Organic Chemistry of Explosives

Nitramine rearrangement 145

yields a nitrosophenol, which is then oxidized to a nitrophenol on reaction with nitric acid; the

latter step regenerates nitrous acid and so the process is autocatalytic. The nitration of phenols

viain situnitrosation–oxidation with dilute nitric acid is often very selective for thepara-

isomer. The result is similar to that obtained in the phenol substrate is first treated with nitrous

acid and the resulting nitrosophenol isolated and oxidized to the nitrophenol. Occasionally a

catalytic amount of sodium nitrite is added to dilute nitric acid to increasepara-selectivity.^135

It is important to note that nitrous acid catalyzed nitrations are specific to certain groups of

compounds. Nitrous acid can retard the nitration of many aromatic compounds by lowering the

concentration of the nitronium cation. However, under the conditions described i.e. dilute nitric

acid solutions, the concentration of active nitrating species (probably the nitracidium ion) is

so low that nitrosation competes. The nitration of phenol ethers and other reactive compounds

is also catalyzed by nitrous acid.

4.5 Nitramine rearrangement

The nitration of anilines with mixed acid often leads to theN-nitration of the amino group

and the formation of nitramines. The nitramines formed in these reactions can undergo acid-

catalyzed rearrangement to the ring nitrated aniline in a reaction known as the Bamberger

rearrangement.^136 The Bamberger rearrangement probably proceeds via an electrocyclic rear-

rangement and is usually effected by suspending the substrate in neat sulfuric acid at ambient

or subambient temperature. Reactions are usually very selective for theo-isomer. Significant

amounts of thep-isomer can be formed at low solution acidities although such conditions

generally result in much lower yields.^137

Depending on the amine substrate and the nitrating conditions used, it is not uncommon for

an intermediate nitramine to undergo direct rearrangement to the ring-nitrated product without

prior isolation, in which case, the formation of the nitramine as an intermediate can only be

postulated. Due to the higho-selectivity often observed with this type of reaction theo/p-ratio

can be very different to that where the aromatic ring is directly nitrated.

Amine substrates whose rings are strongly deactivated with nitro groups areN-nitrated

with relative ease; 2,4-dinitro-N-methylaniline undergoesN-nitration on treatment with 70 %

nitric acid at room temperature.^138 It is known that theN-nitration of anilines is favoured by

the presence of a large excess of nitric acid.

NH 2

O 2 N NO 2

NHNO 2

O 2 N NO 2

NO 2

NO 2

NH 2

NO 2

NO 2

NO 2

O 2 N

O 2 N

30

35 31

HNO 3 , H 2 SO 4

52 %

Figure 4.15

The nitration of 3,5-dinitroaniline (30) to 2,3,4,5,6-pentanitroaniline (31) is known to in-

volve the formation of the intermediate nitramineN,2,3,4,5-pentanitroaniline (35).^39 ,^139 ,^140

It is also known that the metal salts of arylnitramines show more of a tendency to undergo

Bamberger rearrangement than the free nitramines.^141 Nielsen and co-workers^139 modified

the original synthesis^140 of 2,3,4,5,6-pentanitroaniline by adding a lead salt to facilitate the

rearrangement of the intermediate nitramine to the ring nitrated product.