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Which of the structures given below do NOT follow isoprene rule ?

  • A.A cyclohexanol ring carrying a methyl group, a hydroxyl group and an isopropyl group, drawn with wedge and hash bonds
  • B.Camphor, a bicyclic ketone with three methyl groups
  • C.An open-chain carboxylic acid with methyl branches, two trisubstituted double bonds and an isopropylidene end
  • D.An open-chain carboxylic acid with a methyl on the carbon bearing COOH, two double bonds and an isopropylidene end

Answer: D

The structure that does not follow the isoprene rule is D.

The isoprene rule says a terpenoid carbon skeleton is built from 2-methylbutane (C5) units joined head to tail. Written from the carboxyl carbon, D is HOOC-C(CH3)=CH-CH2-CH2-CH=C(CH3)2: the first methyl sits on C-2, directly beside the COOH. In a head-to-tail chain such as geranic acid the methyl branches fall on C-3 and C-7, so D has its first branch one carbon too close to the carboxyl end and the units do not link head to tail.

A is menthol and B is camphor, both monoterpenes. C is a chain with methyl branches at the spacing the rule requires (as in farnesoic acid), so A, B and C all follow the rule.

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