Discussion
The sequence of three steps involved in the following conversion is
(Conversion: benzene into 1-isopropyl-3-propylbenzene, a benzene ring carrying an isopropyl group and an n-propyl group meta to each other.)
Answer: B
The sequence is (i) Friedel-Crafts acylation; (ii) Friedel-Crafts alkylation; (iii) Reduction.
The two groups in the product are meta to each other, so the first group introduced must direct the second to the meta position. An acyl group does that; an alkyl group directs ortho/para.
- Friedel-Crafts acylation with propanoyl chloride gives propiophenone; the acyl group also avoids the rearrangement that direct n-propylation would suffer.
- Friedel-Crafts isopropylation then places the isopropyl group meta to the acyl group.
- Clemmensen or Wolff-Kishner reduction turns C=O into CH2, giving the n-propyl group.
The key rests on this directing-group logic; in the laboratory, alkylating an acyl-deactivated ring is sluggish. Reducing before the second step (option C), or starting with alkylation (A, D), gives an ortho/para product.
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