But-1-yne is converted by (i) NaNH2, (ii) bromoethane, (iii) H2 with Lindlar catalyst, (iv) Br2 in CCl4. The final product is

a racemic mixture of (3R,4R)- and (3S,4S)-3,4-dibromohexane

an equimolar mixture of all three stereoisomers of 3,4-dibromohexane

a racemic mixture of (3R,4R)- and (3S,4S)-3,4-dibromohexane

only the (3R,4R) enantiomer, which is optically active

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JEE Advanced levelSingle correct optionDifficulty 3 of 3hc124

But-1-yne is converted by (i) $NaNH X_{2}$ , (ii) bromoethane, (iii) $H X_{2}$ with Lindlar catalyst, (iv) $Br X_{2}$ in $CCl X_{4}$ . The final product is

Aan equimolar mixture of all three stereoisomers of 3,4-dibromohexane
Bmeso-(3R,4S)-3,4-dibromohexane
Ca racemic mixture of (3R,4R)- and (3S,4S)-3,4-dibromohexane
Donly the (3R,4R) enantiomer, which is optically active

Answer: C

Steps (i)-(ii) give hex-3-yne; Lindlar reduction gives (Z)-hex-3-ene by syn addition. Bromine adds anti through a bromonium ion. Anti addition to a cis-alkene gives the (R,R)/(S,S) pair in equal amounts (racemate); trans-alkenes would give the meso compound. The attack on a symmetric bromonium ion is equally likely at either face, so the racemate is formed.

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But-1-yne is converted by (i) NaNHX2​, (ii) bromoethane, (iii) HX2​ with Lindlar catalyst, (iv) BrX2​ in CClX4​. The final product is

Answer: C

More questions on Alkynes: acidity, preparation and reactions

But-1-yne is converted by (i) $NaNH X_{2}$ , (ii) bromoethane, (iii) $H X_{2}$ with Lindlar catalyst, (iv) $Br X_{2}$ in $CCl X_{4}$ . The final product is