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1、279 CHAPTER 12 REACTIONS OF ARENES: ELECTROPHILIC AROMATIC SUBSTITUTION SOLUTIONS TO TEXT PROBLEMS 12.1The three most stable resonance structures for cyclohexadienyl cation are The positive charge is shared equally by the three carbons indicated. Thus the two carbons ortho to the sp3-hybridized carb
2、on and the one para to it each bear one third of a positive charge (?0.33). None of the other carbons is charged. The resonance picture and the simple MO treatment agree with respect to the distribution of charge in cyclohexadienyl cation. 12.2Electrophilic aromatic substitution leads to replacement
3、 of one of the hydrogens directly attached to the ring by the electrophile. All four of the ring hydrogens of p-xylene are equivalent; so it does not matter which one is replaced by the nitro group. CH3 CH3 p-Xylene HNO3 H2SO4 CH3 CH3 NO2 1,4-Dimethyl-2- nitrobenzene HH H H H H H ? HH H H H H H ? HH
4、 H H H H H ? BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website 280REACTIONS OF ARENES: ELECTROPHILIC AROMATIC SUBSTITUTION 12.3The aromatic ring of 1,2,4,5-tetramethylbenzene has two equivalent hydrogen substituents. Sul- fonation of the ring leads to replacement of one of them by SO3H.
5、12.4The major product is isopropylbenzene. Aluminum chloride coordinates with 1-chloropropane to give a Lewis acid/Lewis base complex, which can be attacked by benzene to yield propylbenzene or can undergo an intramolecular hydride shift to produce isopropyl cation. Isopropylbenzene arises by reacti
6、on of isopropyl cation with benzene. 12.5The species that attacks the benzene ring is cyclohexyl cation, formed by protonation of cyclohexene. The mechanism for the reaction of cyclohexyl cation with benzene is analogous to the general mech- anism for electrophilic aromatic substitution. 12.6The pre
7、paration of cyclohexylbenzene from cyclohexene and benzene was described in text Sec- tion 12.6. Cyclohexylbenzene is converted to 1-phenylcyclohexene by benzylic bromination, fol- lowed by dehydrohalogenation. ? BenzeneCyclohexeneCyclohexylbenzene H2SO4NaOCH2CH3 N-Bromosuccinimide (NBS), benzoyl pe
8、roxide, heat 1-Phenylcyclohexene1-Bromo-1- phenylcyclohexane Br H H H H H H H H ?H? ? BenzeneCyclohexyl cation Cyclohexadienyl cation intermediate Cyclohexylbenzene H H H H ? H HOSO2OH CyclohexeneSulfuric acidCyclohexyl cationHydrogen sulfate ion OSO2OH ? ? CH3 Isopropyl cation CH H CH2ClAlCl3 hydri
9、de migration ? ?CH3CHCH3AlCl4? ? CH3CH2CH2Cl 1-Chloropropane ? AlCl3 Propylbenzene (20% yield) CH2CH2CH3 BenzeneIsopropylbenzene (40% yield) CH(CH3)2 H3CCH3 CH3H3C 1,2,4,5-Tetramethylbenzene SO3 H2SO4 2,3,5,6-Tetramethylbenzene- sulfonic acid SO3H H3CCH3 CH3H3C BackForwardMain MenuTOCStudy Guide TOC
10、Student OLCMHHE Website 12.7Treatment of 1,3,5-trimethoxybenzene with an acyl chloride and aluminum chloride brings about FriedelCrafts acylation at one of the three equivalent positions available on the ring. 12.8Because the anhydride is cyclic, its structural units are not incorporated into a keto
11、ne and a car- boxylic acid as two separate product molecules. Rather, they become part of a four-carbon unit attached to benzene by a ketone carbonyl. The acyl substituent terminates in a carboxylic acid func- tional group. 12.9(b)A FriedelCrafts alkylation of benzene using 1-chloro-2,2-dimethylprop
12、ane would not be a satisfactory method to prepare neopentylbenzene because of the likelihood of a carbocation rearrangement. The best way to prepare this compound is by FriedelCrafts acylation fol- lowed by Clemmensen reduction. 12.10(b)Partial rate factors for nitration of toluene and tert-butylben
13、zene, relative to a single position of benzene, are as shown: The sum of these partial rate factors is 147 for toluene, 90 for tert-butylbenzene. Toluene is 147?90, or 1.7, times more reactive than tert-butylbenzene. (c)The product distribution for nitration of tert-butylbenzene is determined from t
14、he partial rate factors. Ortho:? 2( 9 4 0 .5) ? 10% Meta:? 2 9 (3 0 ) ? 6.7% Para:? 7 9 5 0 ? 83.3% CH3 4242 58 2.52.5 C(CH3)3 4.54.5 75 33 AlCl3Zn(Hg), HCl ? Benzene2,2-Dimethylpropanoyl chloride (CH3)3CCCl O 2,2-Dimethyl-1- phenyl-1-propanone (CH3)3CC O Neopentylbenzene (CH3)3CCH2 AlCl3 ? Benzene4
15、-Oxo-4-phenylbutanoic acid CCH2CH2COH OO Succinic anhydride O O O AlCl3 ? 1,3,5-Trimethoxybenzene OCH3 CH3O OCH3 3-Methylbutanoyl chloride (CH3)2CHCH2CCl O Isobutyl 1,3,5-trimethoxyphenyl ketone OCH3 CH3O OCH3 CCH2CH(CH3)2 O REACTIONS OF ARENES: ELECTROPHILIC AROMATIC SUBSTITUTION281 BackForwardMain
16、 MenuTOCStudy Guide TOCStudent OLCMHHE Website 12.11The compounds shown all undergo electrophilic aromatic substitution more slowly than benzene. Therefore, CH2Cl, CHCl2, and CCl3are deactivating substituents. The electron-withdrawing power of these substituents, and their tendency to direct incoming elec- trophiles meta to themselves, will increase with the number of chlorines each contains. Thus, the substituent that gives 4% meta nitration (96% ortho ? para) contains the fewest chl
