But-2-yne is converted into P by (H2…

But-2-yne is converted into P by (H2, Lindlar catalyst) followed by Br2 in CCl4, and into Q by (Na, liquid NH3) followed by Br2 in CCl4. Which…

Single correct option

But-2-yne is converted into

\mathbf{P}

by (

\mathrm{H_2}

, Lindlar catalyst) followed by

\mathrm{Br_2}

\mathrm{CCl_4}

, and into

\mathbf{Q}

by (

\mathrm{Na}

, liquid

\mathrm{NH_3}

) followed by

\mathrm{Br_2}

\mathrm{CCl_4}

. Which statement about

\mathbf{P}

and

\mathbf{Q}

(the 2,3-dibromobutane products) is correct?

A. $\mathbf{P}$ is a single meso compound; $\mathbf{Q}$ is a racemic mixture; they are diastereomers
B. $\mathbf{P}$ is a racemic mixture of $(2R,3R)$ and $(2S,3S)$ ; $\mathbf{Q}$ is the meso $(2R,3S)$ compound; they are diastereomers
C. Both $\mathbf{P}$ and $\mathbf{Q}$ are racemic mixtures, and the two racemates are enantiomers of each other
D. Both $\mathbf{P}$ and $\mathbf{Q}$ are the same meso compound

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Answer: (B) $\mathbf{P}$ is a racemic mixture of $(2R,3R)$ and $(2S,3S)$ ; $\mathbf{Q}$ is the meso $(2R,3S)$ compound; they are diastereomers

Lindlar hydrogenation is a syn addition of

\mathrm{H_2}

and gives

cis

-but-2-ene;

\mathrm{Na/NH_3}

is a dissolving-metal reduction through the more stable trans-vinyl radical anion and gives

trans

-but-2-ene. Bromine adds anti through a bromonium ion that is opened from the opposite face. Anti addition to the

cis

alkene produces the

(2R,3R)

and

(2S,3S)

pair in equal amounts (bromonium ion can form on either face), i.e. a racemic mixture, so

\mathbf{P}

is racemic. Anti addition to the

trans

alkene gives the

(2R,3S)

compound, which has an internal mirror plane, i.e. meso, so

\mathbf{Q}

is meso. A racemate and a meso form are stereoisomers that are not mirror images of each other, hence diastereomers. Option 1 reverses the stereospecificity; options 3 and 4 ignore it.

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Original question written in the JEE Advanced 2025 Paper 2 pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake