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ChemistryInteger+3 / -1Question 42 of 54

JEE Advanced 2022 · Paper 2, Chemistry question 6

The number of isomeric tetraenes (NOT containing spsp-hybridized carbon atoms) that can be formed from the following reaction sequence is ________.

Starting material 4-(but-2-yn-1-yl)cyclohex-1-ene (a cyclohexene ring bearing a CH2-C≡C-CH3 group on the carbon two positions away from the ring C=C) treated with 1. Na, liquid NH3; 2. Br2 (excess); 3. alc. KOH.
Answer and worked solution

Official answer: 2

Step by step.

  1. Na in liquid NHX3\ce{NH3} reduces the alkyne to the trans alkene; the ring C=C is untouched.
  2. Excess BrX2\ce{Br2} adds across both C=C bonds, giving two vicinal dibromides.
  3. Alcoholic KOH removes two HBr from each dibromide. Products with no spsp carbon (no alkyne, no allene) are conjugated dienes:
    • the ring becomes a cyclohexa-1,3-diene, with the side chain on the sp³ carbon next to the diene;
    • the side chain becomes −CH=CH−CH=CHX2\ce{-CH=CH-CH=CH2}.

Count. The tetraene is 5-(buta-1,3-dien-1-yl)cyclohexa-1,3-diene. Its internal side-chain double bond can be E or Z, so there are 2 isomeric tetraenes. The ring carbon bearing the side chain is a stereocentre, but the product is formed racemic from racemic starting material.

Topics:AlkynesGeometrical isomerismElimination reactions
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