1559Tch22389-40g11/7/0515:40Pag0402 EQA 402 chapter 22 CHEMISTRY OF BENZENE SUBSTITUENTS:ALKYLBENZENES,PHENOLS,AND BENZENAMINES on the benzene ring.catalyzed by acid. d典2日 Dmigration to give the rearranged aromatic product 9点-g Dcan also be lost in the final step,giving a phenol lacking deuterium S5.(a)Diels-Alder) (b)H2.Pd/C.CHCH2OH CHO Now what?Work backward from the product 点心瑞心 one-step S2 displacement mechanism at a benzene carbon.54. (a) Benzene rings containing only alkyl substituents are not very susceptible to attack by nucleophiles or radicals. Electrophilic attack is the most reasonable pathway. (b) Formation of the oxacyclopropane probably involves electrophilic attack of a reagent like H2O2 (derived from O2) on the benzene ring, catalyzed by acid. The final phenol probably arises from reversal of the last two steps. The oxacyclopropane ring can always close again, but eventually the carbocation reacts via an alternative pathway, involving D migration to give the rearranged aromatic product. D can also be lost in the final step, giving a phenol lacking deuterium. 55. (a)  (Diels-Alder) (b) H2, Pd/C, CH3CH2OH (c) (d) Now what? Work backward from the product. 56. The normal “backside displacement” geometry associated with an SN2 transition state is impossible to achieve at a carbon atom in a benzene ring. There is, in fact, no evidence to support the occurrence of the one-step SN2 displacement mechanism at a benzene carbon. O O H H Wittig twice 2 CH2 P(C6H5)3 Cope rearrangement  CHO CHO CH2OH CH2OH O O O R HO R D H D  HO 
H OH R D O
H   R D R R D D H O R OH D  HO OH2
H2O 402 • Chapter 22 CHEMISTRY OF BENZENE SUBSTITUENTS: ALKYLBENZENES, PHENOLS, AND BENZENAMINES 1559T_ch22_389-408 11/7/05 15:40 Page 402