94 STRUCTURE AND PREPARATION OF ALKENES: ELIMINATION REACTIONS (b, c) Both 1-propanol and 2-propanol give propene on acid-catalyzed dehydration OH 1-Propanol Propene (d) Carbon-3 has no hydrogens in 2, 3, 3-trimethyl-2-butanol. Elimination can involve only the hydroxyl group at C-2 and a hydrogen at C-1 CHCH CH CH CH -CH3 2, 3, 3-Tnimethyl-2-butanol 2.3.3-Trimethyl-1-butene 5.13(b) Elimination can involve loss of a hydrogen from the methyl group or from C-2 of the ring in 1-methylcyclohexanol CH I-Methylcyclohexanol disubstituted clohexane I- Methyl ed alkene kene According to the Zaitsev rule, the major alkene is the one corresponding to loss of a hydrogen from the alkyl group that has the smaller number of hydrogens. Thus hydrogen is removed from the methylene group in the ring rather than from the methyl group, and l-methylcyclo- hexene is formed in greater amounts than methylenecyclohexane (c) The two alkenes formed are as shown in the equation double bond double bond The more highly substituted alkene is formed in greater amounts, as predicted by Zaitsev's rule 5.14 2-Pentanol can undergo dehydration in two different directions, giving either 1-pentene or 2-pentene. 2-Pentene is formed as a mixture of the cis and trans stereoisomers CH H CHaCHCH, CH, CH3 CH =CHCHCH CH,+ CHCM x A 2-Pentanol 1-Pentene cis-2-Pentene trans.2-Pentene Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website(b, c) Both 1-propanol and 2-propanol give propene on acid-catalyzed dehydration. (d) Carbon-3 has no hydrogens in 2,3,3-trimethyl-2-butanol. Elimination can involve only the hydroxyl group at C-2 and a hydrogen at C-1. 5.13 (b) Elimination can involve loss of a hydrogen from the methyl group or from C-2 of the ring in 1-methylcyclohexanol. According to the Zaitsev rule, the major alkene is the one corresponding to loss of a hydrogen from the alkyl group that has the smaller number of hydrogens. Thus hydrogen is removed from the methylene group in the ring rather than from the methyl group, and 1-methylcyclo￾hexene is formed in greater amounts than methylenecyclohexane. (c) The two alkenes formed are as shown in the equation. The more highly substituted alkene is formed in greater amounts, as predicted by Zaitsev’s rule. 5.14 2-Pentanol can undergo dehydration in two different directions, giving either 1-pentene or 2-pentene. 2-Pentene is formed as a mixture of the cis and trans stereoisomers. 1-Pentene cis-2-Pentene CH2 CHCH2CH2CH3 C C CH3 H CH2CH3 H trans-2-Pentene C C CH3 H H CH2CH3 H heat 2-Pentanol OH CH3CHCH2CH2CH3 OH H H Compound has a trisubstituted double bond Compound has a tetrasubstituted double bond; more stable H2O H2O H3C OH 1-Methylcyclohexanol CH2 Methylenecyclohexane (a disubstituted alkene; minor product) CH3 1-Methylcyclohexene (a trisubstituted alkene; major product) H heat 2,3,3-Trimethyl-2-butanol No hydrogens on this carbon 2,3,3-Trimethyl-1-butene C HO CH3 CH3 H3C C CH3  C C CH3 CH3 CH3 CH3 CH3 H2C H heat H heat 1-Propanol Propene CH3CH2CH2OH CH3CH CH2  2-Propanol CH3CHCH3  OH 94 STRUCTURE AND PREPARATION OF ALKENES: ELIMINATION REACTIONS Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website