Page 823 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

84
806 The reaction rates of toluene and benzene with i-propyl chloride or t-butyl chloride 85
in nitromethane can be fit to a third-order rate law.
CHAPTER 9
Aromatic Substitution Rate = k AlCl i-PrCl ArH
3
Rates that are independent of aromatic substrate concentration have been found
for reaction of benzyl chloride catalyzed by TiCl or SbF in nitromethane. 86 This
5
4
can be interpreted as resulting from rate-determining formation of the electrophile,
presumably a benzyl ion pair. The reaction of benzyl chloride and toluene shows
a second-order dependence on the titanium chloride concentration under conditions
where there is a large excess of hydrocarbon. 87 This is attributed to reaction through
a 1:2 benzyl chloride-TiCl complex, with the second TiCl molecule assisting in the
4
4
ionization reaction:
Rate = k PhCH Cl TiCl 2
2
4
All these kinetic results can be accommodated by a general mechanistic scheme
that incorporates the following fundamental components: (1) complexation of the
alkylating agent and the Lewis acid; in some systems, there may be an ionization of
the complex to yield a discrete carbocation; (2) electrophilic attack on the aromatic
reactant to form the cyclohexadienylium ion intermediate; and (3) deprotonation. The
formation of carbocations accounts for the fact that rearrangement of the alkyl group
is observed frequently during Friedel-Crafts alkylation.

–
+
(1) R X + MY n R X -M Y n
+
–
(2) R X -M Y n R + + [MY X] –
n
+
–
R X -M Y n R
(3) Z + or Z + H
R +
R
(4) Z + Z R + H +
H
Absolute rate data for the Friedel-Craft reactions are difficult to obtain. The
reaction is very sensitive to the effects of moisture and heterogeneity. For this reason,
most of the structure-reactivity trends have been developed using competitive methods,
rather than by direct measurements. Relative rates are established by allowing the
electrophile to compete for an excess of the two reactants. The product ratio establishes

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85
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87
F. P. DeHaan, W. D. Covey, R. L. Ezelle, J. E. Margetan, S. A. Pace, M. J. Sollenberger, and D. S. Wolfe,
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