenium carbocation) stablished by resonance. NO2 + HNO3 conc. conc. H2SO4 330 K OH NO2 + 2H2SO4 OH2+ nitronium ion NO2 fuming HNO3 conc. H2SO4 + H3O++ 2H2SO NO2 –CH3 and –OCH3 groups are activating groups hence, nitration of toluene and methoxy benzene (anisole) is easier than that of benzene. CH3 CH3 + CH3CO- O - NO2 + CH3COOH acetyl nitrate toluene NO2 toluene (CH3CO)2O+N2O5 acetic anhydride OCH3 OCH3 N2O5, (CH3CO)2O NO2 + CH3COOH methoxybenzene From Diazonium salts H3C N2+Cl HNO2, Cu2O + H3C N2 + HCl + NO2 Oxidation Amino compounds : (using trifluroperbenzoic acid) NH2 NHCOCH3 CH3COCl acetylation to protecting ring NHCOCH3 HNO3 NHCOCH3 NO2 o- NHCOCH3 + NO2 p- AROMATIC HYDROCARBON Properties    Nitro-compounds are yellow crystalline solids (nitrobenzene is yellow liquid) Steam-volatile, denser than water in which they are in soluble Nitro group is attacked by Grignard reagent, nitro-substituted aryl halides cannot be used for the preparation of these reagents. O2N Br Mg/ether X No reaction There is also partial positive charge on the ortho and para-positions. Thus, –NO2 activates the benzene nucleus for nucleophilic substitutions at the ortho and para-positions. Electrophilic substitutions (SE) Further nitration to form trinitrobenzene takes place under drasitic conditions. Presence of electron-releasing group activates benzene nucleus for SE. Toluene can be nitrated only once but nitro group introduced retards a second nitration on the same ring. CH3 CH3 NO2 conc. HNO3/H2SO4 343 K NO2 NO2 2,4-dinitrotoluene NO2 O2N conc. HNO3/H2SO4 393 K CH3 NO2 NO2 2,4,6-trinitrotoluene NO2 Br2 /Fe   Br m-bromonitrobenzene  -NO2 group also deactivatives benzene nucleus alkyl for alkylation by Fridel-Crafts reaction. Nucleophilic Substitution Reactions  Substitution of hydrogen by a nucleophile does not ocur in benzene itself, but the presence of one nitro group is sufficient to activate the o- and p-positions. NaOH slow + OH NO2 NO2 + OH NO2 fast H NO2 NO2 NO2 NO2 OH K3F 3(CN) 6 NaOH NO2 NO2 + NO2 OH 2, 4-(main) dinitrophenol 2, 6- Cl OH H OMe Cl Cl Cl OMe NO2 NO2 NH2 NO2 NO2 NH3 NO2 NO2 aq. KOH OH alcoholic Aromatic Sulphonic Acids They are derivatives of aromatic hydrocarbons in which one or more hydrogen of the benzene ring have been replaced by the function – SO3H, called the sulphonic grouop. SO3H benzene sulphonic acid SO3H SO3H NO2 CH3 p-toluene sulphonic acid m-nitrobenzene sulphonic acid CH3 O O S OH O S OH O benzene sulphonic acid (carbon bonded to sulphur) O a sulphate (an ester) (carbon bonded to oxygen) SO2Cl SO2Cl benzene sulphonyl chloride p-toluene sulphonyl chloride (Hinsber'gs reagent) (tosy chloride) AROMATIC HYDROCARBON Source Direct sulphonation SO3H SO3 300-330 K + H2SO4 conc. CH3 CH3 SO3 273 K + H2SO4 + o-isomer SO3H (main) In the sulphonation, electrophile is SO3 formed as : SO3 + H3O+ + HSO4 2H2SO 4 Although it appears neutral, sulphur carries positives charge due to resonance : O O O S O O S O O O H SO3 SO3 S O H SO3 H SO3 H SO3 + HSO4 SO3 + H2SO4 SO3 SO3H + H2O + H3O + Properties Colourless crystalline deliquescent solid. Very soluble in water and solution is strongly acidic. SO3H + H2O SO3 + H3O+ They are stronger acid than carboxylic acid. SO3H (Ka = very large) COOH (ka = 6.3 × 10-5) H SO3 Formation of diphenyl sulphone SO3H + SO2 +H3SO4 Desulphonation The knocking out of sulphonic acid group from benzene nucleus is called desulphonation. H SO3H  H   SO3H + H   + SO3H X  H2SO4 SO3H + OH– (from water)  The desulphonation is very useful for preparation of certain isomers which are otherwise not obtained in pure state. Reactions of sodium salt of benzene sulphonic acid (conversioin into other derivatives) SO3Na OH NaOH fuse SH ONa OH H NaOH + SO3Na NaSH, fuse NH2 NaSH, fuse thiophenol COONa CN HCOONa, fuse NaCN, fuse Source (a) Nitrobenzene can be reduced under different media to produce aniline and releated compounds. NO 2 Sn/HCl NH2 + H2O NO2 NH2 + TiCl3 + HCl TiCl4 + H2O CH 3 CH3 NO2 (b) NH2 NO2  + 3 NH4Hs  NO2 + 3 NH3 + 3S + 2H2O NH4 HS is suitable reagent where only one –NO2 (in stages) is to be reduced. CH3 NH2 NO2 CH3 NH 4HS  NH2 NO2 2, 4, dinitrotoluene CH3 SnCl2 /HCl   NO2 NH2 AROMATIC HYDROCARBON Action of NH4HS and SnCl2/HCl is specific. NH4HS reduces – NO2 para to –CH3 while SnCl2/HCl reduces – NO2 ortho to –CH3. CHO CHO alkaline Fe or NO2 SnCl2/HCl 2+ NH2 Alkaline Fe does not reduce – CHO into – CH2OH.SnCl2/HCl can also be used here. 2+ (c) Cu2O pressure, 437 K Cl + NH3 NH3 + HCl C–Cl bond is stable, hence ammonolysis of aryl halide takes place under high temperature and pressure. However electron withdrawing group in benzene nucleus makes C–Cl bond highly reactive. NO 2 O2N Cl NO 2 NH3   NH2 O2N NO2 NO 2 OH+NH3 O CNH2 (d) ZnCl2 573 K NH 2+H 2O Br2/aq.NaOH, Hofmann degradiation NH2 When the migrating group is aryl, the rate of the Hofmann degradation reaction is increased by the presence of electron releasing substituens in the aromatic ring. G  OBr CONH2   NH2 G : = – OCH3 > – H > – Cl > – NH2 (e) COOH (f) COOH (g) N=N N3H, D (Schmidt) conc. H2SO4 azobenzene NH 2+CO2+N2 H2, Ni 313 K, 15 atm FeCl3 + NH2OH   (h) CH3 CH3 N HCl NHHCl (i)    N, N-dimethyl aniline hydrochloride NH NH hydrazobenzene H2, Ni 313 K, 15 atm NH2 NH 2+H 2O hydroxylamine CH3 NH 2+CO2+N2    CH3 NH2HCl CH3 CH3 NH2  OH   CH3 CH3 This is called Hofmann-Martius rearrangement and alkyl group preferentially migrates to p-position and if it is occupied, then to o-position. reduction NC   (j) NHCH3 2° (N-methyl aniline) NH 2 (k) RX OH NHR 2° amine R RX OH N 3° amine Cl NO2 OH   + CH3NH2  NO2 R NHCH3 NO2 NO2 2° amine Some other methods are very similar to that used for aliphatic amines. Properties Physical  Colourless liquid/solids, but due to air oxidation, they turn brown. NH2 O air   colourless   O Slightly soluble in water (polar solvent) but highly soluble in benzen and other organic solvents. High b.p. due to intermolecular H-bonding-N, N-dimethyl aniline hs lower b.p. due to lack of H-bonding but molecular weight is also important affecting b.p. CH3 N: CH3 no intermolecular H-bonding possibl H H N H  H N H N H intermolecular H-bonding in aniline Para-substiuted aniline, being the most symmetric, have the highest melting points. Thus p-toluidine is solid at room temperature while o-toluidines are liquids. AROMATIC HYDROCARBON Relations of the –NH2 group (a) Methylation (Alkylation) OH   NH2 + CH3I  NHCH3 OH CH3I N CH3 CH3I CH3 CH3 N CH3 I CH3 quaternary ammonium salts (b) Arylation Arylation of aniline with C6H5Cl is difficult. NH2+ Cl 473K, pressure CuCl NH diphenyl amine NH3Cl NH2+ (c) 413K pressure NH4Cl Acylation Arylation of aniline with C6H5Cl is difficult. NH2+CH3COCl NHCOCH3 N-acetyl aniline (acetanilide) Acetyl group deactivates –NH2 group, whenever aniline is subjected to nitration, acetylation is necessary to protect the ring from being oxidised. (d) Sulphonylation NH2+ SO2Cl benzene sulphonyl chloride (Hinsberg reagent) NHSO2 N-phenyl benzene sulphonamide (soluble in alkali) 2° amine also reacts but the product is insoluble in alkali. This provides a method (called Hinsberg method) of separation of mixture of 1°, 2° and 3° amines. (e) Carbyl Amine Reaction Only 1° amines give this reaction when unpleasant smell of isocyanides is obtained on heating a mixture of alcoholic KOH, CHCl3 and 1° amine.  NH 2+CHCl3+ 3 KOH   NC+3KCl + 3H 2O (f) Formation of Schiff base N=HC NH 2+OHC H 2O   NH2 Schiff base These Schiff bases are easily hydrolysed to the free maines and so their formation offers a mean of ‘protecting’ an amino group (during nitration). (g) Oxidation K 2Cr2O7 /H 2SO 4 O NH2  O p-benzoquinone NH2 NO2 NO Caro's acid H2SO4   NO2 p-nitro aniline (h) NO2 dil.HNO3 NO2 p-nitroso nitrobenzene p-dinitrobenzene Reaction with HNO2 1° amines form diazonium salts at low temperature (273–278K). A reaction in which – NH2 group is converted into  diazo group (  N  N) is called diazotisation. Diazonium salts are stable in cold aqueous solution. 273K  HNO2 + NaCl NaNO2 + HCl  273K NH 2+HCl   + 273K NH 3 +HNO2   + NH3 Cl N NCl benzene diazonium chloride Aniline can be converted into so many other compounds through the formation of diazonium salts. N2+ Cl OH+N2+HCl H3PO2 + H2O +H3PO3 + HCl Cl + N 2 CuBr + HBr (Sandymeyer) Br + N 2 Cl + N 2 CaCN + HCN CN + N2 AROMATIC HYDROCARBON N2+ Cl KI I+N 2+KCl HBF4 N 2 BF4 F+N2+BF 3 NO2 NaNO2/Cu NH 2 N=N NH2 p-amino azobenzene (dye) More reacious are given in sec. 16.8 (diazonium salts) 2º amines form nitrosamine (yellow oily liquid) whichis stable at room temperature. However on reaction with HCl in ether and alcohol – NO group nigrates to para-position (Fischer-Hepp) NH+HNO2 N NO CH3 CH3 HCl,ether/C2H5 OH ON NHCH3 p-nitroso-N-methyl aniline 3° amines form p-nitroso derivatives H3C N CH3 H3C + HNO2 (strongly p-activating) N CH3 HCl H 3C CH3 + H2O NO NO p-nitroso, N, N-dimethyl amine N(CH3)2 N N(CH3)2 N(CH3)2 NO + H NO Hydrolysis of p-nitrosos derivative gives 2° amines H 3C H 3C N NO+H2O NaOH ON OH+ p-nitrosophenol H 3C H 3C NH 2° amine This provides a path of the preparationof 2° amines   NH2+K  + NH K + 1/2 H2 This represent acidic nature of aniline. Hypochlorous acid also reacts with aniline in which active H is replaced : NH 2+HOCl NHCl+H2O N-Chloroaniline Electrophilic substitution in benzene nucleus –NH2 group is o, -p directing and activating group. :NH2 :NH2 NH2 NH2 NH2 Paraderivative Electrophile will attack here Hence, –NH2 group activatives benzene nucleus for ortho and para-electrophilic attack. (a) Bromination NH2 +3Br2(aq) Br NH2 Br +3HBr2 Br –NH2 group is greatly activating group hence reacton takes place rapidly. However if ring is deactivated by acetylation, o-and p-isomers are obtained. NH2 NHCOCH3 Br 2 CH3COCl acetylation acetanilide (b) NHCOCH3 Br NH2 +p- o- Br +p- (steam volatile) Nitration Direct nitration of aniline with nitric acid gives a complex mixture of mono-,di-and tri-nitro compounds and oxidation products. If –NH2 group is protected by acetylation and then nitrated, p-isomer is the main product. NH2 NH2 NHCOCH3 CH3COCl HNO3 NO2 p-nitroaniline (c) Sulphonation NH2 NH3HSO4 NH2 NHSO3H rearrangement SO3H p-amino benzene sulphonic acid (sulphonic acid) AROMATIC HYDROCARBON Like antranilic acid, sulphanilic acid also exists as Zwitter ion due to inernal neutralisation. NH3 NH2 SO3H SO3 Zwitter ion Distinction between Aliphatic and Aromatic Amines NH2 S. No. Test 1. Diazotisation Diazonium salt is formed SE 3. Basic nature – NH is replaced by – OH + N2 (reaction with HNO2) 2. CH3NH2 which forms azo dye with phenolic compounds can be halogenated, sulphonated, nitrated. less basic than CH3NH2,NH3 no more basic than NH3 Phenolic compounds Phenols are the compounds of the general formula ArOH, where, Ar is phenyl or substitued phenyl. Phenols differs from alcohols in having the –OH group attached directly toan aromatic ring. OH OH phenol o-cresol OH CH3 OH OH catechol OH OH COOH OH salicylic acid resorcinol Source From Cumene (Isopropyl benzene) +CH3CH2CH2Cl AlCl3 CHCH3 CH3 (Cumene) +CH3CH=CH2 O CH3CCH3 + acetone AlCl3 OH H2O, H+ phenol OH  Cl H 2O,H ,   Cl CH3 C O OH CH3 cumene hydroperoxide Hydrolysis of Diazonium salts N2+HSO4 O2 air + N2 OH resorcinol Diazonium salts are obtained from aniline and its derivatives by a process called diazotisation. NH2 OH N2Cl CH3 NaOH2 + HCl 273 K +N2+HCl CH 3 CH3 m-toluidine Alkali Fusion of Benzene Sulphonic Acid salts ONa OH SO3Na NaOH 573K  +NaOH  + H CH3 CH3 CH3 Hydrolysis of Aryl Halides Halogen atom attached to benzene nucleus does not give SN2 reaction since C–X bond is stable due to resonance. However, in presence of electron withdrawing group in o- and p-positions, SN reaction is favoured and phenolic compounds are formed. Cl NaOH high, temp, pressure no reaction Cl NO 2 OH OH NaOH NO2 NaOH SN Cl NO2 (Dow's process) ONa NO2 HCl OH (i) NaOH (ii)HCl NO2 NO2 NO2 Distillation of Phenolic Acids OH OH COOH ONa OH  NaOH   H  Salicylic acid Oxidation of Grignard Reagent MgBr O2MgBr O2   MgBr OMgBr 2 OH  H3O   AROMATIC HYDROCARBON Middle Oil of Coil - Tar Distillation Middle oil of coal-tar distillation has naphthalene and phenolic compounds. Phenolic compounds are isolated in following step : Step-I : Middle oil is washed with H2SO4. It dissolves basic impurities like pyridine (base). Step-II : Excessive cooling separates naphthalene (a low melting solid) Step-III : Filtrate of step II is treated with aqueous NaOH when phenols dissolves as phenoxides. Carbon dioxide is then blown through the solution to liberate phenols. CO 2 ,H 2O ONa + H 2O   OH + NaOH OH + Na 2CO 2 Step-IV : Drude phenol (of step III) is subjected to fractional distillation. 450K Crude phenols fractional distillation phenol 463 - 476K o, m, p-cresols 484 - 508K xylo(hydroxy xylenes) Properties Physical Similest phenols are liquids or low-melting solids. They are capable of forming intermolecular H-bonding among themselves and with water. Thus, – thy have high b.p. – they are soluble in water. – Due to intermolecular H-bonding and high dipole moment, m.p. and b.p. of phenol are much higher than that of hydrocarbon of comparable molecular weights. Solute mol. wt. b.p. m.p. phenol 94 g mol–1 455 K 314 K toluene 92 g mol–1 384 K 178 K Comparison of properties of nitrophenols Isomer b.p. at 760 mm Solubility g/100 g o-nitrophenol 373 K 0.2 volatile in steam p-nitrophenol decomposes 1.69 non-volatile in steam m-nitrophenol 467 K 1.35 non-volatile in steam Form the above table, it is clear that o-nitrophenol has much lower b.p. and much lower solubility in water than its isomer and is only steam-volatile. –m and p–isomer have high b.p. because of intermolecular H-bonding and high solubility in water is againdue to crossed intermolecular H-bonding with H2O. o-isomer has intramolecular H-bonding due to which association with different molecules decreases hence, b.p. is low giving volatile character to it and also H-bonding with H2O molecules is not possible which makes it water soluble to a very small extent. –Steam distillation depends upon a substance having an appreciable vapour pressure at the b.p. of water, by lowering the v.p., intermolecular hydrogen bonding inhibits steam distillation of the m-and p-isomer. O N O H O H O O N O intermolecular hydrogen bonding in p-nitrophenol molecules O H O C H H N O O H intramolecular hydrogen bonding in o-nitrophenol (chelation) chelation in o-hydroxybenzaldehyde Acidity of Phenols And Effect of Substituents On It OH O + OH stronger acid O OH + H2O +H2CO3 weakter acid stronger acid +H2O weakter acid Phenols are converted into their salts by aqueous NaOH, but not by aquesou bicarbonates. The salts are converted to free phenols by aqueous mineral acids (HCl, etc.,) carboxylic acid or carbonic acids. Phenol is more acidic than alcohols but less than carboxylic acid. Enhanced acidity of phenol is due to (a) polar effect (b) resonance effect. resists cleavage O H (phenol) H O R (alcohol) easily leaved AROMATIC HYDROCARBON C6H5 - (phenyl) group is electron-withdrawing and phenoxide ion is more resonance stabilised than phenol, hence following equilibrium is displaced towards the right where stable phenoxide ionis formed. Alkoxides are stabilised neither by resonance nor by the polar effect of benzene rings or double bonds. O cyclohexanolate anion (no polar or resonance effect) • electron-attracting substituents like –X, or –NO2 increase the acidity of phenols are electron-releaseing substituents like –CH3 decrease acidity. G : – NO2, –X, –NR3+, –CHO, –COX, –COOR, –CN Ka (acidity constant) and pKa of different substituted phenola are given in table. The marked lower acidity in case of meta-isomer is explained by the fact that m-nitrophenoxide ion is stablised by inductive effect only and no resonance effect operates with –NO2 group. O O N=O N=O no other resonance with –NO2 group O O • Reactions in which phenol is a source of active H are : OH+Na OH+RMgI ONa+1/2 H 2 OMgI + RH Formation of Complexes with FeCl3 Phenols form coloured iron complexes when related with neutral FeCl3 solution. The formation of iron complexes is attributed to the existence of keto-enol tautomerism in phenols. Phenol predominantly exists in enolic form, hence colour formation is used to identify phenols. OH O enol keto OH 6 3– +FeCl3 3H + Fe O + 3HCl + 6 enol Other Reaction of –OH Groups OH + PCl3 OH Cl+POCl3+HCl ONa OR NaOH RX SN This is called Williumson’s synthesis. The phenoxide ionis a nucleophile and will replace halogen of the alkylhalide. OH+CH2N2 OCH3+N2 anisole O OH+CH3CCl or (CH2CO)2O O OCCH3 base phenyl acetate O OH + O CCl OC phenyl benzoate O OH CrO2Cl2 oxidation +H2O O p-benzoquinone (or quinone) AROMATIC HYDROCARBON OH O + 2AgOH +2Ag+2H2O OH O quinol quinone Electrophilic Substitution In Benzene Nucleus –OH group is o- and p-directing and activating group for SE reactions. O H O H OH O H OH O H OH OH and + –H H Eu o- stable by resonance ( -complex) Halogenation With excess of Br2 solution : OH Br2 + Br2 Br +3H OH Br+ + Br3– Br + Br 2, 4, 6-tribromophenol OH OH + Br2 CS2 or CHCl3 273 K OH Eu H Eu and + Eu+ O H OH Br + O-bromophenol Br p-bromophenol (main) Eu p- In presence of NaOH, phenoxide ion is formed whichis mroe reactive than phenol towards electrophilic aromatic substitution. O OH NaOH H2O O O Br2 + Br – Br Br2 Br Br Br Br Sulphonation H2SO4 + H2SO4 SO3H+ + HSO4– + H2O OH OH + SO3H+ SO3 H + OH SO3H opphenol sulphonic acid At room temperature, ortho-isomer while at 373 K the para-isomer is the main product. Nitration –OH group is activating group, hence, nitration is possible with dil. HNO3. HNO3 + HNO3 NO2+ + NO3— + H2O OH OH + NO3+ CHCl3 288 K OH NO2 + O- nitrophenol NO2 p- HNO3 is oxidising agent and may lead to cleavage of benzene ring hence, further nitration is done carefully. However ring can be deactivated by sulphonation and then nitration will give picric acid in better yield. OH OH + H2SO4 SO3H SO3H HNO3 O2N OH NO2  NO2 2, 4, 6-trinitrophenol (picric acid) AROMATIC HYDROCARBON Nitrosation HNO2 + H2SO4 NO+ HSO4– + H2O OH OH OH NO + + NO+ NO2 opnitrosophenol (90%) Nitrosophenol ca be oxidised to –NO2 by HNO3; this provides a path of preparing nitrophenol. OH NO OH HNO3  NO2 Alkylation (Friedel-Crafts Reaction) OH ONa ONa CH3I AlCl3 NaOH benzene OH CH3  o- and p- OH + CH3 CH3 C OH CH3 CH3 H 70% H2SO4 AlCl3 o- cresol CH3 H3C C CH3 OH + H2O p-tert-butylphenol Acylation (Friedel-Crafts Reaction) OH OH O + CH3CCl AlCl3 COCH3 and p- o-acetylphenol  Intermediate of this reaction is CH 3C  O (acylium ion). Fries rearrangement When esters or phenols are heated with AlCl3 the acyl group migrates from the phenolic oxygen to an ortho or paraposition of the ring thus, yielding a ketone. This reaction is called the Fries rearrangement and appears to involve generation of an acylium ion, RCO+, which then attack the ring as in ordinary Friedel-Cracft acylation. O OH O CC2H5 C2H5COCl AlCl3 CS2 OH O CC2H5 + p-isomer (non volatile) o-hydroxy phenyl ethyl ketone (volatile in steam) Claisen rearrangement OCH2CH=CH2 OH 473 K (diphenyl ether) allyphenyl ether CH2CH=CH2 o-allyl phenol This is the arrangement of allyl ethers to allyl phenols. Allyl group migrates to ortho-position. If ortho-position is already occupied, para-isomer is obtained. * OCH2CH=CH 2 CH3 H3C H3C 473K OH CH3   CH2CH=CH2 * In this Claisen rearrangement, carbon which gets attached to benzene nucleus is vinylic and no allylic. Coupling reactions (Azo dye formation) Phenol reacts with aryl diazonium salts in presence of alkali at low temperature of form form azo dyes. N2Cl + OH (i) NaOH N=N + (ii) H 273 K OH p-hydroxyazobenzene Reimer-Tiemann Reaction OH OH (a) + CHCl3 (i) aq. NaOH, 330 K (ii) H+ OH CHO + salicyaldehyde (steam volatile) CHO A substituted benzal chloride is initially formed, but is hydrolysed by the alkaline medium. OH O CHCl3 , OH    (b) — CHCl2 O  OH   — CHO O H  — CHO AROMATIC HYDROCARBON Kolbe reaction (Carbonation) Treatment of the ssalt of a phenol with CO2 under pressure brings about substitution of the carboxyl group, –COOH, for hydrogen of the ring. This is called Kolbe reaction. ONa ONa 400 K, 4 7 atm. + CO2  COONa ONa  H  COOH salicyclic acid This provides a path of conversion of phenol into hydroxy carboxylic acids. It seems likely that CO2 attaches itself initially to phenoxide oxygen rather than to the ring. O OCONa ONa OH    + CO 2 COONa At high temperature (525 – 575 K), p-isomers is obtained. O O +C=O O OH H C O O — COO OH — H  O H-atom attached to the ortho-positoin combines with phenoxide oxygen. Salicyclic acidis used to prepare aspirin, salol and picric acid : COOH Gattermann Reaction (Formylation) OH OH +HCN + HCl CHO ZnCl2 H3O+ This reactionis used to prepare aldehyde when activating groups like –OH, –NH2 are present. ZnCl :OH  2  CH  NH  ZnCl3 Cl – CH = NH  HCN + HCl OH  CH  NH   H CH=NH OH CH=NH OH H3O   CHO Condensation Reaction (a) Condensation of phenol with HCHO in presence of acid of acid forms bakelite. OH OH  H +HCHO  CH2OH HCHO/H  + p-  -CH2 OH CH2 o-hydroxy benzyl alcohol CH2 OH CH2 CH2 OH CH2- CH2 bakelite (b) Condensation of phenol with phthalic acid in presence of concentration H2SO4 forms phenolphthalein (an indicator) which gives pink colour with NaOH (phthalein test of dibasic acid) O C OH C OH O phthalic acid    H OH O C O C C O H C O OH OH O OH phenolphthalein AROMATIC HYDROCARBON OH S. No. 1. 2. Test CH3CH2OH Neutral FeCl3 Azo dye test violet colour with benzene diazonium chloride is formed no reaction no dye formation N = N Cl + NaOH, 273 K N=N 3. 4. 5. Phthalein test Litmus test Iodoform test OH is given turns blue not given no given no change is given Sandymeyer Reaction Nucleophiles such as CN–, Cl– etc., replace the diazonium group if appropriate cuprous salt is added to the solution containing diazonium salt. CuCl, HCl N2+,Cl–   Cl + N2 CuBr, HCN N2+,Cl–    Br + N2 CuCN, HCN N2+,Cl–   – CN + N2 Sandmeyer reaction involves : CuCl + Cl–  CuCl2– slow N2+ + CuCl2–   .+ CuCl O2N + N N: + O 2     O2N .+ N + CuCl 2 Cl + CuCl N=N OH COO COO alizarin yellow Diazo coupling often takes place in basic solutions since due to deprotponation of the phenolic –OH group or benzene sulphonic acid and carboxylic groups, activate the aromatic ring for SE reaction. Common azo dyes have sulphonate/carboxylate groups to enhance solubility of the azo dye in water and to help bind the dye to the polar surface of common fibres such as cotton and wool. Carbonyl Compounds CHO benzaldehyde (oil of bitter almonds) CHO COCH3 CHO OH acetophenone CH3 salicylaldehy p-tolualdehyde O C benzophene Source From Toluene Rosenmund Reaction COCl CHO Pd-BaSO4 + H 2 catalyst moderator + HCl NO 2 NO 2 Acyl chlorides can be reduced to aldehyde by treating them with lithium tri-ter-butoxy aluminium hydride, LiAlH(OC(CH3)3]3, at 195 K. O C Cl O CH (i) LiAlH[OC(CH3 )3], diethyl ether, 195 K (ii) H2O The reduction is brought by the transfer of a hydride ion from the aluminum atom to the carbonyl carbon of the acyl chloride. Subsequent hydrolysis frees the aldehyde. Reimer-Tiemann reaction is used to prepare phenolic aldehydes OH OH + CHCl3 + 3KOH (aq) CHO + 3KCl + 2H2O    o(and p-) AROMATIC HYDROCARBON Friedel-Crafts Acylation COC 6H5 COC 6H5 C6H5COCl COCH3 CH3COCl AlCl3 AlCl3 (CH 3CO) 2O, AlCl3 This reaction cannot be used on strongly deactivated aromatic systems. Gattermann-Koch Reaction CHO + CO + HCl AlCl3   + CO + DCl AlCl3 + HCl CDO   + HCl Thus, HCl (reactant) and HCl (product) are different. H of HCl (reactant) and CO appear in the form of –CHO in the benzene nucleus. It is confirmed by the fact that using isotopic DCI instead of HCl, would form –CDO and no –CHO. This reaction also succeeds only with benzene and acitvated benzene derivative. Gattermann-Aldehyde Synthesis CHO + HCN + HCl + H2O AlCl3   + NH4Cl Stephan Reaction CN CH = NH SnCl 2 /HCl  CH = NH.HCl SnCl2/HCl CHO SnCl 4 H2O 2 Grignard Reagent MgBr O CHO HCOC2H5 Oxidation of Benzyl Chloride CH2Cl Pb(NO3 )2   CH H O N O CHO + HNO2 O Using Diazonium Salts N CH = NOH – NCl + CH2 = NOH benzaldoxime formaldoxime CH 2OH MnO2 CHO H2O CHO Properties Physical C6H5CHO is a colourless liquid with smellof bitter slmond, spari9ngly solublein water but readilyin ehtanol and ether. C6H5COCH3 is crystalline solid and is steam volatile, soluble readily in ethanol and ether but sparingly soluble in water. It produces natural type of sleep when administered orally. Ortho-quinones, particularly ortho-benzoquinone are less stable than their para-quinone isomers. It is due to the fact that in ortho-isomer, the C = O bond dipoles are nearly aligned and these have a repulsive destabilising interaction. In para-isomers, these dipoles are farther apart. O O O o-benzoquinone (bond dipoles nearly aligned) O p-benzoquinone (bond dipoles farther apart) Chemical Aromatic carbonyl compounds with following structure O CR R=H = CH 3(alkyl) = C 6H5(aryl) Conjugation of the carbonyl carbon with the aryl ring reduces the electrophilic reactivity of the carbonyl carbon atom due to delocalisation of -electrons. Hence, aromatic aldehydes and ketones are less reactive than aliphatic classes. Benzaldehyde, like aliphatic aldehydes, reduces Tollen’s reagent (silver-mirror test). However, it does not reduce Fehling’s solution. (a distinction from aliphatic class). They slow elimination and addition reaction of aliphatic classes.  > C = NG + H2O > C = O + H2NG    C = O + HCN  C OH CN However, acetophenone does not form bisulphite complex, probably due to steric hindrance. no reaction COCH3 NaHSO3 CHO CH OH SO3Na Benzaldehyde can react with hydrazine to form benzylideneazine. CHO + H2O – NH2 + OHC CH = N – N = HC benzylideneazine Oxidation CHO + O2 hv Tollen's reagent COOH AROMATIC HYDROCARBON Reduction CHO + 2H Zn/HCl CH 2OH LiAlH 4 Benzyl alcohol CHO + 2H Zn/HCl 2 CH CH OH OH pinacol (hydrobenzoin) COCH3 Zn(Hg) / conc. HCl Clemmensen CH2CH3 (i) NH2NH2 (ii) glycol/OH– (Wolff-Kishmer) Cannizzaro Reaction CHO 2 CH2OH + NaOH COONa + • It is given by those aldehydes which do not have H at -carbon, and also by HCHO. • It is a disproportionation reaction. • Benzoin condensation When benzaldehyde is heated with aqueous ethanolic NaCN or KCN, it dimerises to form an a hydroxy ketone ketone called benzoin, and reaction is called benzoin condensation. It involves self condensation of an aromatic aldehyde in the presence of CN– as catalyst. H – CN , H2O C2H5OH C+H–C O O H C–C OH O It resembles Aldol condensation of aliphatic aldehyde in which -H condenses, but –CHO group is free. H O CH 3C + H – CH 2CH NaOH CH3CHCH2CHO OH O Perkin Reaction It is the condensation reaction in which aromatic aldehyde is heated with an anhydride of an aliphatic acid in the presence of sodium salt of the same acid form -unsaturated acid. O O CHO + CH 3C – O – CCH 3 CH3COONa acetic anhydride CH = CHCOOH + CH3COOH cinnamic acid However, with cylic anhydride (succinic anhydride), -unsaturated acid is formed. CH2CO CHO + CH2CO O CH3COONa CH = CHCH2COOH Witting Reaction It converts the carbonyl group ofa ketone or aldehyde into a new double bond on reaction with phosphorus ylides. R R C = O + Ph3 – CH 2 — carbonyl compound phosphorus ylide R R C = O + Ph3P – C C = CH2 + Ph3P = O H H C=C H Mixture of cis and trans-isomers often result when geometric isomerism is possible. CHO + H2N CH = N Schiff's base (i) ether CHO + CH2 = CHLi (ii) H3O+ CHO + CH3CH2NH2 NaBH3CN methanol OH CHCH = CH2 1-phenyl-2-propen-1-ol CH2NHCH2CH3 N-ethylbenzylamine Reactions of Benzene Nucleus CHO CHO CHO SO 3H H2SO4 SO3 HNO3 H2SO4 Br2/Fe CHO Br NO2 AROMATIC HYDROCARBON EDUBULL TIPS & FORMULAS QUICK REVIEW OF PREPARATION OF SOME IMPORTANT AROMATIC COMPOUNDS Nut Shell preparation 'Aromatic Halogen Compound' I I2 Cl CuCl + 300°C (i) I AqKI Balz Schemann Reaction Br Br2/FeBr3 HCl +½ O2 Rashing Process Cl Cl2/FeCl3 HgO or H NO3 AlCl3 H 2SO4 (ii) Sn/ HCl (iii) NaNO2/ HCl/ 0°-5° + N COOH CH 3 CH 3Cl HNO3 KMnO4 OH HBF4/ Br Br2 CCl4 Reflux Hunsdieker Reaction Cl NCl CuCl/ HCl Sandmayer Reaction [CUX/ HX] or Cu/ HCl F Ag2O COOAg Gatterman Reaction [CU/ HX] Br CuBr + HBr Cu/ HBr 284 AROMATIC HYDROCARBON QUICK REVIEW OF IMPORTANT REACTIONS OF AROMATIC COMPOUNDS AROMATIC HYDROCARBON Nutshell Review and Preview of Toluene Reactions AROMATIC HYDROCARBON 292 AROMATIC HYDROCARBON Nutshell Review and Review of Benzaldehyde Reactions AROMATIC HYDROCARBON NUTSHELL REVIEW AND PREVIEW OF BENZOIC ACID REACTIONS