See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Concept: In an E2 reaction, elimination occurs in one concerted step where a base abstracts a beta-hydrogen anti-periplanar to the leaving group. The number of distinct alkene products depends on (1) how many different sets of beta-hydrogens are available, (2) whether each elimination gives geometrically distinct (E/Z) isomers, and (3) whether different structural (constitutional) isomers of alkenes can form. Analysis of each option: (a) CH3-CH2-CH2-CH2-Br (1-bromobutane): Only one set of beta-carbons (C2), giving only 1-butene. Very few products. (c) CH3-CH2-CH(Br)-CH2-CH3 (3-bromopentane): Beta-carbons are C2 and C4, but by symmetry both give the same alkene (pent-2-ene, which can be E or Z). So only 2 products (E and Z pent-2-ene). Limited complexity. (d) CH3-C(CH3)(Br)-CH2-CH3 (2-bromo-2-methylbutane): Beta-carbons are C1 (the CH3 directly on C2) and C3. Elimination toward C1 gives 2-methylbut-1-ene (one product). Elimination toward C3 gives 2-methylbut-2-ene (one product, no E/Z possible due to identical groups). Total: 2 structural isomers, limited mixture. (b) CH3-CH2-CH2-CH(Br)-CH3 (2-broмopentane): The chiral center at C2 has beta-carbons at C1 and C3. Elimination toward C1 (removing H from CH3) gives pent-1-ene. Elimination toward C3 gives pent-2-ene, which exists as both E and Z isomers. Additionally, the starting material is chiral (R and S enantiomers both present in racemic mixture), and E2 is stereospecific (anti elimination), meaning each enantiomer gives different stereochemical outcomes. This generates: pent-1-ene, (E)-pent-2-ene, and (Z)-pent-2-ene — giving 3 alkene products, the most among all options. The combination of two different beta-carbon sets plus E/Z isomerism makes option (b) yield the most complex mixture. Why (b) wins: It has two structurally distinct beta-positions (giving constitutional isomers pent-1-ene and pent-2-ene) AND one of those products (pent-2-ene) has E/Z isomerism, resulting in three distinct alkene products total — more than any other option. Therefore, the correct answer is B.