Organic Chemistry of Explosives

32 Synthetic Routes to AliphaticC-Nitro

Trinitroacetonitrile (110), a precursor to dinitroacetonitrile and its derivatives, can be syn-

thesized from the nitration of cyanoacetic acid (109) with a solution of sulfur dioxide and

absolute nitric acid in carbon tetrachloride.^117 This method is particularly attractive because

the trinitroacetonitrile can be kept as a solution in carbon tetrachloride without isolation;

trinitroacetonitrile is hazardous to handle and its vapours are both toxic and lachrymatory.

1.9 Oxidative dimerization

H+

R^2

R^1 R^1

R^2

O 2 N CCNO 2 + 2 SO 4 − (Eq. 1.2)

111

2 R^1 R^2 C = NO 2 − + S 2 O 82 −

Figure 1.46

Kaplan and Shechter^234 found that certain oxidants react with the nitronate salts of secondary

nitroalkanes to yieldvic-dinitroalkanes (111) in a reaction referred to as oxidative dimerization.

These reactions are believed to involve transfer of an electron from the secondary alkyl nitronate

to the oxidant with the production of a nitroalkyl radical. The radical can then dimerize to the

correspondingvic-dinitroalkane (111) (Equation 1.2) or lose nitric oxide to form a ketone via

the Nef reaction (Equation 1.3). Unfortunately, formation of the ketone is a major side-reaction

during oxidative dimerization and is often the major product.

H+

R^1 R^2 C = NO 2 − + 2 S 2 O 82 − + 2 H 2 O R^1 R^2 C = O + 4 H+ + NO 3 − + 4 SO 42 − (Eq. 1.3)

Figure 1.47

Studies into oxidative dimerization have shown that only the persulfate anion is of synthetic

value in these reactions. Reaction pH is also crucial; with reactions proceeding fastest when

a pH of 7.2–9.4 is maintained.^234 The reaction medium becomes more acidic as the oxidation

progresses and needs either buffering or the slow addition of alkali throughout the reaction. If

the reaction medium is allowed to become acidic then the starting nitroalkane is regenerated

and the Nef reaction predominates.

NaOH, NaOAc

(NH 4 ) 2 S 2 O 8 , H 2 O

pH 7.2–9.4, 53 %

48

CH 3

CH 3

CH 3

NO 2

CH 3

2 (CH 3 ) 2 CHNO 2 O 2 N CC

76

Figure 1.48

Oxidative dimerization gives reasonable yields ofvic-dinitroalkanes for some substrates;

2,3-dimethyl-2,3-dinitrobutane (48, 53 %) and 3,4-dimethyl-3,4-dinitrohexane (37 %) are ob-

tained from 2-nitropropane (76) and 2-nitrobutane respectively.^234 However, oxidative dimer-

ization fails to convert 1,1-dinitroethane and trinitromethane into 2,2,3,3-tetranitrobutane and

hexanitroethane respectively. Additionally, oxidative dimerisation is not a feasible route for the

synthesis ofvic-dinitroalkanes from primary nitroalkanes. Although oxidative dimerization is

limited in scope, and yields are often poor, the starting materials are usually inexpensive.