Organic Chemistry

Section 16.2 Reactions of Substituents on Benzene 627

Remember that halo-substituted alkyl groups can also undergo E2 and E1 reactions
(Section 11.8). Notice that a bulky base is used to encourage elimination
over substitution.

Substituents with double and triple bonds can undergo catalytic hydrogenation
(Section 4.11). Addition of hydrogen to a double or triple bond is an example of a
reduction reaction (Section 4.8). When an organic compound is reduced, either the
number of bonds in the compound increases or the number of
or (where X denotes a halogen atom) bonds decreases (Section 20.0).

Recall that benzene is an unusually stable compound (Section 7.11). It, therefore, can
be reduced only at high temperature and pressure.

An alkyl group bonded to a benzene ring can be oxidized to a carboxyl group. When
an organic compound is oxidized, either the number of or (where
X denotes a halogen atom) bonds increases or the number of bonds decreases
(Section 20.0). Commonly used oxidizing agents are potassium permanganate
or acidic solutions of sodium dichromate Because the benzene ring is
so stable, it will not be oxidized—only the alkyl group is oxidized.

Regardless of the length of the alkyl substituent, it will be oxidized to a COOH group,
provided that a hydrogen is bonded to the benzylic carbon.

CH 3

toluene

1. KMnO 4 ,


2. H+

∆

COOH

benzoic acid

(H+,Na 2 Cr 2 O 7 ).

(KMnO 4 )

C¬H

C¬O,C¬N, C¬X

Ni

• 3 H (^2175) °C, 180 atm
  benzene cyclohexane
  Ni
  C
  O
  CH 2
  OH
  H


• H 2
  benzaldehyde benzyl alcohol
  Pt
  CN


• 2H 2
  CH 2 NH 2
  benzonitrile benzylamine
  Pt
  CH CH 2


• H 2
  CH 2 CH 3
  styrene ethylbenzene
  C¬X
  C¬H C¬O,C¬N,
  CHCH 3
  Br
  1-bromo-1-phenylethane
  CH CH 2
  styrene
  tert-BuO−
  (tert-BuO-)
  3-D Molecules:
  Benzyl bromide;
  Styrene;
  Benzonitrile;
  Benzaldehyde