Which of the following alkenes is the major product according to the $\mathrm{Zaitsev}$ rule in the $E2$ dehydrohalogenation of $2-\mathrm{bromobutane}$?

• A. $C_2{H}_4$ (ethylene)
• B. cis-2-butene (${\mathrm{CH}}_3\mathrm{CH}=\mathrm{CH}{\mathrm{CH}}_3$, cis isomer)
• C. 1-butene ($\mathrm{CH}=\mathrm{CH}{\mathrm{CH}}_2{\mathrm{CH}}_3$)
• D. trans-2-butene (${\mathrm{CH}}_3\mathrm{CH}=\mathrm{CH}{\mathrm{CH}}_3$, trans isomer)

Answer: (D)

Verified step by step guidance

1. Identify the substrate and the type of reaction: The substrate is 2-bromobutane undergoing an E2 dehydrohalogenation reaction, which involves elimination of HBr to form an alkene.
2. Recall the Zaitsev rule: In elimination reactions, the major product is usually the more substituted alkene, as it is more stable due to hyperconjugation and alkyl group electron-donating effects.
3. Determine possible alkenes formed by elimination: From 2-bromobutane, elimination can give 1-butene (less substituted), cis-2-butene, and trans-2-butene (both more substituted). Ethylene is not a product here because it does not come from 2-bromobutane.
4. Compare substitution patterns: 1-butene is monosubstituted at the double bond, while 2-butene (both cis and trans) is disubstituted, making 2-butene more substituted and favored by Zaitsev's rule.
5. Consider stereochemistry: Between cis-2-butene and trans-2-butene, the trans isomer is more stable due to less steric hindrance, so trans-2-butene is the major product according to Zaitsev's rule and E2 elimination.