Organic Chemistry of Explosives

194 Synthetic Routes toN-Nitro

Some compounds containing the secondary nitramide group have found limited use as ex-

plosives.N, N′-Dinitro-N, N′-dimethyloxamide (9),N, N′-dinitro-N, N′-dimethylsulfamide (10)

andN, N′-dinitro-N, N′-bis(2-hydroxyethyl)oxamide dinitrate (NENO) (11) are powerful sec-

ondary high explosives. Both (10) (m.p. 90◦C) and (11) (m.p. 91◦C) have conveniently low

melting points which allows the melt casting of charges. NENO is particularly useful in this

respect because its slow crystallization from the molten state results in the formation of ho-

mogeneous charges free of cavities. The facile hydrolysis of secondary nitramides to primary

nitramines in the presence of aqueous acid or base and, in some cases, in prolonged contact

with hot water, has undoubtedly limited their use as practical explosives.

N

NN

N

O

O 2 N

H

H

NO 2

O

H 2 N

N

NH 2

O NO^2

NO 2

H

O 2 N

NN

H H

H NO 2

NN

H

12

nitrourea

13

N,N'-dinitrourea

(DNU)

14

1,4-dinitroglycouril

(DINGU)

15

nitroguanidine

O

Figure 5.5

N-Nitroureas are an interesting group of compounds. The simplest member, ‘nitrourea’ (12),

is a labile substance and readily decomposes in the presence of water.N, N′-Dinitrourea (DNU)

(13), although a powerful explosive, shows similar properties.N-Substituted-N-nitroureas are

more hydrolytically stable. Much interest has focused on the incorporation of theN-nitrourea

functionality into cyclic and caged structures because of the increase in performance observed.

This is due to increased crystal density, which is attributed to the rigidity of the urea function-

ality. TheN-substituted-N-nitrourea functionality is also associated with a low sensitivity to

impact, a property possibly due to intramolecular hydrogen bonding in the nitrourea frame-

work. One such compound, 1,4-dinitroglycouril (DINGU) (14), is classified an insensitive high

explosive (IHE) and exhibits good performance (VOD∼7580 m/s,d= 1 .99 g/cm^3 ).N, N′-

Disubstituted-N, N′-dinitroureas are also associated with high performance. However, many

cyclicN, N′-dinitroureas are hydrolytically unstable and decompose on contact with water.

Such compounds will never find use as practical explosives.

Some compounds can be drawn as primary nitramines or as nitrimines. The two groups are

tautomeric but have very different properties. Nitrimines do not contain acidic hydrogen and

so their solutions are neutral. Nitroguanidine (15) exists in the nitrimine form under normal

conditions, and although its structure can be drawn as a primary nitramine, its properties are not

consistent with such a structure. Nitroguanidine (15) is a compound of some importance in the

explosives industry as a component of triple-base propellants and also as a precursor to other

explosives. The low combustion temperature of nitroguanidine-containing gun propellants

makes them both ‘flashless’ and less erosive to gun barrels, a consequence of the high nitrogen

content of nitroguanidine (CH 4 N 4 O 2 =54 % N). Although nitroguanidine is an explosive, its

fibrous nature imparts an extremely low density to the compound even on compression, and

consequently, it exhibits low performance; this factor alone limits the use of nitroguanidine as

an explosive. If nitroguanidine were to exist as a primary nitramine under normal conditions

it is unlikely it would have found wide applications in explosive technologies.