Page 394 - Mechanism and Theory in Organic Chemistry

Electrophilic Aromatic Substitution 381
highly acidic media nitric acid is completely converted to N02+.141 In fact,
nitronium salts such as N0,+BF4- have actually been isolated and can also be
used for aromatic nitrations.142

That the nitronium ion not only exists but also can be the reactive species
has been shown. For example, the rate of nitration of toluene (and of other aro-
matics) in solutions of nitric acid in nitromethane were independent of the
concentration of t01uene.l~~ Thus the slow step must be the formation of the reac-
tive speciesPrior to attack on the toluene ring. This rules out HNO, as the nitrating
agent. That protonated nitric acid, formed as shown in Equation 7.62, is not the
reactive species follows from the fact that the rate does not become first-order in

toluene when NO3- is added to the reaction. A rate first-order in toluene would
+
be expected if H20-NO, were the nitrating agent, because the equilibrium in
Equation 7.62 would be driven to the left and toluene would have to compete
+
with NO3- for H,O-NO, (see the discussion of the partition effect, p. 385).144
Protonated nitric acid has also been ruled out as the reactive species in aqueous
sulfuric acid. At various acid strengths the rate of nitration correlates with the
acidity function HE, which is defined by equilibria of the type shown in Equation
7.63, rather than with the acidity function H,, defined by equilibria of the type

AOH + SH+ A+ + H,O + S (7.63)
shown in Equation 7.64 (see Section 3.3).145 The fact that nitronium salts are

AOH + SH+ A-OH + S
I
H
excellent nitrating agents is direct proof of the ability of NO2+ to substitute an
aromatic ring.146
Nitronium ion is not, however, invariably the reactive species. For example,
nitric acid in acetic anhydride shows a greater than usual selectivity between
toluene and benzene (see p. 394), indicating that another, less reactive, nitrating
agent is formed.147
Sulfonation of an aromatic substrate to produce ArS03H is usually brought
about by reaction of the aromatic with concentrated sulfuric acid or with sulfur

141 C. K. Ingold and D. J. Millen, J. Chem. Sac., 2612 (1950) and references therein.
G. A. Olah, S. Kuhn, and A. Mlinko, J. Chem. Sac., 4257 (1956).
143 G. Benford and C. K. Ingold, J. Chem. Sac., 929 (1938).
144 E. D. Hughes, C. K. Ingold, and R. I. Reed, J. Chem. Sac., 2400 (1950).
'45 F. H. Westheimer and M. S. Kharasch, J. Amer. Chem. Sac., 68, 1871 (1946).
'46 See note 142.
14'S. R. Hartshorn, R. B. Moodie, and K. Schofield, J. Chem. Sac., B, 1256 (1971).