JEE MainChemistryHaloalkanes and Haloarenes
Consider the dehydrohalogenation of cis-1-bromo-2-methylcyclohexane and trans-1-bromo-2-methylcyclohexane using alcoholic KOH. Identify the major alkene product formed in each case.
Options
- ABoth isomers yield 1-methylcyclohexene as the major product.
- BThe cis-isomer yields 3-methylcyclohexene and the trans-isomer yields 1-methylcyclohexene.
- CBoth isomers yield 3-methylcyclohexene as the major product.
- DThe cis-isomer yields 1-methylcyclohexene and the trans-isomer yields 3-methylcyclohexene.
Correct answer
D. The cis-isomer yields 1-methylcyclohexene and the trans-isomer yields 3-methylcyclohexene.
Step-by-step solution
Dehydrohalogenation via the E2 mechanism requires an anti-periplanar arrangement of the leaving group (Br) and the beta-hydrogen. In a cyclohexane ring, this means both the Br atom and the beta-hydrogen must occupy axial positions. For cis-1-bromo-2-methylcyclohexane: In the chair conformation where Br is axial, the methyl group at C2 is equatorial. This leaves the hydrogen at C2 in an axial position. There are also axial hydrogens at C6. Elimination can occur at either C2 or C6. Following Zaitsev's rule, the more