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1、CHAPTER 10 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS SOLUTIONS TO TEXT PROBLEMS 10.1As 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) 10.2For two isomeric halides to yield the same carbocation on ionizatio
2、n, 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 CH3Cl CH3 Br 3-Bromo-1- methylcyclohexene 3-Chloro
3、-3- methylcyclohexene ? ? C(CH3)2C(CH3)2 H2CCH2 CH3 C ? H2CCH2 CH3 C ? 230 BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website 10.3The allylic hydrogens are the ones shown in the structural formulas. (b) (c) (d) 10.4 The statement of the problem specifi es that in allylic brominations usin
4、g 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 fi rst step. ? H H CyclohexeneBromine atom ?H 2-Cyclohexenyl radical
5、Hydrogen bromide BrHBr ?Br2 H H CyclohexeneBromine ? Br H 3-BromocyclohexeneHydrogen bromide HBr 1-Octene HH 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 carbocatio
6、n CH3 Cl CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS231 BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website 232CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS The allylic radical formed in the fi rst step abstracts a bromine atom from Br2in the second propa- gation step. 10.5Write both res
7、onance 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 unp
8、aired 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 sp3carbon atoms. Humulene CH3 CH3 H3C CH3 CH2CHCH2(CH2)4CH3?
9、BrCH2CHCH(CH2)4CH3CH2CHCH(CH2)4CH3 Br NBS 1-Octene3-Bromo-1-octene1-Bromo-2-octene (cis and trans) CH2CHCH2(CH2)4CH3 1-Octene CH2CHCH(CH2)4CH3CH2CHCH(CH2)4CH3 NBS (CH3)3CCCH2 CH3 2,3,3-Trimethyl-1- butene (CH3)3CCCH2 CH2Br 2-(Bromomethyl)-3,3- dimethyl-1-butene (CH3)3CC CH2 CH3 2,3,3-Trimethyl-1-but
10、ene (CH3)3CC CH2 CH2 (CH3)3CC CH2 CH2 ? 3-BromocyclohexeneBromine atom ? 2-Cyclohexenyl radical Bromine H H Br BrBr Br BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE 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 isolat
11、ed 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. 10.7The more stable the isomer, the lower its heat of combustion. The conjugat
12、ed 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. 10.8Compare the mirror-image forms of each compound for superposability. For 2-methyl-2,3- pentadiene, Rotation of the mirror image 180 around an axis pa
13、ssing 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. Rotate 180?C Mirror image C CH3H H3C CH3 C Reoriented mirror image C C HH3C H3C CH3 C 2-methyl-2,3-pentadiene and Reference
14、 structure C C HH3C H3C CH3 C Mirror image C C CH3H H3C CH3 C H2CCHCH2CHCH2H2CCCHCH2CH3 (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) CC H H3CH CHCH2 CH3(CH2)6CH2CHCCHCHCHCO2CH3 654321
15、(CH3)2CHCH3 CH3 CH3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Cembrene CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS233 BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website Comparison of the mirror-image forms of 2-chloro-2,3-pentadiene reveals that they are not superposable. 2-Chloro-2,3-pentadiene
16、 is a chiral allene. 10.9Both starting materials undergo ?-elimination to give a conjugated diene system. Two minor prod- ucts result, both of which have isolated double bonds. 10.10The 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 w
