Propylbenzene propiophenone

This two-step sequence can synthesize many alkylbenzenes that are impossible to make by direct alkylation. For example, we saw earlier that n-propylbenzene cannot be made by Friedel-Crafts alkylation. Benzene reacts with n-propyl chloride and AICI3 to give isopropylbenzene, together with some diisopropylbenzene. In the acylation, however, benzene reacts with propanoyl chloride and A1C13 to give ethyl phenyl ketone (propiophenone), which is easily reduced to n-propylbenzene. 

The procedure for reducing mixed alkyl aryl ketones is very similar. For instance, propyl-benzene is obtained as follows Propiophenone (25 g) is warmed with amalgamated zinc (100 g) and a mixture of equal parts of water and concentrated hydrochloric acid. A lively reaction sets in after 5 min. Then heating is continued whilst concentrated acid is allowed to drop in during a further 4 h. On working up, 90% (20 g) of propylbenzene, b.p. 155-160°, is obtained. 

In the same way that an aromatic ring activates a neighboring (benzylic) C H toward oxidation, it also activates a benzylic carbonyl group toward reduction. Thus, an aryl alkyl ketone prepared by Friedel-Crafts acylation of an aromatic ring can be converted into an alkylbenzene by catalytic hydrogenation over a palladium catalyst. Propiophenone, for instance, is reduced to propylbenzene by catalytic hydrogenation. Since the net effect of Friedel-Crafts acylation followed by reduction is the preparation of a primary alkylbenzene, this two-step sequence of reactions makes it possible to circumvent the carbocation... 

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