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Identify the most probable product in the given reaction

Isopropylbenzene (cumene) treated with O2 and benzoyl peroxide to give the product

  • A.4-Isopropylphenyl hydroperoxide, OOH on the ring para to the isopropyl group
  • B.2-Isopropylphenyl hydroperoxide, OOH on the ring ortho to the isopropyl group
  • C.2-Phenylpropan-2-yl hydroperoxide, OOH on the benzylic carbon of cumene
  • D.2-Phenylpropyl hydroperoxide, OOH on a terminal CH2 carbon

Answer: C

The product is cumene hydroperoxide, PhC(CH3)2OOH, the structure with OOH on the benzylic carbon (C).

Benzoyl peroxide starts a radical chain. The radical abstracts the weakest C-H bond in isopropylbenzene, the tertiary benzylic C-H, giving a tertiary benzylic radical stabilised by the ring. That radical adds O2 to form a peroxyl radical, which takes a hydrogen from another cumene molecule to give the hydroperoxide and carry the chain on. This is the first step of the cumene process for phenol and acetone.

A and B put OOH on the aromatic ring, but aryl C-H bonds are far too strong for radical abstraction. D puts it on a primary CH2, which would need abstraction of a primary methyl hydrogen instead of the much weaker benzylic one.

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