二、由环引起的顺反异构 Br Br cis-l-bromo-3-chlorocyclobutane trans-1-bromo-3-chlorocyclobutane CH 3 CH3 CH3 CH 3 H cis-1.4-dimethylcyclohexane trans-1.4-dimethylcvclohexane二、由环引起的顺反异构