96 STRUCTURE AND PREPARATION OF ALKENES: ELIMINATION REACTION: This secondary carbocation can rearrange to a more stable tertiary carbocation by a methyl group 2, 2-Dimethylcyclohexyl cation 2. Dimet yertia hexyl cation Loss of a proton from the 1, 2-dimethylcyclohexyl cation intermediate yields 1, 2-dimethylcyclo- hexene HO 1, 2-Dimethylcyclohexyl cation 1. 2-Dimethylcyclohexene 5.17 (b) All the hydrogens of tert-butyl chloride are equivalent. Loss of any of these hydrogens alon with the chlorine substituent yields 2-methylpropene as the only alkene CH C、 CH,CCI HC 2-Methylpropene (c) All the B hydrogens of 3-bromo-3-ethylpentane are equivalent, so that B-elimination can give only 3-ethyl-2-pentene CHCH CH2CH,C—B CHCH=C CHCH (d) There are two possible modes of B-elimination from 2-bromo-3-methylbutane Elimination in one direction provides 3-methyl-1-butene; elimination in the other gives 2-methyl-2-butene CH3 CHCH(CH3) CH,=CHCH(CH3)2+ CH_CHC(CH3) 2-Bromo-3-methylbutane Methy l-2 The major product is the more highly substituted alkene, 2-methyl-2-butene. It is the more stable alkene and corresponds to removal of a hydrogen from the carbon that has the fewer (e) Regioselectivity is not an issue here, because 3-methyl-1-butene is the only alkene that can be formed by B-elimination from l-bromo-3-methylbutane BrCH, CH, CH(CH,)2- CH2CHCH(CH,) 1-Bromo-3-methylbutene 3-Methyl-l-butene Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE WebsiteThis secondary carbocation can rearrange to a more stable tertiary carbocation by a methyl group shift. Loss of a proton from the 1,2-dimethylcyclohexyl cation intermediate yields 1,2-dimethylcyclo￾hexene. 5.17 (b) All the hydrogens of tert-butyl chloride are equivalent. Loss of any of these hydrogens along with the chlorine substituent yields 2-methylpropene as the only alkene. (c) All the hydrogens of 3-bromo-3-ethylpentane are equivalent, so that -elimination can give only 3-ethyl-2-pentene. (d) There are two possible modes of -elimination from 2-bromo-3-methylbutane. Elimination in one direction provides 3-methyl-1-butene; elimination in the other gives 2-methyl-2-butene. The major product is the more highly substituted alkene, 2-methyl-2-butene. It is the more stable alkene and corresponds to removal of a hydrogen from the carbon that has the fewer hydrogens. (e) Regioselectivity is not an issue here, because 3-methyl-1-butene is the only alkene that can be formed by -elimination from 1-bromo-3-methylbutane. 1-Bromo-3-methylbutene 3-Methyl-1-butene BrCH2CH2CH(CH3)2 CH2 CHCH(CH3)2 2-Bromo-3-methylbutane 3-Methyl-1-butene (monosubstituted) 2-Methyl-2-butene (trisubstituted) CH3CHCH(CH3)2 Br CH2 CHCH(CH3)2 CH3CH C(CH3) 2 3-Bromo-3-ethylpentane 3-Ethyl-2-pentene CH3CH2 CH2CH3 CH2CH3 C Br CH3CH CH2CH3 CH2CH3 C C H3C H3C CH3CCl CH2 CH3 CH3 tert-Butyl chloride 2-Methylpropene H3O OH2 H CH3 CH3 CH3 CH3 H 1,2-Dimethylcyclohexyl cation 1,2-Dimethylcyclohexene CH3 CH3 CH3 H CH3 H 2,2-Dimethylcyclohexyl cation (secondary) 1,2-Dimethylcyclohexyl cation (tertiary) 96 STRUCTURE AND PREPARATION OF ALKENES: ELIMINATION REACTIONS Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website