Synthetic Applications of Friedel-Crafts Acylations The Clemmensen Reduction

9 Synthetic Applications of Friedel-Crafts Acylations The Clemmensen Reduction 

Friedel-Crafts acylation followed by ketone reduction is the synthetic equivalent of Friedel-Crafts alkylation. 

The acylium ion, because it is stabilized by resonance, is more stable than most other car-bocalions. Thus, there is no driving force for a rearrangement. Because rearrangements do not occur, Friedel-Crafts acylations followed by reduction of the carbonyl group to a CH2 group often give us much better routes to unbranched alkylbenzenes than do Friedel-Crafts alkylations. 

As an example, let us consider the problem of synthesizing propylbenzene. If we attempt this synthesis through a Friedel-Crafts alkylation, a rearrangement occurs and the major product is isopropylbenzene (see also Review Problem 15.4)  

Isopropylbenzene Propylbenzene (major product) (minor product) 

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