Optically pure (R)-2-bromobutane is…

Optically pure (R)-2-bromobutane is used in the following experiments.

One or more options correct

Optically pure

(R)

-2-bromobutane is used in the following experiments. Select the correct statement(s).

A. Treatment with $\mathrm{NaI}$ in dry acetone gives $(S)$ -2-iodobutane
B. Heating with aqueous ethanol gives $(R)$ -butan-2-ol exclusively, with complete retention of configuration
C. Heating with alcoholic $\mathrm{KOH}$ gives but-1-ene as the major alkene
D. Conversion to the Grignard reagent in dry ether followed by quenching with $\mathrm{D_2O}$ gives optically active 2-deuteriobutane

Show answer and solution

Answer: A

(A) is correct. Iodide in acetone is a strong nucleophile in a polar aprotic solvent, giving a clean

\mathrm{S_N2}

reaction with inversion at the stereocentre. Since

\mathrm{I}>\mathrm{C_2H_5}>\mathrm{CH_3}>\mathrm{H}

has the same priority order as the

\mathrm{Br}

compound, inversion changes the descriptor, so the product is

(S)

-2-iodobutane. (B) is wrong: a secondary bromide in aqueous ethanol reacts through a mix of

\mathrm{S_N1}

and

\mathrm{S_N2}

paths, so the alcohol shows partial racemisation with net inversion, never complete retention. (C) is wrong: alcoholic

\mathrm{KOH}

follows Zaitsev, so but-2-ene (mainly

E

) is the major alkene, not but-1-ene. (D) is wrong: the Grignard reagent forms through radical intermediates and is configurationally unstable, so

\mathrm{D_2O}

quenching gives racemic, optically inactive 2-deuteriobutane.

Original question written in the JEE Advanced 2021 Paper 1 pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake