The following compounds are given to you:
2-bromopentane, 2-bromo-2-methylbutane, 1-bromopentane
(a) Write the compound which is most reactive towards the S2 reaction.
(b) Write the compound which is optically active.
(c) Write the compound which is most reactive towards the β-elimination reaction.
2-bromopentane, 2-bromo-2-methylbutane, 1-bromopentane
(a) Write the compound which is most reactive towards the S2 reaction.
(b) Write the compound which is optically active.
(c) Write the compound which is most reactive towards the β-elimination reaction.
Answer: (a) 1-Bromopentane. (b) 2-Bromopentane (chiral C-2). (c) 2-Bromo-2-methylbutane.
Solution
(a) S2 prefers the least hindered halide: \textbf{1-bromopentane} (primary).
(b) \textbf{2-Bromopentane}: C-2 carries CH, Br, H and CH — four different groups, a chiral centre, so it is optically active.
(c) β-Elimination (E2 with alcoholic KOH) is fastest for the tertiary halide, which also gives the most substituted (Saytzeff) alkene: \textbf{2-bromo-2-methylbutane}.
(b) \textbf{2-Bromopentane}: C-2 carries CH, Br, H and CH — four different groups, a chiral centre, so it is optically active.
(c) β-Elimination (E2 with alcoholic KOH) is fastest for the tertiary halide, which also gives the most substituted (Saytzeff) alkene: \textbf{2-bromo-2-methylbutane}.
CBSE Class 12 Board · 2017 · Subjective