Page 393 - Mechanism and Theory in Organic Chemistry

380 ADDITION AND ELIMINATION REACTIONS

normally carried out in acetic acid. Under these conditions the kinetics are
second-order in bromine; the second molecule of Br, polarizes the first, and the
overall reaction is that of Equation 7.57.
ArH + 2Br2 d + H+ + Bra- (7.57)
ArBr
The addition of I, to the reaction mixture increases the rate, because 1,Br- is
0 0
II II
formed more readily than Br3-. HOBr, CH3COBr and CF3COBr can also all be
used as sources of electrophilic bromine, the last being particularly reactive.134
The attacking species is usually the entire molecule, but Br+ may be formed at
times.135 Lewis acids such as AlCl, catalyze bromination by forming Br+ as in
Equation 7.58.
+
AIC1, + Br, + k~1,~r Br+ (7.58)
Chlorination with molecular chlorine also occurs readily and is usually
first-order in Cl,. Apparently chlorine is electronegative enough so that an addi-
tional C1, is not required to polarize the Cl-C1 bond at the transition state.
Stronger Lewis acids such as FeC1, do, however, catalyze the reaction by assist-
ing in bond polarization. HOCl and CH3COC1 also act as chlorinating agents,
but free C1+ is never formed. The reactive species from HOCl are C1,O (formed
+
by dehydration of two molecules of acid) and H20C1, both of which deliver C1+
to the aromatic .rr system.136
Direct fluorination of aromatic rings is so exothermic that a tarry mixture of
products is obtained. Reaction of benzene with the xenon fluorides, XeF, or
XeF,, does give fluorobenzene, but the mechanism is probably free radical rather
than p01ar.l~~
Nitration of an aromatic ring138 to give ArNO, is most often carried out
with nitric acid in sulfuric acid; however, concentrated nitric acid, aqueous
nitric acid, and nitric acid in polar organic solvents are also commonly used, as is
preliminary nitrosation followed by oxidation of the aromatic nitroso compound
(ArNO). Alkyl nitrates (RONO,) are also nitrating agents in the presence of
some Bronsted and Lewis acids.139
When the reagent used is nitric acid, the attacking species is usually the
nitronium ion, NO,+. That this ion exists has been abundantly proven. For
example, cryoscopic measurements show that each molecule of nitric acid dis-
solved in sulfuric acid gives rise to four ions. This result is best explained by the
equilibria shown in Equations 7.59-7.61.140 Raman spectra also show that in

134 See note 133.
13& See, however, H. M. Gilow and J. H. Ridd, J. Chem. Soc., Perkin Trans. 11, 1321 (1973).
136 C. G. Swain and D. R. Crist, J. Amer. Chem. Soc., 94, 3195 (1972).
13' (a) M. J. Shaw, H. H. Hyman, and R. Filler, J. Amer. Chem. SOC., 91, 1563 (1969); (b) T. C.
Shieh, E. D. Feit, C. L. Chernick, and N. C. Yang, J. Org. Chenr., 35, 4020 (1970).
13~ For a review, see: J. G. Hoggett, R. B. Moodie, J. R. Penton, and K. Schofield, Nitration and
Aromatic Reactiuity, Cambridge University Press, London, 1971.
13~ G. A. Olah and H. C. Lin, J. Amer. Chem. SOC., 96, 2892 (1974) and references therein.
140 R. J. Gillespie, J. Graham, E. D. Hughes, C. K. lngold, and E. R. A. Peeling, Nature, 158, 480
( 1946).