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198 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

A halogen atom in the ortho or para position to the nitro group can readily

undergo nucleophilic displacement. The higher the polarization ability of a halo-

gen, the more readily it enters into substitution reactions. Therefore the common

rule that the atoms of the lighter halogens are more reactive is not followed here.

For example, the reaction of halogeno-2,4-dinitrobenzene with N-alkylaniline in

nitrobenzene solution runs with the greatest rapidity in the case of bromine, and

with the lowest in the case of fluorine, i.e. the reactivity varies according to the

order: Br>Cl>F:

(16)

This was formerly considered to be one of the most typical examples of nucleo-

philic displacement.

However, several exceptions to this rule have recently been observed.

According to Beckwith, Miller and Leahy [22], the rates of the nucleophilic

displacement of halogeno-2-dinitrobenzenes with sodium methoxide or sodium

p- nitrophenate for the F, Cl, Br and I derivatives can be expressed by the figures

2880, 1, 0.690, 0.154 respectively.

However, this sequence may be altered as a result of changes in the nucleo-

philic agent and the solvent. Thus Hammond and Parks [23] noticed the rate of

reaction of halogeno-2,4-dinitrobenzenes with aniline in ethanol to be the highest

with fluorine and the lowest with chlorine : F> Br> Cl. When N-methylaniline in

nitrobenzene solution was used the sequence was Br>Cl>F - as mentioned

above (Fierens and Halleux [24]).

Temperature can also influence the rate of this nucleophilic displacement and

alter the rate of reaction of different halogens. Brieux and Deulofeu [25] made

the important observation that the temperature change may. affect the rate of re-

action of various halogens differently. Thus the halogeno-2,4-dinitrobenzenes

react with piperidine at the rates which can be arranged:

and

as Br>Cl>I at 75°C

as Br>I>Cl at 100°C

This would explain the discrepancy between the data of different authors. (Bre-

win and Turner [26] and Sandin and Liskear [27] examined the rate of reaction

with piperidine in benzene. The temperature of a boiling solution in benzene is

in the region where a change in the order of reactivities may take place.)