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NITRATION AGENTS AND METHODS MORE RARELY USED 105

G. B. Bachman and his co-workers [91] investigated the action of the boron
trifluoride and nitrogen dioxide complex, formed by combining the two com-

pounds at 0°C:

BF 3 + N 2 O 4 -> [F 3 B<-NO 2 ]-NO 2 + (35)

Crystalline complex salt

The complex salt acted on benzene to give nitrobenzene (40% yield) and a small

quantity (1.5%) of m- dinitrobenzene at 0°C. At room temperature only m- di-

nitrobenzene in 7% yield was formed.

The action of this reagent on naphthalene is of interest. A mixture of dinitro-

naphthalene isomers is formed in 65% yield, the ratio of the 1,5- to l,8-isomer

being 2:1, the reverse of the ratio obtained by nitration with the ordinary nitrating

mixture.

NITRATION WITH NITROGEN DIOXIDE IN THE PRESENCE

OF ACTIVATED SILICA

McKee and Wilhelm [92] found that the highest yield of nitration of benzene

with nitrogen dioxide in the vapour phase was obtained at 310°C. In the presence of

activated silica the yield was higher than that obtained without a catalyst and could

amount to 83.6%. Nitrophenol was also formed in a yield rising with tempera-

ture, so that at 330°C it was the principal reaction product. The authors supposed

that the catalytic action of activated silica was probably a specific one and did

not consist only in binding the water formed during the nitration process.

PHOTONITRATION WITH NITROGEN DIOXIDE

Gorislavets [93] claimed that the nitration of benzene or nitrobenzene with

nitrogen dioxide readily occurred on irradiation with rays of certain definite wave-

lengths, which activate nitrogen dioxide.

Thus, benzene was nitrated quantitatively at the wavelengths 4000 - 7000 Å

at a temperature of 55-60°C. The highest yield of dinitrobenzene was obtained

at wavelengths 1800 - 2900 Å. The experiments of other workers did not confirm

his results (Hastings and Matsen [94]).

On the other hand Lippert and Vogel [95] assert that on irradiation with high

intensity ultra-violet light nitro compounds undergo dissociation into free radicals:

ArNO 2 -> Ar + .NO 2 (36)

On the basis of this statement it may be concluded that differences in the results

of various authors may be due to the difference in intensity of applied irradiation.

NITROGEN PENTOXIDE (NITRIC ANHYDRIDE)

As is well known, nitrogen pentoxide is a solid, crystalline, colourless product,

melting at 30°C. Nitrogen pentoxide was first used as a nitrating agent for the O-
nitration of cellulose (Hoitsema [96]; Berl and Klaye [97], and others (see Vol. II)).