chapter 6 REACTIONS OF ALKENES ADDITION REACTIONS SOLUTIONS TO TEXT PROBLEMS 6.1 Catalytic hydrogenation converts an alkene to an alkane having the same carbon skeleton. Since 2-methylbutane is the product of hydrogenation, all three alkenes must have a four-carbon chain with a one-carbon branch. The three alkenes are therefore 6.2 The most highly substituted double bond is the most stable and has the smallest heat of hydrogenation 2-Methy l-2-butene Methyl-1-butene thyl-1-butene Heat of hydrogenation: 112 kJ/mol 118 k/mo 126 k/mol (26.7 kcal/mol) 28.2 kcal/m 30.2 kcal/mol) 124 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
CHAPTER 6 REACTIONS OF ALKENES: ADDITION REACTIONS SOLUTIONS TO TEXT PROBLEMS 6.1 Catalytic hydrogenation converts an alkene to an alkane having the same carbon skeleton. Since 2-methylbutane is the product of hydrogenation, all three alkenes must have a four-carbon chain with a one-carbon branch. The three alkenes are therefore: 6.2 The most highly substituted double bond is the most stable and has the smallest heat of hydrogenation. 2-Methyl-2-butene: most stable (trisubstituted) 112 kJ/mol (26.7 kcal/mol) 2-Methyl-1-butene (disubstituted) 118 kJ/mol (28.2 kcal/mol) 3-Methyl-1-butene (monosubstituted) 126 kJ/mol (30.2 kcal/mol) Heat of hydrogenation: 2-Methyl-1-butene 2-Methyl-2-butene 2-Methylbutane 3-Methyl-1-butene H2 metal catalyst 124 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
REACTIONS OF ALKENES: ADDITION REACTIONS 125 3(b) Begin by writing out the structure of the starting alkene. Identify the doubly bonded carbon that has the greater number of attached hydrogens; this is the one to which the proton of hy drogen chloride adds. Chlorine adds to the carbon atom of the double bond that has the fewer attached hydrogens Chlorine adds to Hydrogen adds to his carbon this carbo H, C 3 CH CH, CCH CH CH 2-Methyl-1-butene 2-Chloro-2-methylbutane By applying Markovnikov's rule, we see that the major product is 2-chloro-2-methylbutane (c) Regioselectivity of addition is not an issue here, because the two carbons of the double bond are equivalent in cis-2-butene Hydrogen chloride adds to cis-2-butene to give 2-chlorobutane H,C CH3 +HC1—CHCH2CHCH is-2-Butene Hydrogen 2-Chlorobutane chloride (d) One end of the double bond has no attached hydrogens other end has one. In accordance with Markovnikov's rule, the proton of hydrogen adds to the carbon that already has one hydrogen. The product is 1-chloro-1-ethylcyclohexane CH CI CHCHE HCl Ethylidenecyclohexane I-Chloro-l-ethylcyclohexane 6. 4(b) A proton is transferred to the terminal carbon atom of 2-methyl-l-butene so as to produce a H CH CH, CH H CHCH 2-Methyl-l-butene Hydrogen Tertiary carbocation Chloride This is the carbocation that leads to the observed product, 2-chloro-2-methylbutane. (c) A secondary carbocation is an intermediate in the reaction of cis-2-butene with hydrogen chloride H3C C-CH,CH3 Cl Capture of this carbocation by chloride gives 2-chlorobutane Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
6.3 (b) Begin by writing out the structure of the starting alkene. Identify the doubly bonded carbon that has the greater number of attached hydrogens; this is the one to which the proton of hydrogen chloride adds. Chlorine adds to the carbon atom of the double bond that has the fewer attached hydrogens. By applying Markovnikov’s rule, we see that the major product is 2-chloro-2-methylbutane. (c) Regioselectivity of addition is not an issue here, because the two carbons of the double bond are equivalent in cis-2-butene. Hydrogen chloride adds to cis-2-butene to give 2-chlorobutane. (d) One end of the double bond has no attached hydrogens, but the other end has one. In accordance with Markovnikov’s rule, the proton of hydrogen chloride adds to the carbon that already has one hydrogen. The product is 1-chloro-1-ethylcyclohexane. 6.4 (b) A proton is transferred to the terminal carbon atom of 2-methyl-1-butene so as to produce a tertiary carbocation. This is the carbocation that leads to the observed product, 2-chloro-2-methylbutane. (c) A secondary carbocation is an intermediate in the reaction of cis-2-butene with hydrogen chloride. Capture of this carbocation by chloride gives 2-chlorobutane. C C H Cl H3C CH3 H H cis-2-Butene Hydrogen chloride Secondary carbocation Chloride Cl C H3C H CH2CH3 C C H3C CH3CH2 H H Cl H 2-Methyl-1-butene Hydrogen chloride Tertiary carbocation Chloride C Cl CH3CH2 CH3 CH3 Hydrogen chloride Ethylidenecyclohexane 1-Chloro-1-ethylcyclohexane CH HCl 3CH CH3CH2 Cl C C H3C CH3 H H cis-2-Butene Hydrogen chloride 2-Chlorobutane HCl CH3CH2CHCH3 Cl C C H3C H H CH3CH2 2-Methyl-1-butene 2-Chloro-2-methylbutane Chlorine adds to this carbon. Hydrogen adds to this carbon. HCl CH3CH2CCH3 Cl CH3 REACTIONS OF ALKENES: ADDITION REACTIONS 125 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
126 REACTIONS OF ALKENES: ADDITION REACTIONS (d) A tertiary carbocation is formed by protonation of the double bond. CHCH CH-CH Tertiary cation Chloride This carbocation is captured by chloride to give the observed product, 1-chloro-l 6.5 The carbocation formed by protonation of the double bond of 3, 3-dimethyl-1-butene is secondary Methyl migration can occur to give a more stable tertiary carbocation CH CCHECH CH2C一CHCH3 CHaC-CHCH CH3 CH H3 CHC一 3-Chloro-2, 2-dimethylbutane The two chlorides are 3-chloro-2, 2-dimethylbutane and 2-chloro-2, 3-dimethylbutane 6.6 The structure of allyl bromide(3-bromo-1-propene)is CH CHCH, Br. Its reaction with hydrogen bromide in accordance with Markovnikov's rule proceeds by addition of a proton to the doubly bonded carbon that has the greater number of attached hydrogens Addition according to Markovnikov's rule: CH=CHCH.Br HBr CH,CHCH,Br Allyl bromide Hy 1. 2-Dibromopropane ddition of hydrogen bromide opposite to Markovnikov's rule leads to 1, 3-dibromopropane Addition contrary to Markovnikov's rule: CH,=CHCH, Br HBr BrCH,CH,Ch, Br Allyl bromide Hydroge 1. 3-Dibromopropane bromide Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
126 REACTIONS OF ALKENES: ADDITION REACTIONS (d) A tertiary carbocation is formed by protonation of the double bond. This carbocation is captured by chloride to give the observed product, 1-chloro-1- ethylcyclohexane. 6.5 The carbocation formed by protonation of the double bond of 3,3-dimethyl-1-butene is secondary. Methyl migration can occur to give a more stable tertiary carbocation. The two chlorides are 3-chloro-2,2-dimethylbutane and 2-chloro-2,3-dimethylbutane. 6.6 The structure of allyl bromide (3-bromo-1-propene) is CH2?CHCH2Br. Its reaction with hydrogen bromide in accordance with Markovnikov’s rule proceeds by addition of a proton to the doubly bonded carbon that has the greater number of attached hydrogens. Addition according to Markovnikov’s rule: Addition of hydrogen bromide opposite to Markovnikov’s rule leads to 1,3-dibromopropane. Addition contrary to Markovnikov’s rule: Allyl bromide CH CHCH2Br 2 Hydrogen bromide HBr 1,3-Dibromopropane BrCH2CH2CH2Br Allyl bromide CH CHCH2Br 2 Hydrogen bromide HBr 1,2-Dibromopropane CH3CHCH2Br Br 3,3-Dimethyl-1-butene Secondary carbocation Tertiary carbocation CH3CCH CH2 CH3 CH3 HCl Cl Cl methyl migration CH3C CHCH3 CH3 CH3 3-Chloro-2,2-dimethylbutane CH3C CHCH3 CH3 Cl CH3 CH3C CHCH3 CH3 CH3 2-Chloro-2,3-dimethylbutane CH3C CHCH3 CH3 Cl CH3 CH3CH H Cl Cl CH3CH2 Tertiary cation Chloride Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
REACTIONS OF ALKENES: ADDITION REACTIONS 127 6.7(b) Hydrogen bromide adds to 2-methyl-1-butene in accordance with Markovnikov's rule when peroxides are absent. The product is 2-bromo-2-methylbutane HBr CH CH,CCH CH, CH, B 2-Bromo-2-methylbutane The opposite regioselectivity is observed when peroxides are present. The product is I-bromo-2 =C.+ HBr peroxid CHa CH,CCH, Br 2-Methyl-1-butene I-Bromo-2-methylbutane (c) Both ends of the double bond in cis-2-butene are equivalently substituted, so that the same product (2-bromobutane) is formed by hydrogen bromide addition regardless of whether the reaction is carried out in the presence of peroxides or in their absence. HBr CH,,CHCH .2-Butene 2-Bromobutane (d) A tertiary bromide is formed on addition of hydrogen bromide to ethylidenecyclohexane in the absence of peroxides CHCH CH CHE HBr Ethylidenecyclohexane I-Bromo-l-ethylcyclohexane The regioselectivity of addition is reversed in the presence of peroxides, and the product is (1-bromoethyl)cyclohex CH, CH Ethy lidenecyclohexane Hydrogen (1-Bromoethyl)cyclohexane 6.8 The first step is the addition of sulfuric acid to give cyclohexyl hydrogen sulfate Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
6.7 (b) Hydrogen bromide adds to 2-methyl-1-butene in accordance with Markovnikov’s rule when peroxides are absent. The product is 2-bromo-2-methylbutane. The opposite regioselectivity is observed when peroxides are present. The product is 1-bromo-2-methylbutane. (c) Both ends of the double bond in cis-2-butene are equivalently substituted, so that the same product (2-bromobutane) is formed by hydrogen bromide addition regardless of whether the reaction is carried out in the presence of peroxides or in their absence. (d) A tertiary bromide is formed on addition of hydrogen bromide to ethylidenecyclohexane in the absence of peroxides. The regioselectivity of addition is reversed in the presence of peroxides, and the product is (1-bromoethyl)cyclohexane. 6.8 The first step is the addition of sulfuric acid to give cyclohexyl hydrogen sulfate. Cyclohexene Cyclohexyl hydrogen sulfate H2SO4 OSO2OH Hydrogen bromide Ethylidenecyclohexane (1-Bromoethyl)cyclohexane CH HBr 3CH CH3CH Br peroxides Hydrogen bromide Ethylidenecyclohexane 1-Bromo-1-ethylcyclohexane CH HBr 3CH CH3CH2 Br Hydrogen 2-Bromobutane bromide cis-2-Butene CH3CH2CHCH3 Br C C H CH3 H CH3 HBr Hydrogen 1-Bromo-2-methylbutane bromide 2-Methyl-1-butene CH3CH2CCH2Br H CH3 C C CH3CH2 CH3 H H HBr peroxides Hydrogen 2-Bromo-2-methylbutane bromide 2-Methyl-1-butene CH3CH2CCH3 Br CH3 C C CH3CH2 CH3 H H HBr REACTIONS OF ALKENES: ADDITION REACTIONS 127 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
128 REACTIONS OF ALKENES: ADDITION REACTIONS 6.9 The presence of hydroxide ion in the second step is incompatible with the medium in which the re- action is carried out. The reaction as shown in step I 1.(CH3),C=CH,+ H,O (CH3)C+ H,O is performed in acidic solution. There are, for all practical purposes, no hydroxide ions in aqueous acid, the strongest base present being water itself. It is quite important to pay attention to the species It are actually present in the reaction medium whenever you formulate a reaction mechanism. 6.10 The more stable the carbocation, the faster it is formed. The more reactive alkene gives a tertiary carbocation in the rate-determining step H3 Tertiary carbocation Protonation of D-CH=CHCH3 give ndary carbocation 6.11 The mechanism of electrophilic addition of hydrogen chloride to 2-methylpropene as outlined in text Section 6.6 proceeds through a carbocation intermediate. This mechanism is the reverse of the El elimination. The E2 mechanism is concerted--it does not involve an intermedia 6.12(b) The carbon-carbon double bond is symmetrically substituted in cis-2-butene, and so the gioselectivity of hydroboration-oxidation is not an issue. Hydration of the double bond gives 2-butanol droboralo CH, CHCH H OH cis-2-Butene 2-Butanol (c) Hydroboration-oxidation of alkenes is a method that leads to hydration of the double bond vith a regioselectivity opposite to Markovnikov's rule 2. oxidation CHOH (d) Hydroboration-oxidation of cyclopentene gives cyclopentanol 1. hydroborate Cyclopentene Cyclopentanol (e) When alkenes are converted to alcohols by hydroboration-oxidation, the hydroxyl group is introduced at the less substituted carbon of the double bond CH,=C(CH,CH3) 2 CHaCHCH(CH,CH3)2 3-Ethyl-2-penten 3-Ethyl-2-pentar Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
6.9 The presence of hydroxide ion in the second step is incompatible with the medium in which the reaction is carried out. The reaction as shown in step 1 is performed in acidic solution. There are, for all practical purposes, no hydroxide ions in aqueous acid, the strongest base present being water itself. It is quite important to pay attention to the species that are actually present in the reaction medium whenever you formulate a reaction mechanism. 6.10 The more stable the carbocation, the faster it is formed. The more reactive alkene gives a tertiary carbocation in the rate-determining step. 6.11 The mechanism of electrophilic addition of hydrogen chloride to 2-methylpropene as outlined in text Section 6.6 proceeds through a carbocation intermediate. This mechanism is the reverse of the E1 elimination. The E2 mechanism is concerted—it does not involve an intermediate. 6.12 (b) The carbon–carbon double bond is symmetrically substituted in cis-2-butene, and so the regioselectivity of hydroboration–oxidation is not an issue. Hydration of the double bond gives 2-butanol. (c) Hydroboration–oxidation of alkenes is a method that leads to hydration of the double bond with a regioselectivity opposite to Markovnikov’s rule. (d) Hydroboration–oxidation of cyclopentene gives cyclopentanol. (e) When alkenes are converted to alcohols by hydroboration–oxidation, the hydroxyl group is introduced at the less substituted carbon of the double bond. 1. hydroboration 2. oxidation 3-Ethyl-2-pentanol C(CH2CH3) CH3CH 2 3-Ethyl-2-pentene CH3CHCH(CH2CH3)2 OH 2. oxidation 1. hydroboration Cyclopentene Cyclopentanol OH CH2 CH2OH H 2. oxidation 1. hydroboration Methylenecyclobutane Cyclobutylmethanol cis-2-Butene 2-Butanol CH3CHCH2CH3 OH C C H H3C H CH3 1. hydroboration 2. oxidation Protonation of gives a secondary carbocation. CH CHCH3 C CH3 CH2 H3O C CH3 CH3 Tertiary carbocation CH2 1. (CH3)2C H3O (CH3)3C H2O 128 REACTIONS OF ALKENES: ADDITION REACTIONS Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website