Organic Chemistry of Explosives

198 Synthetic Routes toN-Nitro

have been reported and these are discussed in Chapter 6 together with other energetic bicyclic

and caged nitramines.

If nitration under acidic conditions could only be used for the nitration of the weakest of

amine bases its use for the synthesis of secondary nitramines would be severely limited. An

important discovery by Wright and co-workers^3 ,^4 ,^22 found that the nitrations of the more basic

amines are strongly catalyzed by chloride ion. This is explained by the fact that chloride ion, in

the form of anhydrous zinc chloride, the hydrochloride salt of the amine, or dissolved gaseous

hydrogen chloride, is a source of electropositive chlorine under the oxidizing conditions of

nitration and this can react with the free amine to form an intermediate chloramine. The

corresponding chloramines are readily nitrated with the loss of electropositive chlorine and

the formation of the secondary nitramine in a catalytic cycle (Equations 5.2, 5.3 and 5.4).^22

The mechanism of this reaction is proposed^22 to involve chlorine acetate as the source of

electropositive chlorine but other species may play a role. The success of the reaction appears

to be due to the chloramines being weaker bases than the parent amines.

2 HCl + 2 HNO 3 + 3 Ac 2 O

AcOCl + R 2 NH

R 2 NCl + HNO 3 + Ac 2 O

R 2 NCl + AcOH (Eq. 5.3)

R 2 NNO 2 + AcOCl + AcOH (Eq. 5.4)

2 R 2 NH + N 2 O 3 + Ac 2 O2 R 2 NNO + 2 AcOH (Eq. 5.5)

2 AcOCl + N 2 O 3 + 4 AcOH (Eq. 5.2)

Figure 5.12 Proposed mechanism for chloride-catalyzed nitration^22 ,^24

Wright illustrated the effectiveness of chloride-catalyzed nitration for a number of amines

of different basicity. Wright showed that weakly basic amines like iminodiacetonitrile and

its dimethyl and tetramethyl derivatives are all nitrated in high yield with nitric acid–acetic

anhydride mixtures in the absence of chloride ion.^3 In contrast, the slightly more basic 3,3′-

iminodipropionitrile is not appreciably nitrated with acetic anhydride–nitric acid, but the in-

clusion of a catalytic amount of the hydrochloride salt of the amine base generates the corre-

sponding nitramine in 71 % yield.^3

Wright found it more suitable to use the nitrate salts of the more basic amines, like in the

case of morpholine, which ignites at ambient temperature in the presence of nitric acid–acetic

anhydride mixtures. Morpholine nitrate is not nitrated with acetic anhydride–nitric acid in the

absence of chloride ion at room temperature, but the addition of 4 mole % of zinc chloride

generates a 65 % yield ofN-nitromorpholine, and this yield rises to 93 % if the hydrochloride

salt of morpholine is directly nitrated.^3 While morpholine nitrate is unaffected by treatment

with acetic anhydride–nitric acid at room temperature, the same reaction in the presence of

ammonium nitrate at 65◦C is reported to yieldN-nitromorpholine in 48 % yield.^23

Wright found that as amine basicity increases a larger amount of chloride ion is needed to

effectively catalyze the nitration. The nitrate salt of diethylamine needs the addition of 4 mole %

of zinc chloride to generate a 60 % yield of the corresponding nitramine.^3 In the extreme case

of diisopropylamine, where the isopropyl groups strongly donate electron density to nitrogen

and make it a relatively strong base, a full equivalent of chloride ion is needed to attain an