Hydrocarbon. Alkene

Hydrocarbon ALKENE Alkene are also called olefins as they forms oily Sol. a and b – Position isomerism compounds on reaction with halogens (oleum, oil a and c – Chain isomerism + fines, forming). a and d – Ring chain isomerism Firstly, Ethylene reacts with Cl2, gives oily liquid b also show Geometrical isomerism ethylene dichloride. That is why ethylene called as olefiant gas. (Olefiant gas, oleum, oil + fiant, make) CH2 CH2 + Cl2 Cl—CH2—CH2—Cl Property of alkene General formula C—C Bond energy C—H Bond energy Bond angle 109°.28 Characteristic reaction C—C Bond length C—H Bond length Hybridisation on C %C : %H Empirical formula Characteristics CnH2n 142 Kcal/mole 98.67 kcal/mole Trigonal planar Electrophilic addition reactions 1.34 A° 1.10 A° sp2 85.71 : 14.29 CH2 Isomerism : Alkenes show chain, Ring chain or functional, Position, Geometrical isomerism and optical isomerism. For more details refer to isomerism provided to you in study material. cis–2–butene trans–2–butene Ex.16 How many minimum carbon atom persent in optically active alkene? Ans. 6C, 3–Methyl–1–Pentene General Methods of Preparation : (1) From Alcohols : Alkenes can be prepared from monohydric alcohols or alkanols by the loss of H2O and the reaction is known as dehydration reaction. For the dehydration, the presence of a -hydrogen in alcohol is necessary.  Alcohol Ex. 15 What is relation between CH3CH CH2 , ? Ring chain isomerism Sol. Ex.16 (a) CH3CH2CH (b) CH3—CH CH2 CH—CH3 (c) (d) The dehydration can be carried with Al2O3 or with mineral acid upon heating. (a) Dehydration with Al2O3 : Ethene is prepared by heating ethanol with Al2O3 at 620 K.  CH2 CH3—CH2—OH  CH2+H2O Ethanol Ethene (b) Dehydration with mineral acid : Alcohols upon heating with conc. H2SO4 form alkenes and the reaction is called acidic dehydration. CH3—CH2—OH  CH2 Define relations between a,b,c,d ? Ethanol CH2+H2O Ethene 161  CH —CH 3 Hydrocarbon Step III : Carbocation is unstable in nature and loses a H+ and changes into ethene in a fast step as follows: CH2 + H2O Propan–2–ol Propene H—CH2—  CH2 + H+   CH 2 Ethene Note: Dehydration of secondary and tert alcohol is best carried out by using dil. H2SO4. Since alkenes  2–Methylpropan–2–ol 2–Methylpropene From the above reactions, it is clear that the order of acidic dehydration in different alcohols is produced from those alcohols have a tendency to form polymers under the influence of concentrated acid. Saytzeff Rule : Cycloalkenes can be prepared in the same way by the dehydration of cycloalkanols. Ex.   Cyclohexanol Cyclohexene Machanism of Reaction: The acidic dehydration of alcohol proceeds through the formation of a carbocation intermediate and is explained as follows Step I : Alcohol being a Lewis base accepts a proton (H+) from the acid in a reversible step as follows:    2–butanol CH3—CH CH— CH3+CH3—CH2—CH main product 1–butene 2–butene 80% 20% CH2  CH3—CH2—CH2—CH2—OH   1–butanol     CH3—CH CH—CH3 + CH3CH2CH 2–butene80% Ethanol (From acid) Protonated ethanol Step II : Due to presence of positive charge on electronegative oxygen, its electron accepting tendency increases. As a result C – O bond becomes weak and cleaves as follows :   CH3— CH2 1–butene 20% Main product Mechanism : Acid catalyzed dehydration of alkanols proceeds via the formation of more stable carbonium ion. CH3CH2CH2 —CH2—  —H+    + H2O CH3CH2CH2CH2  Ethyl carbocation This is a slow and is regarded as rate determining step. Primary Carbonium ion 162 Hydrocarbon  Ex. 10 Carbonium 20 Carbonium more stable   Ex.    CH3CH2CH CH2 1–Butene Ex. butene (minor product) (Saytzeff rule) (2) From Alkyl halide (By dehydrohalogenation): Removal of HX from a substrate by alcoholic KOH or NaNH2     RCH  CH2     Ex.   CH3CH Primary and secondary alkyl halides undergo elimination reaction by E2 mechanism. E1 elimination reactions are shown by tertiary alkyl halides which are capable of producing stable (tert) Carbonium ion on show ionization. CH—CH3 + CH3CH2CH  CH2 (Saytzeff rule) The ease of dehydrohalogenation show the order  For alkyl group E2 mechanism : Those alkyl halides which do not give Stable Carbonium ion on ionization show E2 elimination. For halogen in halide  It is single step and synchronous process. Removal of proton, the formation of multiple bond between C and C and the release of the leaving group X take place simultaneously. (E2 mechanism) 163 Hydrocarbon (3) From Alkyl dihalide (By dehalogenation of Vicinal or Gem dihalide) : Removal of X2 from a substrate by Zn dust or Zn—Cu in alcoholic solution. (a) From Vicinal dihalide : same number carbon alkene, obtained.    CH2 CH2+ZnX2 (b) From gem dihalide : Higher alkene obtained CH3CH CHCH3  CH3—CH For example, CH3—C C—CH 3   But–2–yne trans–but–2–ene Mechanism : The mechanism of Birch reduction is called ion-electron mechanism and proceeds in two steps. In the first step, alkyne takes up an electron from sodium atom to form ion-radical which accepts a H+ ion from ammonia molecule to form a free radical. CH—CH3 + ZnX2 (4) By the controlled hydrogenation of alkynes : Alkynes can be converted into alkenes as a result of controlled reduction in two ways: (a) By the use of Lindlar's catalyst : Lindlar's catalyst is a mixture of palladium catalyst deposited over barium sulphate or calcium carbonate. The catalytic mixture is slightly poisoned by quinoline or sulphur and allows the reduction or hydrogenation of alkyne with hydrogen only upto the alkene stage. The next step in the reaction i.e., hydrogenation of alkene to alkane is not possible since the catalyst is no longer in a position to catalyse the reaction. The addition of hydrogen to the alkyne molecule occurs from the same side of the double bond. It is, therefore, cis addition in nature. This reaction is stereospecific reaction. Ex. CH3—C C—CH 3 + H 2   But–2–yne But–2–yne Ion–radical Free radical The free radical again accepts one electron from the metal atom followed by  ion from ammonia to form the desired product. anion trans–2–Butene (5) By Pyrolysis of ester : cis–but–2–ene In place of Lindlar's catalyst Nickel-boride (Ni–B also called P-2 catalyst) can also be used. (b) By the action of sodium in liquid ammonia This is known as Birch reduction and the product is a trans alkene i.e., the two hydrogen atoms get attached on the opposite side of the double bond.     CH3COOH + CH2 CHR Hoffmann's Rule : Less substituted or less stable alkene is major product. 164 Ex.   In the reaction to form an alkene a -hydrogen from alkyl ester is attracted by oxygen atom of keto group. (a) By this reaction methane cann't be obtained. (b) In this reaction intermediate is transition state and major product is determined by crowding alkyl groups. Hydrocarbon (a) In this reaction –hydrogen of tetra-alkyl ammonium ion is attracted by a base and alkene is formed. (b) In this reaction intermediate is carbanion.So yield of product depends on stability of carbanion. (c) In this reaction Hoffmann's Rule is followed. (d) By this reaction methane can not be prepared.    No –H (7) By Pyrolysis of alkane : (6) By Pyrolysis of tetra alkyl ammonium ion : (8) By Kolbe's method: Electrolysis of potassium or sodium salt of saturated dicarboxylic acid gives alkene. Ex.        Ex. 165 (9) From Grignard reagent: This reaction is used to obtained higher alkenes and it is nucleophilic substitution (NSR). Ex. CH3—CH2—MgCl + Cl—CH2—CH CH2  CH3—CH2—CH2—CH CH2 (10) Pyrolysis of Tri alkyl amine Oxide : (Cope Reaction)    R—CH CH2 Alkene Ex.17 CH 3—Mg—Cl+Cl—CH Product CH 2  Sol. In vinyl chloride there is partial double bond in between – C – Cl so no substitution take place. Hydrocarbon Chemical Properties : Alkenes are more reactive than alkane this is because (a) The  electrons of double bond are located much far from the carbon nuclei and are thus less firmly bound to them. (b)  bond is weaker than  bond and more easily broken. The reactivity order for alkenes CH2 CH2 > R—CH CH2 > R2C CH2 RCH CHR  > R2C CHR > R2C CR2 (Trans < Cis) The reactivity order of alkenes has been delt in terms of heat of hydrogenation of alkene, more is the heat of hydrogenation (H = –ve), more is the reactivity, the reactivity of alkene is however also related to (i) Steric hinderence (ii) Hyperconjugation (iii) Heat of Combustion. Alkenes give the following type of reactions : (a) Addition reaction. (b) Oxidation reaction. (c) Substitution reaction. (d) Polymerization Reaction. (e) Isomerisation Physical Properties : (1) All are colourless and have no characteristic odour. Ethene has pleasant smell. (2) Lower members (C2 to C4) are gases, middle one (C5 to C17) are liquids, higher are solids. (A) ADDITION REACTION : [A1] FREE RADICAL ADDITION (1) Addition of H2 : R—CH  R—CH —CH + CH2+H2  2 3 Heat of Hydrogenation. (3) The B.P., M.P. and specific gravity show a regular increase with increase in molecular weight Note: (a) Reaction is exothermic, It is called heat of hydrogenation. (4) The increase in branching in carbon chain decreases (b) the B.P. among isomeric alkenes. (5) The B.P. and M.P. of alkenes are slightly higher than the corresponding alkanes because the intermolecular forces of attraction are stronger due to the presence of easily polarizable  bond. (6) Insoluble in water because they can not form Hbond with water molecule, they dissolve freely in organic solvent like benzene, CHCl3, CCl4 etc. (c) The process is used to obtain vegetable (saturated fats) ghee from hydrogenation of oil. 166 Hydrocarbon [A2] ELECTROPHILIC ADDITION REACTIONS Because of the presence of >C C< bond in Ex.18 CH2 CH2 + Br2  Products, Ethylene molecules, alkenes generally take part in the addition reactions. what are the products? Sol. + AB  Alkene Attacking molecule Addition product From mechanism point of view, the addition in alkenes is generally electrophilic in nature which means that attacking reagent which carries the initial attack is an electrophile (E+). This is quite expected also as there is high electron density in the double bond. The mechanism proceeds in two steps. Step I : The –electron cloud of the double bond causes the polarisation of the attacking molecule (E–Nu) which cleaves to release the electrophile (  ) for the attack. The double bond simultaneously undergoes electromeric effect and the attack by the electrophile is accomplished in slow step (also called rate determining step) to form a carbocation intermediate.    Addition product Step II : The nucleophile (: ) released in the slow step combines with the carbocation to give the desired addition product in the fast step. (1) Addition of Halogen : It is a electrophilic addition reaction. (Vicinal halides) Similarly CH2 CH2 + Br2 (2) Addition of halogen acid : R—CH CH—R + HX  R—CH CH2 + HX  Note: (i) The order of reactivity of hydrogen halide is : HI > HBr > HCl > HF (ii) Their addition is an example of electrophilic addition. (iii) Addition on alkene proceeds via the formation of more stable carbonium ion. (iv) Addition of HX on unsymmetrical alkenes (R— CH CH 2 ) takes place according to Markownikoff's rule. 167 Hydrocarbon MARKOWNIKOFF'S RULE STATES : (a) First Rule : When molecule of a HX add up on unsymmetrical unsaturated hydrocarbon, the halogen atom goes to the unsaturated carbon atom bearing lesser number of hydrogen atoms. Mechanism :    In vinyl chloride two effects operate simultaneously in opposite direction(i) Inductive effect – electron attracting (-I) effect of chlorine. Mechanism : It is electrophilic addition and is illustrated by the action of HCl to propene.   CH3— —CH3+ Secondary carbocation  (ii) Resonance effect – electron pair releasing (+R) effect of chlorine. The resonance effect is much more than the -I effect of Chlorine at the time of attack.This creates centres of +ve and –ve charges. All polar reagents of the general structure (such as ) add on unsymmetrical unsaturated compound in accordance with Markownikoff's rules. Such additions are called normal Markownikoff's rule, where as additions in the opposite manner are reffered to as abnormal or antimarkowniff's additions. ANTI MARKONIFF'S RULE OR PEROXIDE 2–Chloropropane EFFECT OR KHARASCH RULE  (i) In the presence of oxygen of peroxides the Primary carbocation (CH3—CH2— ) is formed addition of HBr on unsaturated unsymmetrical but only in very small proportion since it is less compound takes place contrary to Markownikoff's stable than the secondary carbocation. rule. This is called peroxide effect and is due to Markownikff's rule can also be stated as : the difference in the mechanism of the addition. (ii) In the normal Markownikoff's addition the The electrophilic addition to unsymmetrical mechanism is ionic. alkenes always occurs through the formation (iii) In the presence of peroxide the addition of of a more stable carbocation intermediate. HBr takes place via free radicals. (b) Second Rule : In the addition of HX to vinyl halide and analogous compounds, the halogen attaches itself to the carbon atom, on which the halogen atom is already present. CH2 CH—Cl+HCl  Ethylidene chloride 168 Mechanism : (i) Chain initiation (a) R—O—O—R  2RO• (b) HBr + RO•  ROH + Br• (ii) Chain propagation Hydrocarbon In the fast step, there is competition between ion and H2O molecule to act as nucleophile but H2O is a better nucleophile. (4) Addition of H2SO4: Alkene react with conc. H2SO4 to produce alkyl hydrogen sulphate. Which gives alcohols on hydrolyses.This reaction used to seprate alkene from a mixture of alkane and alkene. (iii) Chain termination :         R—R R—Br Isopropyl alcohol Br—Br Ex. 19 Why HCl and HI do not give antimarkownikoff products in the presence of peroxides.? Sol. (a) The H—Cl bond is stronger than H—Br. (b) The H-I bond is weaker than H—Br bond. It Ethyl hydrogen sulphate give ethylene when heated is broken by the alkoxy free radicals obtained from 430-440K while ethanol is obtained on boiling it peroxides, but the addition of iodine atom on alkene with water. is endothermic as compared to Br atom therefore iodine atoms so formed combine with each other (5) Addition of water (Hydration of alkenes) : to yield iodine. Propene and higher alkenes react with water in the presence of acid to form alcohol. This reaction is known as the hydration reaction . Intermediate (3) Addition of Hypohalous acid (or X2/H2O, or HOX) : It is a electrophilic addition and follows in this reaction is carbo cation, so rearrangement Markownikoff's rule. will take place. (i)    Carbocation Propene (ii) Ethylene chlorohydrin Propan-2-ol    2–Methylpropene 72-Methylpropan-2-ol 169 Hydrocarbon Mechanism : Ex.   Carbocation (2°)     Propan-2-ol (6) Addition of NOCl (Tilden reagant) : (8) Oxymercuration – demercuration : Mercuric acetate in tetrahydro furan (THF) is treated with an alkene.The addition product on reduction with sodium Boro hydride in aqueous NaOH solution gives alcohol. It follows the markonikoff's rule. CH 3—CH Propylene nitrosochloride (7) Hydroboration : It obeys markoni'koff's rule.Diborane readily reacts with alkenes giving trialkyl boranes. The reaction is called hydroboration. CH2  (i) (AcO) 2 Hg/H 2 O (Mercuric acetate) or (CH3COO)2 Hg/H 2O (ii) NaBH4/NaOH Mechanism :   — CH3—COO + CH3—COOHg+ (Electrophile) Trialkylborane BH3 does not exist or stable as monomer so a solvent THF (tetra hydro furane) is used. Ex.  (CH3—CH2—CH2)3B BHR2 also can be taken. 170 Hydrocarbon Note : Intermediate is cyclic cation so their is no rearrangement. CH2   CH 2 + (CH3CO)2O CH2 CHCOCH3 + CH3COOH CH3COCl + CH2   CH2 CH3COCH2CH2Cl  CH2  (CH3)3CCH2CH2Cl (CH3)OCH2Cl + CH2 CH2  CH3OCH2CH2CH2Cl (CH3)3CCl + CH2 (10) Addition of alkanes (alkylation) : The addition of an alkane to an alkene results in the formation of higher alkane.   Isooctane (an important aviation fuel) (Octane No.=100) Mechanism : (i) (ii) (9) Addition with (CH 3 CO) 2 O, CH 3 COC , (CH3)3CC and CH3OCH2C: Alkenes condense with acetic anhydride in presence a catalyst e.g. ZnCl2 to form unsaturate ketones. Reaction is also called acetylation.Acid chloride, alkyl chlorides and  -halogenated ethers also combine with alkenes in presence of AlCl3. (iii) 171 Hydrocarbon The carbonium ion formed in step (II) then abstract (13) Addition of HCHO : In the presence of mineral  acid HCHO is added to alkene molecule to obtain a hydride ion : from isobutane to give diol. isooctane.The carbonium ion formed in the last step propagates the chain reaction. (11) Hydroformylation or Oxo reaction : Alkenes react with Carbon monoxide and hydrogen at 100 – 150°C temperature and high pressure (200 atm) in the presence of Cobalt catalyst to produce an aldehyde.It does not follows markonikoff's rule. The net reaction is the addition of a H–atom to one of the Olefinic bond and a formyl (–CHO) group to the other. Mechanism:  (12) Alkenylation (Addition of alkene) In presence of H2SO 4 or H3PO 4 at 80°C dimerisation of isobutylene take place gives two isomer of octene. (14) Addition of Carbene : The addition of carbene to alkene is always carried by diazomethane CH2N2 Carbene group obtained from diazomethane is added to alkene and give cycloalkanes. Mechanism : CH2  CH 2 + CH2N2  Since : CH2 is an electrophile (neutral) and there is more electron density on double bond so first attack of :CH2 will be at double bond. CH2 CH 2+CH2N2  pyrazoline 172 Hydrocarbon Ex. 21 How many mole of oxygen is required for complete combustion of 1 mole of Alkene. (15) Addition of HCN: Sol.    keeping in mind, the above equation. (16) Addition of N2O3:  for 2 mole of Alkene, 3n mole of O2 is required for combustion.  for 1 mole of Alkene, ethylene nitrosite (17) Addition of N2O4: mole of O2 is required for combustion. = 1.5n mole of O2 Ans. ethylene nitrosate (B) OXIDATION REACTION : Alkenes are easily oxidised by oxidising agents. Oxidising agents attack on double bond and product formed during oxidation depends on oxidising agents. (1) alkene on combustion gives CO2 and H2O CnH2n + O2 nCO2 + nH2O One mole of alkene requires moles of O2 for complete combustion. Ex.22 30 mL mixture of ethylene and Butylene is burnt in presence of oxygen then 150 mL of oxygen is required, what is the volume of Ethylene & Butylene in mixture. Sol. Let the volume of C2H4 = x mL So volume of Butylene = (30–x) mL For C2H4     from equation  for 1 volume C2H4, 3 volume of O2 is required.  for x mL vol. of C2H4, 3x ml volume of O2 is required. For C4H8 Ex. 20 90 mL of oxygen is required for complete combustion of unsatuarated 20 mL gaseous hydrocarbon, hydrocarbon is? Sol. Following two formulae can be used for solution of the above asked question.   (for Alkene)  (for Alkyne) By putting the values in above formulae we can find the hydrocarbon for which n is natural number.  n = 3 So hydrocarbon is Propene [C3H6]. C4H8 + 6O2  4CO2 + 4H2O  for 1 volume C4H8, 6 volume of O2 is required. " " , 6 (30–x) mL  for (30–x) mL of O2 is required. Total volume of O2 = 3x + 6 (30–x) mL = 150 mL(Given) x = 10  Volume of C2H4 in mixture is 10 mL  Volume of C2H4 in mixture is 20 mL (2) Ozonolysis : (A test for unsaturation in molecule) (i) The addition of ozone on the double bonds and subsequent a reductive hydrolysis of the ozonide formed is termed as ozonolysis. 173 (ii) When ozone is passed through an alkene in an inert solvent, it adds across the double bond to form an ozonide. Ozonides are explosive compound they are not isolated. (iii) On warming with Zn and H2O, ozonides cleave at the site of the double bond, the products are carbonyl compound (aldehyde or ketone) depending on the nature of the alkene. Hydrocarbon It may be noted that reaction with bromine water or Baeyer's reagent detects the presence of double bond (or unsaturation) in an alkene while ozonolysis helps in locating the position of the double bond. In an reduction of ozonide by LiAlH4 or NaBH4 gives corresponding alcohols. Ex.   (3) Hydroxylation : Oxidation of carbon-carbon (iv) Ozonolysis of alkenes helps in locating the position of double bond in an alkene. It can be achieved by joining together the carbon atoms of the two carbonyl compounds formed as the products of ozonolysis with double bond. Ex. double bond to is known as hydroxylation. (a) Oxidation by Baeyer's reagent (A test for unsaturation) : Alkenes on passing through dilute alkaline 1% cold KMnO4 (i.e., Baeyer's reagent) decolourise the pink colour of KMnO4 and gives brown ppt MnO2 and glycol.  Ethanal But-2-ene (b) By OsO4 : Ex. Methanal Propanal But-1-ene (c) By peracid :    Ex.    Propanone 2,3–Dimethyl but–2–ene trans glycol 174 Hydrocarbon (4) Epoxidation : (a) Alkenes reacts with oxygen in the presence of Ag catalyst at 250°–400° C to form epoxide. CH2 CH2  (ii)     (anti addition) (b) Prileschiaev reaction: When an alkene is treated with perbezoic acid an epoxide is formed. Such an epoxidation is known as Prileschiave reactions. RCH CH2 + C 6H5COOOH  Epoxide Emmons have found that perbenzoic oxy trifluoroacetic acid (CF3COO2H) is a very good reagent for epoxidation and hydroxylation. (5) Oxidation by strong oxidising agent (Oxidative cleavage): The alkenes themselves are readily oxidised to acid or ketone by means of acid permagnate or acid dichromate.If HCOOH is formed, it further oxidized to CO2 and H2O. Keep it in mind that no further oxidation of ketones will takes place. CH2 CH2+4[O]  2HCOOH  2CO2 + H2O Ex..(i) CH3CH CH2  CH3COOH+CO2+H2O CH3CH CHCH3  2CH3COOH  (6) Oxidation with retention of Carbon-Carbon bond - (Waker process) :  CH CHO CH2 CH2+H2O2  3 (C) SUBSTITUTION REACTION (allylic substitution) When alkenes are treated with Cl2 or Br2 at high temp., one of their allylic hydrogen is replaced by halogen atom. Allylic position is the carbon adjacent to one of the unsaturated carbon atoms.It is free radical substitution. 175 CH3—CH Ex. nCH2 CH2 + Cl2    ClCH2—CH CH2 + HCl Allyl chloride (3-Chloro-1-propene) N-Bromosuccinimide (NBS) is an important reagent used for allylic bromination and benzlic substitution. Hydrocarbon CH2 (—CH2—CH2—)n ethene Poly ethene used in the manufacture of insulating Coating, sheeting and moulded products.  nCH3—CH CH2   Polypropene or Koylene (Plastic) (E) ISOMERISATION Alkene on heating to 500° to 700 °C or on heating in presence of catalyst [AlCl3 or Al2(SO4)3] undergo isomerisation. Substitution reaction is not given by ethene. CH3CH2—CH CH2    CH3—CH Ex. (i)  1–Butene (ii) CH—CH3 + 2–Butene Isobutylene     USES : (1) In plastic formation. (2) In oxy ethylene welding (3) As food preservatives and ripening fruits. 3° more stable (4) As general anaesthetic (C2H4 with 10% O2) (D) POLYMERIZATION (5) In preparation of mustard gas (i) Two or more than two molecules of same compound unit with each other to form a long chain molecule with same empirical formula. This long chain molecule having repeating structural units called 2,2' or (, ') dichloro diethyl-sulphide polymer, and the starting simple molecule as (mustard gas) monomer and process is called addition polymerization. DIENES (ii) Molecular weight of polymer is simple multiple Dienes are the unsaturated hydrocarbons with of monomer. carbon-carbon double bonds in their molecules. (iii) Polymerization can be carried out by free These are represented by the general formula radical or ionic mechanism.  which means that they are isomeric with (iv) The presence of oxygen initiates free radical alkynes (functional isomers). However, their mechanism. properties are quite different from those of alkynes. (v) Addition polymerization can also be carried Depending upon the relative positions of the two out by ionic mechanism by using Ziegler - Natta double bond, dienes are classified in three types : Catalysts (R3Al+TiCl4) 176 Hydrocarbon Penta-1, 4–diene (isolated diene) has only two contributing structures. Isolated dienes or non conjugated dienes : In an isolated diene, the two double bonds are separated by more than one single bond. For example,  (I) Penta–1,4–diene Buta-1,3-diene Properties of Conjugated Dienes : 2-Methylpenta-1,3-diene Commulate dienes : In this case, the two double bonds in the molecules are present at adjacent positions. For example, Propa-1,2–diene Since the carbon atom C3 is not involved in any resonance, the contributing structures are less in number are compared to the conjugated diene. The isolated diene is, therefore, less stable than a conjugated diene. 3–Methylpenta–1, 4–diene Cunjugated dienes : In a conjugated diene, the two double bonds are present in the conjugated or alternate position and are separated by a single bond. Buta–1, 2–diene Comparison of relative stabilities of isolated and conjugated dienes : Resonance Theory : The relative stabilities of the two types dienes can also be justifid on the basis of the theory of resonance. Penta-1,3-diene (conjugated diene) is a hybrid of the following contributing structures. (II) 1. The properties of the isolated dienes are similar to those of simple alkene but those of conjugated dienes are somewhtat modified because of delocalisation of the -electron charge. However, they also participate in the addition reactions. The important chemical characteristics of the conjugated dienes are briefly discussed. Addition Reaction : Conjugated or 1,3–dienes take part in the addition reactions which can proceed by electrophilic as well as free radical mechanism depending upon the nature of the attacking reagent and the reaction conditions. (A) Electrophilic Addition Reactions : The electrophilic addition is illustrated by the attack of halogen and halogen acid on buta–1,3–diene, a conjugated diene. (a) Addition of halogen : If one mole of halogen attacks per mole of the diene, two types of addition products are formed. There are 1, 2 and 1, 4 addition products. For example,   The delocalisation of -electron charge because of resonanace decreases the energy of the molecule or increases its stability. 1, 2–addition is a normal addition in which one mole of halogen has been added to one of the double bond. But 1, 4-addition is somewhat unexpected. 177 Mechanism : The addition is electrophilic in nature and the halogen molecule (bromine) provides the electrophilie for the attack. Hydrocarbon The carbocation gets resonance stabilised as follows (I) (II) – The attack of Br ion on the carbocation (I) gives 1,2-addition product whereas the attack on the carbocation (II) yields 1,4-addition product.  Buta-1, 3-diene Carbocation (2°)  The 2° carbocation get stablised by resonance as follows – 1,2–Addition product    I 1,4–Addition product II  The attack of ion on carbocation (I and II) (B) Free Radical Addition Reaction : The addition to conjugated dienes can also proceed by free radical mechanism provided it is carried in the presence of a suitable reagent which can help in forming a free radical. However, the addition also yields 1,2 and 1,4 addition products. The free radical addition is illustratated by the attack of bromotrichloromethane (BrCCl 3) on buta–1,3– diene in the presence of an organic peroxide such as benzoyl peroxide.  (b) Addition of H – X : Buta–1,3–diene Bromotri- CH2 CH—CH CH2  ? Mechanism : The addition is electrophilic in nature as  ion is the electrophile.  ` Mechanism : The mechanism of addition is free radical in nature which is initiated by benzoyl free radical. It is explained in the following steps : Step I : Generation of free radical C 6H5CO—O—O—COC6H 5   C6 H5—CO— C 6H5—CO—  Carbocation (2°) chloromethane        —COC 6 H5 +CO 2 C 6H5—Br +  Trichloromethyl free radical 178 Step II.  Attack of free radical on buta-1, 3-diene +CH2 CH—CH CH 2  Cl3C—CH2—  —CH Hydrocarbon The addition product is called Diel Alder Adduct. For example. CH2 + Free radical (secondary)  The free radical gets resonance stabilised Cl3C—CH2—  —CH CH2  Cl3C—CH 2—CH CH—  Step III. Change of free radical into addition product Buta–1,3–diene Ethene Cyclohexene (Diel Alder adduct) +   The free radicals take up from the attacking reagent to give the desired addition products.  (1,2-Addition product) Buta–1,3–diene Propenal (Acrolein) 3-cyclohexene carbaldehyde Volume of hydrocarbon will be given and volume of O2 for complete combustion will also be given. What is hydrocarbon is to be asked ? 2. Cyclo-Addition Reaction (Diel Alder Reactions): Cyclo-addition reactions are one of the most important reactions of conjugated dienes. Cycloaddition involves the combination between a conjugated diene (4-electron system) and a compound containing a double bond (2-electron system) called dienophile which means a diene loving or attracting molecule. As a result, a six membered ring gets formed and the reaction is therefore known as cyclo-addition reaction. It is quite often termed as (4 +2) cyclo-addition reaction because four 4-electron system adds to a two 2 electron system. The reactions of this type are known as : Diel Alder Reaction. The above question may be solved with the help of following three formulae. Formula No. 1  (for Alkane) Formula No. 2  (for Alkene) Formula No. 3  (for Alkyne) 179 Hydrocarbon Ex. 23 How much propanol is required for dehydration to get 2.24 litre of Propene at N.T.P. if yield is 100%. Sol.     Molecular weight of propanol = 60 from the equation given above we can see that from dehydration of 1 mole or 60 gram of propanol we get 1 mole (22.4lit.) of propene as product.  22.4 litre of C3H6 can be get from dehydration of 60 g of propanol.  1 litre of propene can be get from dehydration of g of propanol  2.24 litre of propene can be get from dehydration of g of propano =6g 180