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SYNTHETIC APPLICATIONS OF FRIEDEL-CRAFTS ACYLATIONS: THE CLEMMENSEN AND WOLFF-KISHNER REDUCTIONS Rearrangements of the carbon chain do not occur in Friedel-Crafts acylations. The acylium ion, because it is stabilized by resonance, is more stable than most other carbocations. 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 CH2CH2 group often give much better routes to unbranched alkylbenzenes than do FriedelCrafts alkylations. The carbonyl group of an aryl ketone can be reduced to a CH 22 group. (FIGURE CANNOT COPY) As an example, let us consider the problem of synthesizing propylbenzenc. If we attempt this synthesis through a Friedel-Crafts alkylation, a rearrangement occurs and the major product is isopropylbenzene (see also Practice Problem 15.4 ): (FIGURE CANNOT COPY) Isopropylbenzene (major product) Propylbenzene (minor product) By contrast, the Fricdel-Crafts acylation of benzene with propanoyl chloride produces a ketone with an unrearranged carbon chain in excellent yield: (FIGURE CANNOT COPY) Propanoyl chloride Ethyl phenyl ketone (90%)(90%) This ketone can then be reduced to propylbenzenc by scveral methods, including the Clemmensen reduction (Sect. 15.7A15.7A ) and the Wolff-Kishner reduction (Sect. 15.7B15.7B ). A The Clemmensen Reduction One general method for reducing a ketone to a methylene group-called the Clemmensen reduction- consists of refluxing the ketone with hydrochloric acid containing amalgamated zinc. [Caution: As we shall discuss later (Scction 20.4B20.4B ), zinc and hydrochloric acid will also reduce nitro groups to amino groups.] A Clemmensen reduction (FIGURE CANNOT COPY) Ethyl phenyl ketone Propylbenzene (80%)(80%) In general, (FIGURE CANNOT COPY) B The Wolff-Kishner Reduction Another method for reducing a ketone to a methylene group is the Wolff-Kishner reduction, which involves heating the ketone with hydrazine and base.
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