IIT-JEE 2007 · Paper 1 · Chemistry Q27

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ChemistrySingle correct+3 / -1Question 27 of 66

The reagent(s) for the following conversion, 1,2-dibromoethane → ethyne (H-C≡C-H), is/are

alcoholic KOH

alcoholic KOH followed by

\ce{NaNH2}

aqueous KOH followed by

\ce{NaNH2}

\ce{Zn/CH3OH}

Answer and worked solution

Official answer: B

Two eliminations. Alcoholic KOH removes one HBr to give vinyl bromide; the second elimination from the vinylic halide needs a stronger base, $\ce{NaNH2}$ :
$\ce{BrCH2CH2Br ->[alc.\ KOH] CH2=CHBr ->[NaNH2] HC#CH}.$

Aqueous KOH would substitute (giving the glycol), and Zn would give ethene.

(B)

Topics:Elimination reactions Alkynes

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IIT-JEE 2007 · Paper 1, Chemistry question 27