The Alkylation of Benzene by Acylation-Reduction

A Friedel-Crafts alkylation cannot produce a good yield of an alkylbenzene containing a straight-chain alkyl group because the incipient primary carbocation will rearrange to a more stable carbocation. 

Besides avoiding carbocation rearrangements, another advantage to preparing alkyl-substituted benzenes by acylation-reduction rather than by direct alkylation is that a large excess of benzene does not have to be used (Section 19.8). Unlike alkyl-substituted benzenes, which are more reactive than benzene, acyl-substituted benzenes are less reactive than benzene, so they will not undergo a second Friedel-Crafts reaction (Section 19.14). 

We have seen that hydrazine reacts with a ketone to form a hydrazone (Section 17.10). Hydroxide ion and heat differentiate the Wolff-Kishner reduction from ordinary hydra-zone formation. 

The ketone reacts with hydrazine to form a hydrazone following the mechanism for imine formation on page 811. 

Hydroxide ion removes a proton from the NH2 group of the hydrozone. The reaction requires heat because this proton is only weakly acidic. 

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