Organic Chemistry of Explosives

Direct nitration of amines 195

5.3 Direct nitration of amines

5.3.1 Nitration under acidic conditions

R NHNO 2

H+

RNN

OH

O

N 2 O +ROH (Eq. 5.1)

Figure 5.6

The direct nitration of a primary amine to a nitramine with nitric acid or mixtures containing

nitric acid is not possible due to the instability of the tautomeric isonitramine in strongly acidic

solution (Equation 5.1). Secondary amines are far more stable under strongly acidic conditions

and some of these can undergo electrophilic nitration with nitric acid in a dehydrating medium

like acetic anhydride.

Experimentally, it is found that only weakly basic amines can be nitrated under highly

acidic conditions.^2 ,^3 Strong amine bases are largely protonated under such conditions with

the positive charge on nitrogen repelling electrophilic nitrating species, whereas weaker bases

have a higher proportion of free amine base present under the same conditions. It follows

that very weakly basic amines or those with a poor affinity for protons are nitrated more

readily than stronger bases.^2 ,^3 This is reflected in the fact that treatment of iminodiacetonitrile,^3

piperidine,^4 ,^5 dimethylamine,^4 ,^5 and diisopropylamine^3 with a mixture of nitric acid in acetic

anhydride generates the corresponding secondary nitramines in 93 %, 22 %, 6 % and negligible

yields, respectively. Electron-withdrawing groups on the carbonαto the amino group reduce

proton affinity and help nitration. Accordingly, amines of the type RCH 2 NHCH 2 R, where

R=CN, COOH or CONH 2 , are readily nitrated with nitric acid–acetic anhydride mixtures.^6

A number of other amines containing electron-withdrawing groups undergo facileN-

nitration (Table 5.1). The directN-nitration of aniline derivatives is limited to amines of

(CF 3 ) 2 NH (CF 3 ) 2 NNO 2

(CF 3 CH 2 ) 2 NNO 2

NO 2

NO 2

O 2 N NHCH 2 CF 3

NO 2

NO 2

O 2 N

CH 2 CF 3

NO 2

N

N

N

H

NO 2

H

F 3 C

N

N

N

NO 2

NO 2

O 2 N

F 3 C

Table 5.1

N-Nitration of amines with nitric acid and its mixtures

Entry Substrate Conditions Product Yield (%)

70 % HNO 3 , TFAA 787

(CF 3 CH 2 ) 2 NH.HNO 3 HNO 3 , Ac 2 O84^8

HNO 3 , H 2 SO 4 989

1

2

3

100 % HNO 3 ,

TFAA

4 7010

N

CF 3 CF 3