CHAPTER 10 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS SOLUTIONS TO TEXT PROBLEMS 10.1 As noted in the sample solution to part(a), a pair of electrons is moved from the double bond toward the positively charged carbon. (b) H,C →HC-C=CH C(CH3)2 C(CH3) 10.2 For two isomeric halides to yield the same carbocation on ionization, they must have the same carbon skeleton. They may have their leaving group at a different location, but the carbocations must become equivalent by allylic resonance. Cl CH 3-Bromo-I 3-Chloro-3. methylcyclohexane Not an allylic carbocation 4-Bromo.I 230 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
CHAPTER 10 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS SOLUTIONS TO TEXT PROBLEMS 10.1 As noted in the sample solution to part (a), a pair of electrons is moved from the double bond toward the positively charged carbon. (b) (c) 10.2 For two isomeric halides to yield the same carbocation on ionization, they must have the same carbon skeleton. They may have their leaving group at a different location, but the carbocations must become equivalent by allylic resonance. CH3 Br CH3 4-Bromo-1- methylcyclohexene Not an allylic carbocation CH3 CH3 Cl CH3 CH3 Br 3-Bromo-1- methylcyclohexene 3-Chloro-3- methylcyclohexene C(CH3)2 C(CH3)2 H2C CH2 CH3 C H2C CH2 CH3 C 230 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS 231 Not an allylic carbocation 5-chloro CH CH ot an 10.3 The allylic hydrogens are the ones shown in the structural formulas. I-Methylcyclohexene 2, 3, 3-Trimethyl-1-butene 1-Octene 10.4 The statement of the problem specifies that in allylic brominations using N-bromosuccinimide the active reagent is Br,. Thus, the equation for the overall reaction is H Br2 B 3-Bromocyclohexene Hydrogen The propagation steps are analogous to those of other free-radical brominations. An allylic hydrogen is removed by a bromine atom in the first step ·Br: H+H—Br Cyclohexene Bromine Hydrogen Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
10.3 The allylic hydrogens are the ones shown in the structural formulas. (b) (c) (d) 10.4 The statement of the problem specifies that in allylic brominations using N-bromosuccinimide the active reagent is Br2. Thus, the equation for the overall reaction is The propagation steps are analogous to those of other free-radical brominations. An allylic hydrogen is removed by a bromine atom in the first step. H H Cyclohexene Bromine atom H 2-Cyclohexenyl radical Hydrogen bromide Br H Br Br2 H H Cyclohexene Bromine Br H 3-Bromocyclohexene Hydrogen bromide HBr 1-Octene H H 2,3,3-Trimethyl-1-butene CH3 H H H H CH3 1-Methylcyclohexene CH3 1-Bromo-3- methylcyclohexene Not an allylic carbocation CH3 Br CH3 5-Chloro-1- methylcyclohexene Not an allylic carbocation CH3 Cl CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS 231 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
232 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS The allylic radical formed in the first step abstracts a bromine atom from Br, in the second propa Br 2-Cyclohexenyl 3-Bromocyclohexene Bromine 10.5 Write both resonance forms of the allylic radicals produced by hydrogen atom abstraction from the CH CH CH, (CH3)3CC (CH3)3CC →(CH3)3CC H 2, 3,3-Trimethyl-l-butene Both resonance forms are equivalent, and so 2, 3, 3-trimethyl-l-butene gives a single bromide on treatment with N-bromosuccinimide(NBS) (CH3)3CC (CH3)3CC=CH2 CH CHB ethyl-l-butene Hydrogen atom abstraction from 1-octene gives a radical in which the unpaired electron is delocalized between two nonequivalent positions CH=CHCH,(CH2),CH,CH2=CHCH(CH2),CH, CH,CH=CH(CH,)4CH3 1-Octene Allylic bromination of 1-octene gives a mixture of products NBS CH2CHCH,(CH,)4CH3 CH2=CHCH(CH2)4CH3 BrCH- CH=CH(CH,)4CH3 3-Bromo-1-octene I-Bromo-2-octene(cis and trans) 10.6(b) All the double bonds in humulene are isolated, because they are separated from each other by one or more spcarbon atoms CH HC Humulene Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
232 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS The allylic radical formed in the first step abstracts a bromine atom from Br2 in the second propagation step. 10.5 Write both resonance forms of the allylic radicals produced by hydrogen atom abstraction from the alkene. Both resonance forms are equivalent, and so 2,3,3-trimethyl-1-butene gives a single bromide on treatment with N-bromosuccinimide (NBS). Hydrogen atom abstraction from 1-octene gives a radical in which the unpaired electron is delocalized between two nonequivalent positions. Allylic bromination of 1-octene gives a mixture of products 10.6 (b) All the double bonds in humulene are isolated, because they are separated from each other by one or more sp3 carbon atoms. Humulene CH3 CH3 H3C CH3 CH2 CHCH2(CH2)4CH3 BrCH2CH CH(CH2) CH2 CHCH(CH2 4CH3 )4CH3 Br NBS 1-Octene 3-Bromo-1-octene 1-Bromo-2-octene (cis and trans) CH2 CHCH2(CH2)4CH3 1-Octene CH2 CHCH(CH2)4CH3 CH2CH CH(CH2)4CH3 NBS (CH3)3CC CH2 CH3 2,3,3-Trimethyl-1- butene (CH3)3CC CH2 CH2Br 2-(Bromomethyl)-3,3- dimethyl-1-butene (CH3)3CC CH2 CH3 2,3,3-Trimethyl-1-butene (CH3)3CC CH2 CH2 (CH3)3CC CH2 CH2 3-Bromocyclohexene Bromine atom 2-Cyclohexenyl radical Bromine H H Br Br Br Br Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS 233 (c) The C-1 and C-3 bonds of cembrene are conjugated with each other. CH (CH3)2CH CH The double bonds at C-6 and C-10 are isolated from each other and from the conjugated diene (d) The sex attractant of the dried-bean beetle has a cumulated diene system involving C-4, C-5, and C-6. This allenic system is conjugated with the C-2 double bond CH3(CH2)CH, CH=C=CHCH=CHCO, CH 10.7 The more stable the isomer, the lower its heat of combustion. The conjugated diene is the most stable and has the lowest heat of combustion the cumulated diene is the least stable and has the highest heat of combustion HC H,C=CHCH, CH=CH, H,C=C=CHCH, CH3 (E)-1, 3-Pentadiene 1. 4-Pentadiene Most stabl 3186 kJ/mol (68.9 kcal/mol) 3251 (761.6 kcal/mol) (777.1 kcal/mol) 10.8 Compare the mirror-image forms of each compound for superposability. For 2-methyl-2, 3- pentadiene 2-methyl-2, 3-pentadiene H-C H,C. CH3 H-C H C unit demonstrates that the reference structure and its mirror image are superposable he Rotation of the mirror image 180 around an axis passing through the three carbons of th H C. CH CH H CH Mirror image 2-Methyl-2, 3-pentadiene is an achiral allene Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(c) The C-1 and C-3 double bonds of cembrene are conjugated with each other. The double bonds at C-6 and C-10 are isolated from each other and from the conjugated diene system. (d) The sex attractant of the dried-bean beetle has a cumulated diene system involving C-4, C-5, and C-6. This allenic system is conjugated with the C-2 double bond. 10.7 The more stable the isomer, the lower its heat of combustion. The conjugated diene is the most stable and has the lowest heat of combustion. The cumulated diene is the least stable and has the highest heat of combustion. 10.8 Compare the mirror-image forms of each compound for superposability. For 2-methyl-2,3- pentadiene, Rotation of the mirror image 180° around an axis passing through the three carbons of the C?C?C unit demonstrates that the reference structure and its mirror image are superposable. 2-Methyl-2,3-pentadiene is an achiral allene. C Rotate 180 Mirror image C H CH3 H3C CH3 C Reoriented mirror image C C H3C H H3C CH3 C 2-methyl-2,3-pentadiene and Reference structure C C H3C H H3C CH3 C Mirror image C C H CH3 H3C CH3 C H2C CHCH2CH CH2 H2C C CHCH2CH3 (E)-1,3-Pentadiene Most stable 3186 kJ/mol (761.6 kcal/mol) 1,4-Pentadiene 3217 kJ/mol (768.9 kcal/mol) 1,2-Pentadiene Least stable 3251 kJ/mol (777.1 kcal/mol) C C H H3C H CH CH2 CH3(CH2)6CH2CH C CHCH CHCO2CH3 6 54 3 2 1 (CH3)2CH CH3 CH3 CH3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Cembrene CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS 233 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
234 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS Comparison of the mirror-image forms of 2-chloro-2,3-pentadiene reveals that they are not superposable. 2-Chloro-2, 3-pe ne is a chiral allene -Chloro-2, 3-pentadiene CH Rotate180° HC H Reference structure Mirror image 10.9 Both starting materials undergo B-elimination to give a conjugated diene system. Two minor prod ucts result. both of which have isolated double bonds H,C=CHCH, CCH, CH X=OH 3-Methyl-S-hexen-3-ol X= Br 4-Bromo-4-methyl-1-hexene Faster Slower CH CH H,C=CHCH=CCH,CH3 H,C=CHCH,C=CHCH3 H,C=CHCH, CCHaCH3 2-Ethyl-1, 4-pentadiene (mixture of E and Z isom (mixture of E and Z isomers: major product) 10.10 The best approach is to work through this reaction mechanistically. Addition of hydrogen halides always proceeds by protonation of one of the terminal carbons of the diene system. Protonation of C-1 gives an allylic cation for which the most stable resonance form is a tertiary carbocation. Pro- tonation of C-4 would give a less stable allylic carbocation for which the most stable resonance form is a secondary carbocation H,C 尤C-CH=CH2 H-C H,C=CCH=CH, (CH3),CCH=CH, H3C thyl-13-butadiene =CH-CH, H,C Under kinetically controlled conditions the carbocation is captured at the carbon that bears the great est share of positive charge, and the product is the tertiary chloride. Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
Comparison of the mirror-image forms of 2-chloro-2,3-pentadiene reveals that they are not superposable. 2-Chloro-2,3-pentadiene is a chiral allene. 10.9 Both starting materials undergo -elimination to give a conjugated diene system. Two minor products result, both of which have isolated double bonds. 10.10 The best approach is to work through this reaction mechanistically. Addition of hydrogen halides always proceeds by protonation of one of the terminal carbons of the diene system. Protonation of C-1 gives an allylic cation for which the most stable resonance form is a tertiary carbocation. Protonation of C-4 would give a less stable allylic carbocation for which the most stable resonance form is a secondary carbocation. Under kinetically controlled conditions the carbocation is captured at the carbon that bears the greatest share of positive charge, and the product is the tertiary chloride. H2C CH2 CH3 CCH (CH3)2CCH CH2 Cl HCl C CH CH2 H3C H3C C CH CH2 H3C H3C 2-Methyl-1,3-butadiene 3-Chloro-3-methyl-1- butene (major product) X OH 3-Methyl-5-hexen-3-ol X Br 4-Bromo-4-methyl-1-hexene CHCH2CCH2CH3 X H2C CH3 H2C CHCH CCH2CH3 CH3 H2C CHCH2C CHCH3 CH3 H2C CHCH2CCH2CH3 CH2 4-Methyl-1,3-hexadiene (mixture of E and Z isomers; major product) 4-Methyl-1,4-hexadiene (mixture of E and Z isomers; minor product) 2-Ethyl-1,4-pentadiene (minor product) Faster Slower Reference structure C C H3C H Cl CH3 C C C H3C H H3C Cl C Mirror image C C H CH3 Cl CH3 C Rotate 180 Reoriented mirror image and 2-Chloro-2,3-pentadiene 234 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website