Reaction Mechanism

Organic Chemistry – Some Basic Principles and Techniques REACTION MECHANISM ORGANIC CHEMISTRY The study of hydrocarbons and the compounds which would be thought of as their derivative. NATURE OF COVALENT BOND HYBRIDISATION In hybridisation phenomenon, the orbitals of different shapes, but almost of equal energies blend up to give the same number of new orbitals of another shape and of identical energies (give stronger, more directional and stable bonds) e.g. sp3, sp2, sp A covalent bond is formed by the progressive overlapping of two bonding orbitals. For the stable bond formation, the condition is that the electrons Example :1 2 3 should have opposite spins. SIGMA BOND () 4 5 1 2 3 4 (a) CHC–CH=CH–CH3 and (b) CH2C=CH–CH3 The molecular orbital formed by the overlapping of (hybridisation state of carbon, number of  and -bonds) two s-atomic orbitals or one s-and one p-orbital is (a) C1 , C2, C3, C4 C5 called a sigma bond. Overlapping of hybridised sp, sp, sp2, sp2 sp3 orbitals also leads to the formation of sigma bond.  2, 2, 3, 3, 4 -bonds are stronger as they result from the effective  2, 2, 1, 1, 0 axial overlapping. (b) C1 , C2 C3 , C4 Order of strength of -bond :sp2, sp, sp2, sp3 (i) sp3 –sp3 > sp3–sp2> sp2–sp2>sp2–sp>sp–sp (ii) sp3–s>sp2–s > sp–s  3, 2, 3, 4  1, 2, 1, 0 (iii) sp3–p>sp2–p>sp–p (iv) p–p>s–p>s–s The relative energies of different orbitals :More is the s-character associated with the orbital less will be its energy p > sp3>sp2>sp>s PI-BOND The molecular orbital formed by the sideways overlapping of two p-atomic orbitals is called Pi bond. Relative to -bond , -bond is weaker as there is only partial overlapping in the later case. -electrons are mobile. Hence -bond is more reactive as compared to sigma bond. -bond is always accompanied by the formation of a -bond. GEOMETRY OF CARBON Four valencies of a carbon atom are directed towards the four corners of a regular tetrahedron with carbon atom situated at the centre (Le Bel and vant Hoff, angle 109°28'). The square pyramidal and square planar structures of carbon have not been accepted BOND LENGTH The distance between the nuclei of the two bonded atoms is termed as bond length e.g. C–C bond length (i) C2H6(1.54Å) (ii) C2H4 (1.34 Å ) (iii) C2H2 (1.20 Å) (iv) C6H6 (1.39 Å) Organic Chemistry – Some Basic Principles and Techniques BOND DISSOCIATION ENERGY Order of bond length (i) C–C > C–N > C –O Bond energy may be defined as the energy released when two atoms get bonded or the energy needed to break the bond to form neutral atoms. (ii) C=C > C=N > C=O (iii) C C > C  N Example : (iv) (i) C–C (80 k.cal–mole–1) (v) =C=C=> –C  C– (ii) C=C (142 k.cal – mole–1) (vi) (iii) CC (190 k.cal– mole–1) –C > C  C= Order of bond energy :- (vii) =C–C= > =C–C  | | (i) CC > C=C>C–C | (viii) C– Cl [ in CH2Cl – CH2Cl] > C – Cl (ii) H–H > O –H > C–H (iii) CN > C=N > C–N [ in CH2 =CH –Cl] (iv) C–Cl > C –Br > C–I (v) C–O (ketone) > C=O [CH3CHO] > C=O [HCHO] (C–Cl bond acquires double bond character) (ix) C = C bond length : CH2=CH2>CF2=CF2 [ more electronegative atom F is attached to C–atom] (x) C–H bond length : [order] C H>C H>C (sp3) (s) (sp2) (s) (sp) (s) (xi) C (1.08Å) H > O H H (0.97Å) | (vi) =C–H > = C–H >–C–H | (vii) C=O (CO2) > C=S (CS2) (viii) O=O > N = N (xi) O–H > N –H > C–O > C–N [110.6] [93.0] [85.5] [73.0] (k.cal-mole–1] Values of bond dissociation energy (xii) C = C > C = N > C = O (k.cal mole–1) 1.34Å 1.29Å 1.22Å –2 3 (xiii) C–O bond is same in CO (xiv) All the C–C bond distance in benzene are equal [ 1.39 Å] (resonance). In acetate ion both the C–O bonds are of equal length [1.26 Å] (resonance) Determination of bond length : X-ray analysis, spectroscopy | C–H [99.0] N=N [100.3] O–H [110.6] C=O (CO2) [192] N–H [93.6] C=O (ketone) [179] C–O [85.5] C=O [HCHO] [166] C–Cl [81.1] C=N [147.2] C–Br [68.0] N=O [145.1] C–I [51.1] C=S (CS2) [128] O–F [45.2] CC [200] O–Cl [52.1] CN [212.4] O–Br [48.2] C=C [142] C–C [80] O–O [119] Organic Chemistry – Some Basic Principles and Techniques Homolysis of a sigma bond forms free radical BOND ANGLE intermediate species The angle between two adjacent bonds, in a molecule is known as bond angle, e.g. A : B  A   B Compound Angle Number Homolysis of a pi bond forms a biradial intermediate (i) acetylene H–C–C (180°) two (ii) ethylene H–C–C (120°) six (iii) ethane H–C–C (109º 28') twelve species   A = B  A  B CARBOCATIONS (iv) benzene eighteen (v) alcohol R–O–H (105.5°) one (vi) ether R–O–R (110°) one COVALENT BOND BREAKING (1) Heterolysis, heterolytic fission or heterolytic cleavage (2) Homolysis, homolytic fission or homolytic cleavage 1. Heterolytic Fission When the covalent bond breaks in such a way that the both the electrons of the bond pair remain with only one of the two atoms, the process is called heterolysis or heterolytic fission or ionic fission or unsymmetrical fission or polar cleavage. Heterolysis of a sigma bond forms a cation and an anion as ionic intermediate species. or When a carbon atom in an organic compound is covalently bonded to a more electronegative atom. Z, the bonding electrons are more shifted towards Z than carbon atom. Therefore, this covalent bond has greater tendency to undergo heterolysis in such a way that the electron pair goes to Z which develops negative charge while carbon atom develops positive charge as shown below The species formed by heterolysis of a covalent bond in an organic compound and having positively charged carbon atom is known as a carbocation The positively charged carbon atom in an alkyl carbocation has six electrons in its outermost energy level and this carbon atom is in a state of sp2 hybridisation. Due to sp2 hybridisation, the geometry around the positive carbon atom is trigonal and the value of bond angles is 120° as shown below. It is clear that heterolysis of a pi bond should form a bipolar species, as follows. 2. 1. Homolytic Fission When the covalent bond breaks in such a way that the electrons of the bond pair are distributed between the two atoms i.e. each atom gets a share of one electron, the process is called homolysis or homolytic fission or nonionic fission or symmetrical fission or free radical cleavage. Type or carbocations Carbocations are classified into three categories. (i) Primary or 1° carbocations (ii) Secondary or 2° carbocations (iii) Tertiary or 3° carbocations Organic Chemistry – Some Basic Principles and Techniques (i) Primary or 1° carbocations have positive charge (i) Stability of Alkyl Carbocations on primary carbon atom. For example The order of stability of alkyl carbocations is as follows  CH3 Tertiary > Secondary > Primary Methyl carbocation  CH3 – CH2 Ethyl carbocation  CH3–CH2– CH2 This order of stability is explainable on the basis of increase in the electron-donor (or + I effect due to hyperconjugation) with increase in the number of alkyl groups on the cationic carbon atom. as shown below n-Propyl carbocation  CH3–CH2–CH2– CH2 n-Butyl carbocation CH3 | tert-Butyl Isopropyl carbocation carbocation (Tertiary) (Secondary)  CH3–CH– CH2 Isobutyl carbocation (ii) Secondary or 2° carbocations have positive charge on secondary carbon atom. For example  CH3– CH –CH3 Isopropyl carbocation  CH3–CH2– CH –CH3 sec-Butyl carbocation (iii) Tertiary or 3° carbocations have positive charge on tertiary carbon atom. For example Ethyl Methyl carbocation carbocation (Primary) (Primary) (ii) Stability of Aryl Carbocations The increase in the number of phenyl groups on the cationic carbon atom results in tremendous increase in the stability of the aryl carbocations. This is because of resonance due to which the positive charge of carbon atom gets delocalized to o-, p– and o –positions of the benzene rings.  (tert. Butyl carbocation)  (C6H5)3 C > (C6H5)2 CH > C6H5 CH2 > CH2   = CH – CH2 > 3° > 2° > 1° > CH3 (1°) 2. Stability of Carbocations [Stability  No. of resonating structures] Carbocations are reactive due to positive charge on     the carbon atom. Increase in positive charge Examples : CH3 , CH3 CH2 , (CH3)2 CH , (CH3)3 C increases their reactivity and decreases their stability. 3. Organic Chemistry – Some Basic Principles and Techniques Resonance in benzyl carbocation : Formation of carbonium ion  (i) (CH3)3C–Cl  (CH3)3 C + Cl– [Heterolytic fission ]  H O H (ii) CH3OH    CH3 OH2  CH3  2 [Protonation]   H (iii) CH3–CH = CH2  CH3 – CH – CH3 [Protonation] (iv) Abstraction of halide ion by lewis acid [Four structures] Obviously benzyl carbocation is more stable as compared to allyl carbocation. (ii) Stability  Reduction of positive charge on the carbon Lewis acid Lewis acid CARBANIONS (v) Removal of N2 from diazonium cation Reactions : (i) Nucleophilic substitution of 3° alkyl halide (ii) Markownikoff reaction When a carbon atom is bonded to a more electron donor atom or group Z. in an organic molecule. The shared pair of electrons is more shifted towards the carbon atom. This bond during chemical reaction can undergo heterolysis in such a way that the electron pair remains with carbon atom and Z does not receive the share of the electron pair. Thus carbon atom acquires a positive charge and Z acquires a negative charge as shown below (iii) Dehydration of alcohols (iv) Hydrolysis of acetyl chloride Examples :conc. H SO Ethyl alcohol   ethene 170 2 4 Mechanism  H  CH3– CH3–CH2OH     H O H  CH3 – CH2   CH2 = CH2 CH2 OH2  2 Resonance in allyl carbocation : [Two structures] The organic species obtained as above, having negative charge on carbon atom is known as carbanion. The negatively charged carbon atom of a carbanion has eight electrons in its outermost energy level. The carbanion carbon atom of an alkyl carbanion is in a state of sp3 hybridisation. The geometry of the methyl carbanion is trigonal pyramidal similar to that of ammonia molecule. Organic Chemistry – Some Basic Principles and Techniques 1. Types of carbanions (ii) (i) Primary Carbanions CH3– Ethyl Methyl carbanion CH3–CH2– n-propyl carbanion –CH3   CH2  carbanion 2     (v) CH2  CH2  NO2  CH3  CH  NO2  CH3 2 CH CH3–CH2– Isopropyl carbanion  (iv) CH2  CHO  CH3 (ii) Secondary Carbanions CH3–  (iii) –CH3   (vi) F  CH2  Cl  CH2 sec-Butyl carbanion  (vii) Resonance exists in CH2 –CN, hence it is (iii) Tertiary Carbanions stable carbanion CH3– –CH3 | CH3 (viii) Carbanion in which negative charge and double bond are in conjugation tert-Butyl carbanion 2. Note : Carbanions are nucleophile Stability of Carbanions Carbanions are reactive due to negative charge on 3. the carbon atom. Increase in negative charge increases their reactivity and decreases their stability. Similarly decrease in negative charge decrease the reactivity of the carbanions and increases their stability. If the hydrogen atom present on the negatively charged carbon atom of a carbanion is substituted by an electron-donor group. There is an increase in the electron density on the carbanion carbon and thus decrease in the stability. – (i) (ii) Reactions : > 1° > 2° > 3° – Formation of Carbanion – – Examples : CH3 , CH3 2 CH,CH3 CH2 ,CH2 CHO (i) Aldol condensation (ii) Claisen condensation (iii) Decarboxylation (i) Example : Explanation :- Stability of alkyl carbanions can be explained by inductive effect. Greater the number of alkyl group [+ I effect] attached to the carbon atom bearing negative charge, lesser is the stability.  3° < 2° < 1° < CH3 soda  lim e RCOOH    RH Organic Chemistry – Some Basic Principles and Techniques Reactions ORGANIC FREE RADICALS When a carbon atom in an organic compound is covalently bonded to an atom or a group. Z which is electroneutral or very weakly electron-donor or very weakly electron-acceptor. The electron pair of the covalent bond is almost equally shared by C and Z. Such a covalent bond has a tendency to undergo homolysis during chemical reaction to form electroneutral species. Each having one unpaired electron and are known as free radicals. The reactive intermediate organic species having an odd or unpaired free electron on a carbon atom is called an organic free radical. Example (i) Chlorination of alkanes (ii) Pyrolysis of alkanes (iii) Wurtz reaction (iv) Anti-Markownikoff rule (v) Kolbe electrolytic synthesis (vi) Polymerisation initiated by free radical Example CH4 + Cl2 CH3Cl + HCl Step - I [initiation] Cl2  Cl  + Cl  Step - 2 [propagation] CH4+Cl   CH3  + HCl and CH3  + Cl2  CH3Cl + Cl  1. Example –C  , CH 3  , (C 6H 5)3 C  , CH 2 = CH–CH 2  , (CH3)2 CH  , CH3CH2  Step-3 [termination] Formation of free radicals or CH3  + Cl   CH3 – Cl (i) Cl2 Cl  + Cl  [ Homolytic fission] CH3  +CH3CO  CO +  CH3 (ii) CH3COCH3 (iii) (C2H5)4 Pb Pb + 4C2H5  (iv) CH3–N=N–CH3  N2+2CH3  (v) C6H5– –O–O– –C6H5    2C6H5COO   2C6H5  +2CO2 (vi) Fe+2 + H2O2  Fe+3 + OH + HO  [Fenton's reagent] – (vii) RCOO  RCOO  + e  R  + CO2 Salient features (i) Free radical reactions are catalyzed by light, heat etc. (ii) Free radical reactions proceed in vapour phase or in nonpolar solvents. (iii) Free radical reactions are frequently autocatalytic anode Cl  + Cl   Cl2 or CH3  + CH3  2.  CH3 – CH3 Order of stability of free radicals (C6H5)3 C  > (C6H5)2CH  > C6H5CH2  > CH2 = CH –CH2  > 3° > 2° > 1° >CH3  Explanation – Lesser is the bond dissociation energy, greater is the ease with which the free radical is formed (greater its stability) CH3 – H  CH3  + H  , H = 435kJ CH3 – CH2 –H H = 411kJ (CH3)2 CH–H H = 395kJ (CH3)3 C– H H = 381kJ    CH3 –CH2  + H  , (CH3)2CH  + H  , (CH3)3C  + H  , It is apparent that 3° free radical is formed easily. Order of the potential energy of the free radicals t-butyl < isopropyl < ethyl < methyl Organic Chemistry – Some Basic Principles and Techniques Example-1 : Photolysis of diazomethane forms a carbene Stability on the basis of resonance species, called methylene intermediate. Stability Number of resonating structures  CH 2N2 :CH2 + N2 Triphenyl methyl free radical has the maximum number of resonating structures. Hence it is the most Example-2 : Reaction of metallic zinc on methylene dibromide or diiodide also forms methylene stable free radical intermediate Resonance in allyl free radical CH2Br2 :CH2 + ZnBr2 Resonance in benzyl free radical : The carbon atom of the methylene intermediate referred to in the above two examples has two bond pairs and two nonbonding electrons. The two odd electrons occupy two separate orbitals and have parallel spins. Therefore such a methylene species may be regarded as a biradical that can act as a normal free radical in two steps as follows (i)  CH2  3. + Methylene biradical Geometry (i) Trigonal planar (ii)  Z   CH2 – Z Free radical Intermediate free radical – Z +  Z  Z – CH2 – Z Product Example-3 Example-4 CH2=C=O  :CH2 + CO ketene  + OH  dichlorocarbene : CCl3  : Cl  :CCl2  (CH3)3 C or (C6H5)3 C [sp2] H2O + :CCl3 ; Two forms of carbenes  (ii) Planar : CH2 = C –CH3 [sp2] (iii) Free radicals are electrophiles INTERMEDIATE CARBENE SPECIES The central carbon atom of a carbene species is neutral or uncharged. This carbon atom has two sigma bonds and two nonbonding electrons. i.e. there are six electrons or an incomplete octet in its outermost energy level. Reactions (i) Hofmann carbylamine reaction (ii) Riemer Tiemann reaction (iii) Formation of cyclopropane from ethene and diazomethane Example CH2N2  :CH2 + N2 CH2=CH2 + :CH2  INTERMEDIATE NITRENE SPECIES Organic Chemistry – Some Basic Principles and Techniques Examples : Positively charged species In a nitrene species the central nitrogen atom is neutral i.e. it bears no charge. This neutral nitrogen atom has one sigma bond and two lone pairs, i.e. a total of six electrons in three pairs in its outermost energy level. In order to complete its octet the nitrogen atom of a nitrene species tries to acquire two electrons from somewhere and form a covalent bond thereby behaves as an electrophile.       H , CH3 , H3O+, NO2 , NO , Br , C6H5 N2 ,   CH3 CO , CH3 N2 Neutral Species : :CH2 , C2H5  , AlCl3, BF3 , ZnCl2 , SO3. FeCl3, Cl, Br, I, CH3  , SnCl4, PCl3, CO2, SO2 Starred atom : Example Note : In *I – Cl , chlorine is more electronegative than iodine [EN(Cl) > EN(I) ] as such an electron deficient centre is created on iodine indicated by star. Reaction : Hofmann bromide reaction. Formation - Amide + bromine + caustic potash + Br2 + 2OH  For SO3, electrophilic nature can be understood by its structure.   + 2H2O + 2Br Clearly a positive centre is developed at the site of the sulphur atom. (Acylnitrene) Rearrangement 2. Nucleophile A – B + Y  A – Y + B Y – Nucleophile ELECTROPHILIC AND NUCLEOPHILIC REAGENTS These are negatively charged ions or neutral molecule with unshared pair of electrons. They are electron rich or Lewis bases. Examples - Negatively charged ions Electrophile 1. Nucleophile Neutral Species - Electrophile  A–B+ X   A–X+ B  X - Electrophile These are positively charged ions or neutral molecules containing electron deficient atom. They are Lewis acids. , Starred atom : , R2O ,R2S : * * C2H5 L i , C2H5 – MgBr , * NaBH4, L iAlH4 3. Organic Chemistry – Some Basic Principles and Techniques Reaction INDUCTIVE EFFECT In fact the reaction is the mutual attack of an Electron displacement along a carbon chain due to electrophile on nucleophile or vice-versa. the presence of an attached substituent is known as electrophile + nucleophile  product inductive effect. It is a permanent effect and decrease rapidly with the length of carbon chain. (CH3)3 C + OH  (CH3)3 –OH t-butyl alcohol CH3 CCCC Z 4 3 2 1 + CN  CH3CN methyl cyanide 1. CH3 – CH2 + :NH3  CH3CH2NH2 + H 4. ethylamine The strength of electrophile follows the order (i) CH3 > CH3 CH2 > (CH3)2 CH > (CH3)3 C 5. (ii) NO2 > NO (iii) Cl > Br > I The strength of nucleophile follows the order – 6 – – (i) O C2H5 > OH > CH3CO O (ii) I– < Br– < Cl– (iii) CH3– > NH2– > OH– > F– Electrophile and nucleophile at a glance S.No. Electrophile Nucleophile 1. Accepts the Supplies the electron pair electrons pair 2. Electron deficient Electron rich 3. Attacks the points Attacks the of high electron point of low density electron density 4. Lewis acid Lewis base 5. Possesses an Possesses an empty orbital to electron pair receive the which is loosely electron pair held and can be supplied easily 6. Usually positively Usually charged negatively species charged species 7. Forms an extra Increases its bond with the covalency by nucleophile one unit Inductive effect is of following types (i) Negative Inductive Effector - I Effect Some atoms or groups have a greater tendency to attract the shared electron of the covalent bond. Such atoms or groups acquire partial negative charge by receiving electron density from the covalent bonds of the chain. Therefore, these are classified as the groups exerting negative inductive (–I) effect. For example    NH3 ,  NR 3 , –NO2 , – CN , – COCl, –COOH –CHO , –COOR, –F, –Cl, –Br, –I, –CCH –CH=CH2, –CO–R, –OH, –SH, –NH2 , –NHR, –NR2 , –NH–CO–R, –CF3, –CCl3, etc. The decreasing order of negative inductive effect of some important atoms and groups is given below Order of - I effect    NH3 > N (CH3)3 > – NO2 > – CN > –F > –Cl > –Br > –I > –CF3 > –CCl3 > –OH > –OCH3 > – C6H5 > –H (ii) Positive Inductive Effect or +I Effect Some groups are electron donor and therefore acquire partial positive charge by increasing electron density in the covalent bonds of a chain. Such groups exert positive inductive (+I) effect. Alkyl radicals are the best examples of this type of groups. The decreasing order of positive inductive effect of some alkyl groups with respect to hydrogen is given below. Order of +I Effect (CH3)3C–> (CH3)2CH – > CH3CH2 –> CH3 – > H– 3° 2° 1° 1° 2. Organic Chemistry – Some Basic Principles and Techniques Examples [applications] :Ex.51 Discuss the decreasing order of acid strength of (i) HCOOH > CH3COOH trihalogenated derivatives of acetic acid. (ii) CCl3COOH > CHCl2COOH > Sol. The acid strength of the trihalogenated acetic acids CH2ClCOOH > CH3COOH increases with increase in the –I effect of the halogen (iii) CH3CH2 COOH > 2CH2CH2COOH substituents. Three fluorine atoms have greater –I (iv) effect than the other three halogens. Therefore trifluoroacetic acid ionises more than the other analogues. Thus, the decreasing order of acid strength is as follows CH2OH > CH3CH2OH (v) p-Nitrophenol > Phenol (vi) CH3CH2OH > (CH3)2CHOH > (CH3)3COH (vii) CH2OH > CH2OH > CH2OH (viii) HCOOH > CH3COOH > C2H5 COOH > n-C3H7 COOH CF3 –COOH > CCl3–COOH > CBr3 –COOH > CI3 –COOH Ex.52 Discuss the relative order of acid strength of acetic acid and benzoic acid. Sol. Benzoic acid is a stronger acid than acetic acid. This is because methyl group present in acetic acid has a weak donor (+I) effect and phenyl group present in benzoic acid has a weak acceptor (–I) effect 3. Order of basic character :(i) NH3 > ClNH2 (ii) (CH3)2NH > CH3NH2 (iii) C2H5NH2 > C6H5NH2 4. Other examples :(i) Order of acid strength : CH4 < CH3I < CH3Br 5. < CH3Cl < CH3F (ii) Order of acid strength of -halo acids -fluoroethyl alcohol > -chloroethyl alcohol > -bromoethyl alcohol (iii) Order of acid strength of aromatic alcohols : p-nitrobenzyl alcohol > benzyl alcohol (iv) Order of acid strength in water (a) HCCl3 > CH2Cl2 > CH3Cl > CH4 (b) HCOOH > CH3OH > CH4 Ex.50 Why is formic acid stronger than acetic acid ? Sol. In formic acid, COOH group is bonded to the hydrogen atom, which does not exert any inductive effect. In acetic acid, COOH group is bonded to a methyl group, which exerts +I effect. The acetate ion formed due to ionisation of acetic acid is more reactive due to increase in the electron density on oxygen atom. This makes acetate ion more reactive than formate ion. Thus acetate ions react faster with hydrogen ions shifting the equilibrium in backward direction. Thereby decreasing the concentration of hydrogen ions. Therefore acetic acid is weaker than formic acid. C6H5 – COOH > CH3 – COOH Comparison of Base strength Aliphatic amines are stronger bases than aromatic amines. The basic behaviour of amines is due to availability of lone pair of electrons present on the nitrogen atom. The positive inductive effect of methyl group increases electron density on nitrogen atom, due to which electron pair on nitrogen atoms is more readily available to the attacking acid. Therefore, alkylamines are more basic than ammonia. In aromatic amines, the NH2 group is bonded to the aromatic ring, which exerts a weak –I effect. That is why, aniline is a weaker base than ammonia. R – NH2 > H–NH2 > C6H5 – (+I effect of R) (No effect of H) (–I effect of C6H5) However, the major cause of much lower base strength of aniline than ammonia is resonance. A partial positive charge develops on the nitrogen atom of aniline due resonance, thereby the availability of electron density on nitrogen atom in aniline decreases. Organic Chemistry – Some Basic Principles and Techniques ELECTROMERIC EFFECT Transfer of an electron pair from one location to the Ex.53 Indicate resonating structural formulae of carbon other in the structural formula of an organic dioxide compound under the influence of a reagent, is known as electromeric effect. Electromeric effect is a temporary effect that does not take place of its own, Sol. but occurs on the demand of the attacking reagent. This effect vanishes on the completion of the reaction. The electromeric effect is symbolized by a curved 1. Resonance Energy Due to resonance, the per mole energy of the arrow ( ). Electromeric effect can normally take resonance hybrid of a molecule or ion is less than place in the following three ways the per mole energy calculated for the most stable (a) Transfer of pi electron pair to the adjacent atom of the canonical structures. This energy difference is in the form of nonbonded electron pair known as resonance energy. Higher the value of resonance energy, greater is the resonance stabilization. In the above representation, the two electrons of the pi bond situated between atoms A and B get transferred to atom B as lone pair. Ex.54 What is the increasing order of resonance (b) Transferred of nonbonded electron pair to the stabilization of benzene, naphthalene and anthracene same atom in the form of pi electron pair Sol. The values of resonance energy for benzene, (c) Transfer of pi electron pair to the adjacent atom in the form of pi electron pair RESONANCE Many polyatomic molecules and ions having pi bonds can be represented by more than one structural formula,. But none of these structural formulae truly stands for that molecule or ion. because none of them can explain its properties. These hypothetical or 2. nonexistent structural formulae are known as resonating structures or contributing structures or canonical structures. The actual or existent structure is the hybrid of all the canonical structures and is called resonance hybrid structure. This property is known as resonance, In resonance, the positions of the nuclei of the atoms do not change, but the pi electrons are continuously moving or delocalized. Thus, the delocalization of pi electrons is the cause of resonance. Since the pi electrons constantly move, it becomes more difficult for the reagent to attack then and therefore the molecule or ion undergoing resonance acquires greater stability. This is called resonance stabilization. naphthalene and anthracene are 36, 76 and 85 Kcal per mole respectively. It is clear from these values that the resonance stabilisation of naphthalene is more than that of benzene and less than that of anthracene. Thus, the increasing order of resonance stabilisation is as follows. Benzene Naphthalene Anthracene Resonance Effect or Mesomeric Effect (R or M Effect) When an atom or a group of atoms or a substituent present in a molecule exerts electromeric displacement of a p* i electron pair or a lone pair and thus initiates resonance in the rest of the molecule. The phenomenon is known as Mesomeric effect or resonance effect. The electron pair donor substituent is said to exert positive resonance (+R) effect or positive mesomeric (+M) effect. The electron pair acceptor substituents are said to exert negative resonance (–R) or negative mesomeric (–M) effect. Organic Chemistry – Some Basic Principles and Techniques (i) Groups Exerting +R or +M Effect Following are a few examples of the groups exerting positive mesomeric effect. –OH , –NH2, –OR, –NHR, –NR2, –O, –CH3, –Cl If a +M group is present on benzene ring, it increases electron density on o–and p-location, due to which the attacking electrophile largely forms a mixture of o– and p– electrophilic substitution products. Therefore in aromatic electrophilic substitution reactions, the groups exerting +M effect are mainly o- and p- directing, as illustrated below by taking the example of phenol. Few points [M and R effect ] This effect operates in unsaturated molecules in which – There is a conjugated system of double bond. – Negative charge is in conjugation with double (or multiple) bond. – Lone pair of electron in conjugation with double (or multiple) bond. There is complete transfer of electron pair during the operation The reactivity of compounds is affected by the presence of .. (ii) Groups Exerting –R or –M effect Following are a few examples of groups exerting negative mesomeric effect. –NO2 , –CN, –CHO , –COCH3 , .. –NO2, –CN , > C = O , – C l , NH2 etc. groups. There is relay of -electron from one end to the other end through a conjugated system of double bond. Other Illustrations : Order of stability of canonical forms : When a substituent exerting –M effect is bonded to (a)Vinyl chloride : the benzene nucleus, it reduces electron density at H2C=CH – the ortho and para locations. Therefore, the I electrophile cannot attack the positively charged    H2 C –CH = H2 C – CH = C l : ortho and para positions, but can collide with the electroneutral meta position slowly. Hence, in II III III – least stable [carbon is electron deficient] aromatic electrophilic substitution reactions the II – less stable [charge separation ] groups exerting –M effect exhibit meta-directing I – stable [no charge separation] influence. This is illustrated below by taking the example of nitrobenzene.  III < II < I –COOH, –SO3H, –CF3, –CCl3 Organic Chemistry – Some Basic Principles and Techniques (b) Explanation of Order of Stability of Alkyl Carbocations HYPERCONJUGATION OR BAKER-NATHAN EFFECT In an organic molecule, the transfer of a sigma electron pair (heterolysis) or a single electron from a sigma bond (homolysis) may also be responsible for initiation of resonance in the rest of the molecule. This is referred to as hyperconjugation or BakerNathan effect. It was also called no-bond resonance because in the canonical structure of the molecule, no bond is shown between the two atoms from where the sigma electrons/s has/ have been transferred. It can also be called – conjugation because a sigma electron/s is/are transformed into a pi pair in this type of resonance. However, for the first time, Mulliken in 1941 gave the name hyperconjugation to this phenomenon. Ex.55 (I) (II) In the above structural part (I) , the sigma electron pair present between H and C–1 atoms shifts between C–1 and C–2 atoms as pi electron pair. Simultaneously, the pi electron pair present between C-2 and C-3 shifts to C-3 as a lone pair. As a result of these electron pair transfers. positive charge develops on hydrogen atom and negative charge develops on C-3 atom. Note that no bond is shown between H and C–1 atom in the canonical structural formula (II) 1. Applications of Hyperconjugation (a) Explanation of Electron Donor Property of Alkyl Radicals The cause of electron donor effect or +I effect of alkyl radicals is actually hyperconjugation. The stability of alkyl carbocations decreases in the order 3° > 2° > 1° . For example  Larger the number of locations for delocalisation of the positive charge in the structural formula of the carbocation, lower is the reactivity or higher is the stability of that carbocation. Methyl carbocation is least stable amongst alkyl carbocations, because of only one possible location for positive charge. Thus, methyl carbocation has only one structural formula Ethyl carbocation is more stable than methyl carbocation, because of four possible locations for its positive charge i.e., one carbon atom and three -hydrogen atoms. Thus, ethyl carbocation can be represented by the following four structural formulae. H H H C H H C H H H H C H H C H (c) Explanation of Stability of Alkyl Free Radicals The stability of alkyl free radicals is also similar to that of alkyl carbocations. i.e. 3° > 2° > 1° , because here also methyl, ethyl, isopropyl and tert-butyl free radicals can also be represented by one, four, seven and ten structural formulae. respectively.  Ex.   (CH3)3 C > (CH3)2 C H > CH3 – CH2 > C H3    (CH3)3 C > (CH3)2 C H > CH3 – C H2 > C H3 Organic Chemistry – Some Basic Principles and Techniques Example : STERIC HINDRANCE On account of the presence of bulkier groups at the reaction centre, they cause mechanical interference and with the result that the attacking reagent finds it difficult to reach the reaction site and thus slows down the reaction. This phenomenon is called steric (i) hinderance. (C2H5)3 N SE , C6H6 + Br+  C6H5Br + H+ SE , CH4 + Cl2 CH3Cl + HCl SN reaction or nucleophilic substitution reaction     R – X  Nu  R – Nu + X Example :1. SN , CH3Br + CN–  CH3CN + Br– : is less basic than (CH3)3 N alkyl halide nucleophile : Protonation easier : (Smaller size of CH3 group) Here : R– alkyl group X– – leaving group (halide ion) [(CH3)3] N–H]+ Nu : attacking species [ It is a weaker nucleophile, being conjugate base of hydrohalic acid ] Example : Protonation difficult : [Bigger size of CH3CH2 group]+ [(CH3CH2)2N – H]+ ORGANIC REACTIONS AND MECHANISM Types of reaction :Organic substrate + Reagent  Product Mainly four types of reactions are (a) Substitution (b) Addition (c) Elimination (d) Rearrangement or isomerisation (a) [SN 1] or unimolecular nucleophilic Substitution Reaction [substrate–3° or tertiary halide] Alkaline hydrolysis of tertiary butyl chloride – (CH3)3 C–Cl + OH   (CH3)3 C–OH + Cl Mechanism Step - 1 (Slow) 1. Substitution Reaction It involves replacement of one substituent by another A–B +x – y A–x +B– y (CH3)3 C–Cl    (CH3)3 C +C l Step - 2 (fast)  (CH3)3 C + OH   (CH3)3 C–OH t – butyl alcohol Rate equation :V = k [ t - butyl halide] Molecularity - 1, Organic Chemistry – Some Basic Principles and Techniques Note :-Here the carbonium ion is planar [sp hybridised] Orbital picture of transition state : The nucleophile can attack the planar carbonium on either side to give t-butyl alcohol. 2 at the point of at the point of bond making bond breaking (c) SN 1 or SN 2 reaction [Substrate-2° halide] Non polar medium .................. SN2 reaction If the three groups attached to positively charged carbon are different than the product will be racemic. (b) [SN2] or Bimolecular Nucleophilic Substitution Reaction [substrate- methyl or 1° halide] Alkaline hydrolysis of methyl chloride : CH3Cl + OH–  CH3OH + Cl– Mechanism : (Concerted one step process) Rate equation : V = k [CH3Cl] [ OH–] Molecularity - 2 Polar medium ................... SN1 reaction Transition state and Intermediate Transition state : A transition state refers to an imaginary state and cannot be isolated (SN2) Intermediate : Intermediate is a stable real species and can be isolated under specific condition [SN1 (carbocation)] (ii) SE reaction or electrophilic substitution reaction Benzene usually show electrophilic substitution reaction Here the starred carbon (in benzene) C* is in sp2 hybridisation state. Electrophile attacks and arenium ion is formed as an intermediate. Note : In the slow or rate determining step two species are taking part. Hence it is a bimolecular reaction. Further OH– attacks from the rear (back) side and as such inversion takes place. Supposing in place of methyl chloride there is optically active primary halide then the product obtained will be reverse of the original substrate as inversion takes place. Since in the rate determining step two species electrophile and benzene participates so it is a bimolecular reaction. Rate = k [benzene]. [electrophile] Organic Chemistry – Some Basic Principles and Techniques (iii) Free radical substitution SF – Chlorination of methane is the example of free radical substitution Methylation of benzene Example 1. CH3 Cl / AlCl3 C6H6     C6H5CH3 Mechanism Step-1 : CH3–Cl + AlCl3 Reaction :CH4 + Cl2 –  Mechanism :- Step-2 (Slow) (1) Step-1 (Chain initiation step) Cl  + Cl  Cl–Cl (2) Step-2 (Chain propagation step)  CH3 + C6H5 – H  Cl  + CH4  CH3  + HCl CH3  + Cl2  CH3Cl + Cl  carbocation (-complex intermediate)  (3) Step-3 (Chain termination step) CH3  + CH3   Step-3 : H  AlCl 4  AlCl3 + HCl CH3  + Cl  Nitration of benzene : Example 2. CH3 – Cl + HCl    CH3 – CH3 CH3–Cl Cl  + Cl   Cl2 HNO 3 / H2SO 4 C6H6     C6H5NO2 + H2O Mechanism : Step-1 : 2H2SO4 + HO – NO2  2. Addition reaction : Usually unsaturated molecule undergoes addition reaction. In this type of reaction one -bond is broken and two -bonds are formed   NO2 + 2HSO4– + H3 O [1 , 1]  Step-2 : N O 2  C6H5  H  [3] (i) Electrophilic addition reaction Substrate - Alkene, Alkyne   C 6H5NO 2  H Step-3 : (–complex) H2SO4 Example 3 : Sulphonation of benzene : conc . H SO C6H6     C6H5 – SO3H + H2O Mechanism : 2 4    H2O + SO3 Step-1 : H2SO4  Step-2 : SO3 + C6H5 – H   –    C6H5 – S O3 + H   C6H5SO3H (–complex) trans-addition It should be noted that : – When polar molecular attacks the double bond of alkene, then -electrons of the double bond shift to one of the carbon atom due to electromeric effect , the electrophile attaches to that carbon atom and carbocation is formed Organic Chemistry – Some Basic Principles and Techniques – The nucleophilic part of the reagent attaches to Example :- Addition of HCN at acetone : positively charged carbon and forms the end product. Mechanism : This is of course a trans-addition. Base in    + CN– HCN small – Since the initial attack is of a electrophile this is quantity H referred as electrophilic additio Other example :H CH2 = CH2 + H –OH   CH3 –CH2OH (iii) Free radical addition : Anti-Markownikoff's reaction- peroxide effect Addition of HBr to propene in presence of peroxide Peroxide CH3 –CH=CH2 + HBr   CH3–CH2–CH2Br Mechanism :. Homolysis    2RO (ii) Nucleophilic addition reaction [ Substrate - Aldehyde, Ketone ] (i) Chain initiation :RO. + HBr  ROH + Br. (ii) Chain propagation : (two steps) General Mechanism >C = O + E – Nu  > –OE CH3–CH=CH2 + Br.   CH3 – C H –CH2Br 2° free radical  CH3– C H – CH2Br + HBr  CH3–CH2– CH2Br + Br. (iii) Chain termination :  Br. + Br. or 2CH3 – C H – CH2Br attack Oxygen bearing negative charge less stable more stable (forms) (does not form)  > C – OE | Nu or  CH3 – C H –CH2Br + Br. Addition of Br2 to Ethene ; H2C=CH2 + Br2 Mechanism : Br2 2Br. CH2 = CH2 + Br  Br  – CH2 – Br   Br–CH2–CH2–Br  CH2 3. Organic Chemistry – Some Basic Principles and Techniques Elimination reaction : Elimination is reverse of (i) E2 reaction : Elimination bimolecular addition. In this type of reaction two bonds are – Reaction involves one step only broken and 1  bond is formed. – Base pulls out a proton from the -carbon atom, A= B + x – y – Simultaneously, a halide ion leaves and the double bond is formed (three ) (one– and one–) (one–) (i) Dehydration : Conc. H2SO 4   CH =CH + H O CH3CH2OH  2 2 2 170 (ii) Dehydrobromination : Rate equation : alc . KOH CH3–CH2Br    CH2 = CH2 + HBr v = k [ halide ] [:B] (iii) Debromination : Molecularity - 2 Zn  CH = CH + ZnBr CH2Br–CH2Br  2 2 2  (ii) E1 reaction :- Elimination unimolecular CH3CHBr2 + 2Zn + Br2CHCH3  CH3–CH = CH – CH3 + 2ZnBr2 Step-1 Heterolysis of substrate gives carbocation and halide ion (slow) (iv) Dehydrogenation : Step-2 Carbocation gives up proton to a base immediately and alkene is formed Cu   CH –CHO + H CH3–CH2OH 300 3 2  Examples of Cycloalkenes : (v) Dehydration of Cyclohexanol : (vi) Dehydrocholorination of clohexylchloride Rate equation : v = k [RX] Molecularity - 1 (v) Debromination of 1,2-dibromocyclohexane 4. These reactions have been divided into two classes (1) E2 reaction (2) E1 reaction Rearrangement reaction : Substituents change their positions Examples : (i) Phenylhydroxylamine Organic Chemistry – Some Basic Principles and Techniques REACTION AT A GLANCE Class of compounds Type (i) Alkene, alkyne Electrophilic addition (ii) Aldehyde, ketone Nucleophilic addition (iii) Aldehyde Nucleophilic substitution (iv) Aromatic compounds Electrophilic substitution p-Amino phenol (v) Alkane Free radical substitution (ii) Acetyl nitrene Methyl isocyanate (iii) Sodium phenyl carbonate Sodium Salicylate MARKOWNIKOFF'S RULE AND PEROXIDE EFFECT Markownikoff's Rule : When an unsymmetrical reagent adds to an unsymmetrical unsaturated hydrocarbon, the negative part of the addendum adds on to the carbon containing less number of hydrogen atom. (CH3)2C=CH2 + HCl  (CH3)2C–CH3 | Cl Mechanism : (iv) [3° carbonium ion ] Sulphamic acid (v) Sulphanilic acid H–N = C = O  N  C – O – H Isocyanic acid Cyanic acid (vi) CH3–CH2– C H2  CH3 – C H–CH3 1° Carbonium 2° Carbonium (CH3)2 C – CH3 + C l (CH3)3C–Cl Second rule : In the addition of HCl to vinyl chloride the chlorine, attaches itself to the carbon on which the chlorine atom is already present. CH2=CHCl + HCl  CH3CHCl2 Mechanism : (vii) p-Hydroxy acetophenone (viii) Methyl ethyl ketoxime N–Methyl propionamide Peroxide effect : In the presence of air or peroxides addition of hydrogen bromide to unsymmetrical unsaturated hydrocarbon takes place reverse to that of Markownikoff's rule [Kharash effect] (CH3)2C = CH + HBr (CH3)2 CH–CH2Br It should be noted that HCl and HI do not show peroxide effect.