Page 822 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

Table 9.8. Partial Rate Factors for Hydrogen Exchange 805
for Some Substituted Aromatic Compounds
SECTION 9.4
Substituent f o f m f p
Specific Electrophilic
CH a 3 330 7.2 313 Substitution Reactions
F b 0.136 - 1.70
Cl b 0.035 - 0.161
OPh c 6900 ∼0.1 31,000
Ph d 133 < 1 143
a. C. Eaborn and R. Taylor, J. Chem. Soc., 247 (1961).
b. C. Eaborn and R. Taylor, J. Chem. Soc., 2388 (1961).
c. R. Baker and C. Eaborn, J. Chem. Soc., 5077 (1961).
d. C. Eaborn and R. Taylor, J. Chem. Soc., 1012 (1961).

Among the many experimental results pertaining to hydrogen exchange, a most
79
important one is that general acid catalysis has been demonstrated, a finding that is in
accord with a rate-limiting proton transfer step. Since proton removal is partially rate
determining, hydrogen exchange exhibits an isotope effect. A series of experiments
using both deuterium and tritium labels arrived at k /k = 9 0 for the proton-loss step
H D
80
for 1,3,5-trimethoxybenzene. A substantial isotope effect has also been observed for
the exchange process with azulene. 81

9.4.4. Friedel-Crafts Alkylation and Related Reactions

The Friedel-Crafts reaction is a very important method for introducing alkyl
substituents on an aromatic ring by generation of a carbocation or related electrophilic
species. The usual method of generating these electrophiles involves reaction between
an alkyl halide and a Lewis acid. The most common Friedel-Crafts catalyst for prepar-
ative work is AlCl , but other Lewis acids such as SbF , TiCl , SnCl , and BF can also
3 5 4 4 3
promote reaction. Alternative routes to alkylating species include reaction of alcohols
or alkenes with strong acids.
There are relatively few kinetic data on the Friedel-Crafts reaction. Alkylation
of benzene or toluene with methyl bromide or ethyl bromide with gallium bromide
as the catalyst is first order in each reactant and in the catalyst. 82 With aluminum
bromide as the catalyst, the rate of reaction changes with time, apparently because of
heterogeneity of the reaction mixture. 83 The initial rate data fit the following kinetic
expression:

Rate = k EtBr benzene AlBr 2
3

79 A. J. Kresge and Y. Chiang, J. Am. Chem. Soc., 83, 2877 (1961); A. J. Kresge, S. Slae, and D. W. Taylor,
J. Am. Chem. Soc., 92, 6309 (1970).
80 A. J. Kresge and Y. Chiang, J. Am. Chem. Soc., 89, 4411 (1967).
81
L. C. Gruen and F. A. Long, J. Am. Chem. Soc., 89, 1287 (1967).
82 S. U. Choi and H. C. Brown, J. Am. Chem. Soc., 85, 2596 (1963).
83
B. J. Carter, W. D. Covey, and F. P. DeHaan, J. Am. Chem. Soc., 97, 4783 (1975); cf. S. U. Choi and
H. C. Brown, J. Am. Chem. Soc., 81, 3315 (1959); F. P. DeHaan and H. C. Brown, J. Am. Chem. Soc.,
91, 4844 (1969); H. Jungk, C. R. Smoot, and H. C. Brown, J. Am. Chem. Soc., 78, 2185 (1956).