Besides avoiding carbocation rearrangements, another advantage of 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 15.14). Unlike alkyl-substituted
benzenes, which are more reactive than benzene (Section 16.3), acyl-substituted benzenes
are less reactive than benzene, so they will not undergo additional Friedel–Crafts reactions.
There are more general methods available to reduce a ketone carbonyl group to a
methylene group—methods that reduce all ketone carbonyl groups, not just those that
are adjacent to benzene rings. Two of the most effective are the Clemmensen reduc-
tion and the Wolff–Kishner reduction. The Clemmensen reductionuses an acidic
solution of zinc dissolved in mercury as the reducing reagent. The Wolff–Kishner
reductionemploys hydrazine under basic conditions. The mechanism of
the Wolff–Kishner reduction is shown in Section 18.6.At this point, you may wonder why it is necessary to have more than one way to
carry out the same reaction. Alternative methods are useful when there is another func-
tional group in the molecule that could react with the reagents you are using to carry
out the desired reaction. For example, heating the following compound with HCl (as
required by the Clemmensen reduction) would cause the alcohol to undergo substitu-
tion (Section 11.1). Under the basic conditions of the Wolff–Kishner reduction, how-
ever, the alcohol group would remain unchanged.Alkylbenzenes with straight-chain alkyl groups can also be prepared by means of
the coupling reactions you saw in Section 12.12. One of the alkyl groups of a Gilman
reagent can replace the halogen of an aryl halide.The Stille reactioncouples an aryl halide with a stannane.CCH 3OOHOHZn(Hg), HCl, ∆CH 2 CH 3H 2 NNH 2 , HO−, ∆CH 2 CH 3ClClemmensen
reductionWolff–Kishner
reductionCCH 2 CH 3O Zn(Hg), HCl, ∆CH 2 CH 2 CH 3H 2 NNH 2 , HO−, ∆CH 2 CH 2 CH 3(H 2 NNH 2 )616 CHAPTER 15 Aromaticity • Reactions of Benzene
E. C. Clemmensen (1876–1941)
was born in Denmark and received
a Ph.D. from the University of
Copenhagen. He was a scientist
at Clemmensen Corp. in Newark,
New York.
Ludwig Wolff (1857–1919)was
born in Germany. He received a
Ph.D. from the University of
Strasbourg. He was a professor at the
University of Jena in Germany.
N. M. Kishner (1867–1935)was
born in Moscow. He received a Ph.D.
from the University of Moscow under
the direction of Markovnikov. He was
a professor at the University of
Tomsk and later at the University of
Moscow.
tetrapropylstannanePd(PPh 3 ) 4
THF+Br CH 2 CH 2 CH 3
(CH 3 CH 2 CH 2 ) 4 Sn + (CH 3 CH 2 CH 2 ) 3 SnBr+Br CH 2 CH 3
(CH 3 CH 2 ) 2 CuLi ++CH 3 CH 2 Cu LiBr