Organic Chemistry of Explosives

158 Synthetic Routes to AromaticC-Nitro Compounds

4.8.1 Displacement of halide

4.8.1.1 Substrate reactivity

X

(NO 2 )n

X = I, Br, Cl or F

Nu

Nu

(NO 2 )n + X

Figure 4.29

Many nucleophiles have been used for the displacement of halide anion from nitro-substituted

halobenzenes, including: aliphatic and aromatic amines, ammonia, hydrazine, hydroxide,

alkoxides, phenoxides, halides, azide, sulfides and many more. These reactions proceed via an

addition–elimination mechanism through an intermediate Meisenheimer complex. The rate-

determining step in these reactions is the formation of the Meisenheimer complex, and hence,

any process which lowers the energy of this complex increases the reaction rate. Most organic

textbooks state that the rate of nucleophilic aromatic substitution is directly related to the polar-

ization ability of the halogen substituent, leading to a reactivity order of F>Cl>Br>I. This is

usually the case when reactions are conducted in protic solvents with first row nucleophiles.^201

For example, it is found that 2,4-dinitrofluorobenzene is several thousand times more reac-

tive towards simple oxygen nucleophiles compared to the corresponding chloro- and bromo-

derivatives, with 2,4-dinitroiodobenzene showing the least reactivity.^202 −^204 However, such

rules tend to cover over important and complex issues – there are many examples where the

lighter halogens are the least reactive towards some nucleophiles.^205 Although the theory be-

hind such a rule is very logical, in practice, there are other factors to consider which might

completely alter the relative rates of such reactions. It is found that reaction rate is heavily

dependent on the nature of the substrate and the nucleophile, in addition to the reaction tem-

perature and solvent.^205 de Boer and Dirkx^205 have given a very comprehensive review of the

activating effects of the nitro group in aromatic substitution.

4.8.1.2 The chemistry of picryl chloride

2,4,6-Trinitrochlorobenzene (picryl chloride) (87) can be prepared from the nitration of 2,4-

dinitrochlorobenzene with nitronium tetrafluoroborate^101 or mixed acid composed of fuming

nitric acid and oleum.^50 ,^51 Picryl chloride is also synthesized from the reaction of phosphorous

oxychloride with the pyridinium salt of picric acid.^206

The presence of three nitro groups on the aromatic ring of picryl chloride makes the chloro

group extremely reactive towards nucleophiles. Picryl chloride (87) is hydrolyzed to picric

acid (4) in the presence of hot water or aqueous sodium hydroxide.^207 Aminolysis of picryl

chloride in the presence of primary and secondary amines is complete in minutes at room

temperature.^208 Picryl chloride is therefore a very useful starting material for the synthesis of

a range of other picryl derivatives. The reaction of picryl chloride (87) with ammonia can be

used to synthesize 2,4,6-trinitroaniline (53) (picramide).^209 Treatment of picryl chloride with

alcohols under reflux forms picric acid and the alkyl chloride of the corresponding alcohol,

whereas the same reaction in the presence of alkali metal hydroxides, or the alkoxide anion of