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NITRATION AND NITRATING AGENTS 45

H 2 NO 3 + + (CH 3 CO) 2 O <-> NO 2 + + 2CH 3 COOH (b)

(c)

Such a mechanism accounts for the first-order dependence of the rate on re-

action the nitric acid concentration in presence of sulphuric acid.

A higher-order dependence on nitric acid concentration in the absence of added

strong acid (H 2 SO 4 ) was also observed. This, according to Paul, is a consequence

of the dual role assumed by nitric acid itself providing the proton acquired in step

(a). The nitric acid-acetic anhydride reaction is not a simple one, and requires

more study.
Nitration with mixtures of metal nitrates and acetic anhydride (or acid) was
studied by Menke, and later by Spiegel and Haymann [112] and Bacharach [113].
It has been shown that only the nitrates which readily hydrolyse, such as ferric,
nickel, cobalt, aluminium nitrates, have nitrating properties.

.

Solutions of nitric acid in organic solvents

Nitric acid in organic solvents does not produce the 1050 and 1400 cm

-1

lines.

On examination of absorption spectra in the infra-red, the conclusion has been

drawn that nitric acid in chloroform or carbon tetrachloride solutions is less as-

sociated than when in a sulphuric acid solution (Dalmon [8,9,49]).

It follows from these investigations by Dalmon, that in an ether solution the

nitric acid molecule probably combines with the ether molecule by means of a

hydrogen bond:

Calorimetric measurements by Hantzsch and Karve [22] have shown that

the heat of dilution of nitric acid with ethyl ether is higher than that with water.

Hantzsch therefore suggested that nitric acid forms a compound with ether (prob-

ably through the hydrogen bond) as above. Dioxane also forms a similar com-

pound with nitric acid.

Measurements of the vapour pressures of solutions of nitric acid in ether carried

out by Dalmon and his co-workers confirmed the existence of the compound.

It was shown that the vapour pressure of ether, which at 0°C was 185 mm Hg,

decreased on addition of nitric acid and attained 1 mm Hg when equimolar pro-

portions of the two components were present in the solution.

Ingold, Hughes and Reed [39] have studied the kinetics of nitration with nitric

acid in the presence of nitromethane and found the reaction to be of zero order

in the case of readily nitrated aromatic compound, and of first order with compounds