A Guidebook to Mechanism in Organic Chemistry

Nitration

is thus explained for it contains but little ®N0 2 ; the small amount
that is present is obtained by the two-stage process

fast m

HO—NOa + HN0 3 NO 30 + HO—N0 2

H

HO-LN0 3 +HN0 3 H 3 Offi + NOs©+ ®N0 2

H

in which nitric acid is first converted rapidly into its conjugate acid

and that then more slowly into nitronium ion.

The kinetics of nitration are not easy to follow under normal

preparative conditions for the solubility of, e.g., benzene in nitrating

mixture is sufficiently low for the rate of nitration to be governed by

the rate at which the immiscible hydrocarbon dissolves in the

mixture. This apart, the rate-determining step is, however, almost

certainly the initial attack by ®NOa

rather than the subsequent removal of proton by HS04Q or other

anion. That the latter step cannot be rate-determining has been con­

firmed by studying the nitration of nitrobenzene in which the hydro­

gen atoms have been replaced by deuterium. Nitrobenzene is chosen

rather than benzene itself as the former is more soluble in nitrating

mixture so that the overall rate of the reaction is no longer controlled

by the rate at which it dissolves. Studies of the relative rates of fission

of C—H and C—D in general would lead us to expect an approximately

ten-fold drop in nitration rate on going C 6 H 5 -N0 2 ->-C 6 D 5 -N0 2. In

fact there is no detectable difference in rate, indicating that the fission of

the C—H or C—D bond is not involved in the rate-determining stage

of nitration. The species (V) is thought to enjoy an actual existence—

albeit an extreirlely transient one—in solution and is thus looked upon

as a real intermediate, though a metastable orie>(c/ p. 33), rather than

merely as a transition state. An added significance of the existence of