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AROMATIC NITRO COMPOUNDS 199

Plazek [28,28b] and Plazek and Talik [28a] stated recently that the reactivity

of halogens in nitro derivatives of pyridine is much higher than in the similar benzene

derivatives. Thus, at 20°C where only 0.5% of chloro-2,4-dinitrobenzene was sub-

jected to nucleophilic displacement of chlorine by the amino group, the figure was

98.3% for 2-chloro-3,5-dinitropyridine.

In water, after 5 hr of boiling, the following figures for percentage reacted

were recorded :

ca. 1.0%, of chloro-2,4-dinitrobenzene

26.2% of chloro-2,4,6-trinitrobenzene

98.4% of 2-chloro-3,5-dinitropyridine

99.7% of 2-bromo-3,5-dinitropyridine

Bunnet and Randall [29] suggest a two-stage mechanism involving a nucleo-

phitic reaction of the formation of a metastable intermediate complex according

to the diagrammatic presentation:

(17)

Bases (e.g. alcoholic potassium acetate) were found to catalyse the reaction when

X= F, but they do not catalyse it when X= Cl.

Bunnet and Randall suggest the following equation for the rate of reaction K:

K =

K 1 K 2 + K 1 K 3 (B)

K-1 + K 2 + K 3 (B)

(See also Knowles, Norman and Prosser [159]).

S. D. Ross [30] studied the nucleophilic displacement reaction in aromatic

systems by examining the rate of reaction of chloro-2,4-dinitrobenzene with n-butyl-