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Portmess Homework Q&S 113

Portmess Homework Q&S 113 - tene would be formed Br...

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( b ) (Bromomethyl)cyclohexane is the correct answer. It gives methylenecyclohexane as the only alkene under E2 conditions. 1-Bromo-1-methylcyclohexane is not correct. It gives a mixture of 1-methylcyclohexene and methylenecyclohexane on elimination. ( c ) In order for 4-methylcyclohexene to be the only alkene, the starting alkyl bromide must be 1-bromo-4-methylcyclohexane. Either the cis or the trans isomer may be used, although the cis will react more readily, as the more stable conformation (equatorial methyl) has an axial bromine. 1-Bromo-3-methylcyclohexane is incorrect; its dehydrobromination yields a mixture of 3- methylcyclohexene and 4-methylcyclohexene. ( d ) The bromine must be at C-2 in the starting alkyl bromide for a single alkene to be formed on E2 elimination. If the bromine substituent were at C-3, a mixture of 3,3-dimethyl- and 4,4-dimethylcyclopen-
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Unformatted text preview: tene would be formed. Br CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 base E2 1 3-Bromo-1,1-dimethylcyclopentane 3,3-Dimethylcyclopentene 4,4-Dimethylcyclopentene 2-Bromo-1,1-dimethylcyclopentane 3,3-Dimethylcyclopentene base E2 H H Br CH 3 CH 3 CH 3 CH 3 Base: Br H CH 3 (H) CH 3 H(CH 3 ) H 1-Bromo-3-methylcyclohexene 3-Methylcyclohexene CH 3 4-Methylcyclohexene base E2 1 H H CH 3 (H) CH 3 H(CH 3 ) Br cis- or trans-1-Bromo-4-methylcyclohexane 4-Methylcyclohexene base E2 Br CH 3 CH 2 base E2 1-Bromo-1-methylcyclohexane Methylenecyclohexane CH 3 1-Methylcyclohexene 1 H CH 2 CH 2 Br base E2 (Bromomethyl)cyclohexane Methylenecyclohexane STRUCTURE AND PREPARATION OF ALKENES: ELIMINATION REACTIONS 107...
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