urbaf2

NITRATION AND NITRATING AGENTS

Ba<Pb<NH 4 <Na<K<Ag

47

With KNO 3 and NaNO 3 the yield of nitrobenzene was 70 and 73% respec-

tively, calculated on benzene.

AlCl 3 and BF 3 were shown to be the most efficient catalysts, FeCl 3 was less

efficient and the efficiency of SiCl 4 was the lowest.

The best yield of this nitration reaction is obtained at room temperature and

in the presence of an excess of the aromatic hydrocarbon. An interesting feature

of this nitration is that it is not accompanied by any oxidation processes. The method

may find practical applications.

Topchiev has suggested the following hypothesis concerning the mechanism

of nitration with nitrates in the presence of AlCl 3 or BF 3. Benzene is activated by

aluminium chloride to give an addition product, according to the equation:

C 6 H 6 + AlCl 3 -> (C 6 H 5 AlCl 3 )H (53a)

The product reacts with sodium nitrate, yielding another addition product

which decomposes to give nitro compound:

-> C 6 H 5 NO 2 + NaOH + AlCl 3 (53b)

CONCLUSIONS

It appears that concentrated nitric acid follows a heterolytic dissociation mainly

according to the equation:

NO 2 OH <-> NO 2 + + OH-

The presence of acids stronger than HNO 3 accelerates the heterolysis of nitric

acid into NO 2 + and OH-. It is for this reason that mixed acid is so effective:

HNO 3 + H 2 SO 4 <-> H 2 NO 3 + + HSO 4 -

H 2 NO 3 + + H 2 SO 4 <-> NO 2 + + H 3 O+ + HSO 4 -

In neutral organic solvents (which do not react with the acids) only a second

molecule of nitric acid can supply the necessary proton:

2HNO 3 <-> H 2 NO 3 + + NO 3 -

H 2 NO 3 + <-> NO 2 + + H 2 O

It should, however, be pointed out that most of the evidence presented by a num-
ber of authors, and chiefly by Ingold and his school in support of the nitronium

ion mechanism of nitration, was obtained with homogeneous solutions under con-

ditions that could not be compared directly with the conditions generally applied

in industrial nitrations.