STRUCTURE AND PREPARATION OF ALKENES: ELIMINATION REACTIONS 5.15 (b) The site of positive charge in the carbocation is the carbon atom that bears the hydroxyl group the starting alcohol HC、 t h,o I-Methylcyclohexanol Water may remove a proton from the methyl group, as shown in the following equation HO: H-CH Loss of a proton from the ring gives the major product l-methylcyclohexene C H,O: H H, O++ 1-Methylcyclohexene c) Loss of the hydroxyl group under conditions of acid catalysis yields a tertiary carbocation HSO Water may remove a proton from an adjacent methylene group to give a trisubstituted alkene HO HH +H3O+ Removal of the methine proton gives a tetrasubstituted alkene 一 hO H OH 5.16 In writing mechanisms for acid-catalyzed dehydration of alcohols, begin with formation of the carbocation intermediate CH H Dimethy cyclohexanol 2, 2-Dimethylcyclohexyl cation Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
5.15 (b) The site of positive charge in the carbocation is the carbon atom that bears the hydroxyl group in the starting alcohol. Water may remove a proton from the methyl group, as shown in the following equation: Loss of a proton from the ring gives the major product 1-methylcyclohexene. (c) Loss of the hydroxyl group under conditions of acid catalysis yields a tertiary carbocation. Water may remove a proton from an adjacent methylene group to give a trisubstituted alkene. Removal of the methine proton gives a tetrasubstituted alkene. 5.16 In writing mechanisms for acid-catalyzed dehydration of alcohols, begin with formation of the carbocation intermediate: CH3 CH3 CH3 H CH3 OH H 2,2-Dimethylcyclohexanol 2,2-Dimethylcyclohexyl cation H H2O H H3O OH2 H H H H H H3O H2O OH H H H2SO4 H2O CH3 H H 1-Methylcyclohexene H3O CH3 H H2O Methylenecyclohexane H3O H2O H CH2 CH2 1-Methylcyclohexanol H CH3 H2O H3C OH STRUCTURE AND PREPARATION OF ALKENES: ELIMINATION REACTIONS 95 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
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 Website
This 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-dimethylcyclohexene. 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
STRUCTURE AND PREPARATION OF ALKENES: ELIMINATION REACTIONS (f Two alkenes may be formed here. The more highly substituted one is 1-methylcyclohexene and this is predicted to be the major product in accordance with Zaitsev's rule H I-lodo-1-methylcyclohexane methylenecyclohexane 1-Methylcyclohexene (disubstituted) 5.18 Elimination in 2-bromobutane can take place between C-1 and C-2 or between C-2 and C-3. Ther are three alkenes capable of being formed: 1-butene and the stereoisomers cis-2-butene and trar 2-butene HC CH3 H3C CH3 CHCH,CH3 CH2=CHCH,CH3+ 2-Bromobutane cis-2-Butene As predicted by Zaitsev's rule, the most stable alkene predominates. The major product is trans 2-butene 5.19 An unshared electron pair of the base methoxide(CHao )abstracts a proton from carbon. The pair of electrons in this C-H bond becomes the T component of the double bond of the alkene. The pair of electrons in the C-Cl bond becomes an unshared electron pair of chloride ion CH,O: H CH CH一H+H2C=CCH3)2+Q! 5.20 The most stable conformation of cis-4-tert-butylcyclohexyl bromide has the bromine substituent in an axial orientation. The hydrogen that is removed by the base is an axial proton at C-2. This hydrogen and the bromine are anti periplanar to each other in the most stable conformation H (CH3)C 5.21(a) 1-Heptene is CH2=CH(CH2)4CH, (b)3-Ethyl-2-pentene is CH_ CH=C(CH_CH3) (c) cis-3-Octene is CH, CH CH, CHCHCH (d) trans-1, 4-Dichloro-2-butene is CICH, CHCI Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
( f ) Two alkenes may be formed here. The more highly substituted one is 1-methylcyclohexene, and this is predicted to be the major product in accordance with Zaitsev’s rule. 5.18 Elimination in 2-bromobutane can take place between C-1 and C-2 or between C-2 and C-3. There are three alkenes capable of being formed: 1-butene and the stereoisomers cis-2-butene and trans- 2-butene. As predicted by Zaitsev’s rule, the most stable alkene predominates. The major product is trans- 2-butene. 5.19 An unshared electron pair of the base methoxide (CH3O) abstracts a proton from carbon. The pair of electrons in this C@H bond becomes the component of the double bond of the alkene. The pair of electrons in the C@Cl bond becomes an unshared electron pair of chloride ion. 5.20 The most stable conformation of cis-4-tert-butylcyclohexyl bromide has the bromine substituent in an axial orientation. The hydrogen that is removed by the base is an axial proton at C-2. This hydrogen and the bromine are anti periplanar to each other in the most stable conformation. 5.21 (a) 1-Heptene is . (b) 3-Ethyl-2-pentene is . (c) cis-3-Octene is (d) trans-1,4-Dichloro-2-butene is C ClCH2 H C H CH2Cl C CH3CH2 H C CH2CH2CH2CH3 H CH3CH C(CH2CH3)2 CH2 CH(CH2)4CH3 Br H (CH3)3C (CH3)3C O H H C C CH3 H CH3 Cl CH Cl 3O H H2C C(CH3)2 CH3O 1-Butene cis-2-Butene CH2 CHCH2CH3 C C H3C H CH3 H trans-2-Butene C C H3C H H CH3 2-Bromobutane Br CH3CHCH2CH3 H3C I 1-Iodo-1-methylcyclohexane Methylenecyclohexane (disubstituted) CH2 CH3 1-Methylcyclohexene (trisubstituted; major product) STRUCTURE AND PREPARATION OF ALKENES: ELIMINATION REACTIONS 97 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
98 STRUCTURE AND PREPARATION OF ALKENES: ELIMINATION REACTIONS (e)(z)-3-Methyl-2-hexene is H,C CH, CH, CH3 (f)(E)-3-Chloro-2-hexene is H,C, CH,CH,CI (g) 1-Bromo-3-methylcyclohexene is H3C (h) 1-Bromo-6-methylcyclohexene is (i) 4-Methyl-4-penten-2-ol is H3 CH CHCHC=CH ( Vinylcycloheptane is k) An allyl group is -, CH=CH2. 1, 1-Diallylcyclopropane is CH () An isopropenyl substituent is -C=CH,. trans-1-Isopropenyl-3-methylcyclohexane is CH CH 5.22 Alkenes with tetrasubstituted double bonds have four alkyl groups attached to the doubly bonded carbons. There is only one alkene of molecular formula ChH that has a tetrasubstituted double bond, 2, 3-dimethyl-2-pentene H H, C CHCH 5.23 (a) The longest chain that includes the double bond in(CH_CH,),C=CHCH, contains five carbon atoms, and so the parent alkene is a pentene. The numbering scheme that gives the double bond the lowest number is The compound is 3-ethy l-2-pentene Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(e) (Z)-3-Methyl-2-hexene is ( f ) (E)-3-Chloro-2-hexene is (g) 1-Bromo-3-methylcyclohexene is (h) 1-Bromo-6-methylcyclohexene is (i) 4-Methyl-4-penten-2-ol is ( j) Vinylcycloheptane is (k) An allyl group is @CH2CH?CH2. 1,1-Diallylcyclopropane is (l) An isopropenyl substituent is . trans-1-Isopropenyl-3-methylcyclohexane is therefore 5.22 Alkenes with tetrasubstituted double bonds have four alkyl groups attached to the doubly bonded carbons. There is only one alkene of molecular formula C7H14 that has a tetrasubstituted double bond, 2,3-dimethyl-2-pentene. 5.23 (a) The longest chain that includes the double bond in (CH3CH2)2C?CHCH3 contains five carbon atoms, and so the parent alkene is a pentene. The numbering scheme that gives the double bond the lowest number is The compound is 3-ethyl-2-pentene. H 5 4 1 3 2 C H3C H3C C CH3 CH2CH3 2,3-Dimethyl-2-pentene CH3 CH2 CH3 C CH2 CH3 H CH3CHCH2C CH2 CH3 OH Br CH3 Br H3C C H3C H C CH2CH2CH3 Cl C H3C H C CH2CH2CH3 CH3 98 STRUCTURE AND PREPARATION OF ALKENES: ELIMINATION REACTIONS Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
STRUCTURE AND PREPARATION OF ALKENES: ELIMINATION REACTIONS (b) Write out the structure in detail, and identify the longest continuous chain that includes the double bond CH CH2 CH, CH3 CHCH H,CH The longest chain contains six carbon atoms and the double bond is between C-3 and c-4 The compound is named as a derivative of 3-hexene. There are ethyl substituents at C-3 and C-4. The complete name is 3, 4-diethyl-3-hexene (c) Write out the structure completel H The longest carbon chain contains four carbons. Number the chain so as to give the lowest numbers to the doubly bonded carbons, and list the substituents in alphabetical order. This compound is 1, I-dichloro-3, 3-dimethyl-l-butene. (d) The longest chain has five carbon atoms, the double bond is at C-1, and there are two methyl substituents. The compound is 4, 4-dimethyl-l-pentene (e) We number this trimethylcyclobutene derivative so as to provide the lowest number for the substituent at the first point of difference. We therefore number H C H. rather than H,C The correct IUPAC name is 1, 4, 4-trimethylcyclobutene, not 2, 3, 3-trimethylcyclobutene (f) The cyclohexane ring has a 1, 2-cis arrangement of vinyl substituents. The compound cis-1, 2-divinylcyclohexane (g) Name this compound as a derivative of cyclohexene. It is 1, 2-divinylcyclohexene 5.24 (a) Go to the end of the name, because this tells you how many carbon atoms are present in the longest chain. In the hydrocarbon name 2, 6, 10, 14-tetramethyl-2-pentadecene, the suffix pentadecane"reveals that the longest continuous chain has 15 carbon atoms and that there Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(b) Write out the structure in detail, and identify the longest continuous chain that includes the double bond. The longest chain contains six carbon atoms, and the double bond is between C-3 and C-4. The compound is named as a derivative of 3-hexene. There are ethyl substituents at C-3 and C-4. The complete name is 3,4-diethyl-3-hexene. (c) Write out the structure completely. The longest carbon chain contains four carbons. Number the chain so as to give the lowest numbers to the doubly bonded carbons, and list the substituents in alphabetical order. This compound is 1,1-dichloro-3,3-dimethyl-1-butene. (d ) The longest chain has five carbon atoms, the double bond is at C-1, and there are two methyl substituents. The compound is 4,4-dimethyl-1-pentene. (e) We number this trimethylcyclobutene derivative so as to provide the lowest number for the substituent at the first point of difference. We therefore number The correct IUPAC name is 1,4,4-trimethylcyclobutene, not 2,3,3-trimethylcyclobutene. ( f ) The cyclohexane ring has a 1,2-cis arrangement of vinyl substituents. The compound is cis-1,2-divinylcyclohexane. (g) Name this compound as a derivative of cyclohexene. It is 1,2-divinylcyclohexene. 5.24 (a) Go to the end of the name, because this tells you how many carbon atoms are present in the longest chain. In the hydrocarbon name 2,6,10,14-tetramethyl-2-pentadecene, the suffix “2-pentadecene” reveals that the longest continuous chain has 15 carbon atoms and that there H H H3C H3C H3C 1 4 2 3 H3C H3C H3C 2 3 1 4 rather than 1 2 3 4 5 C H3C H3C CH3 C H C Cl Cl 3 4 2 1 C CH3CH2 CH2CH3 CH3CH2 CH2CH3 C 1 2 5 6 3 4 STRUCTURE AND PREPARATION OF ALKENES: ELIMINATION REACTIONS 99 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website