Organic Chemistry of Explosives

4 Synthetic Routes to AliphaticC-Nitro

The reaction of fuming nitric acid with 2-methyl-2-butene (3) is reported to yield 2-methyl-3-

nitro-2-butene (4).^26 The reaction of alkenes with fuming nitric acid, either neat or in chlorinated

solvents, is an important route to unsaturated nitrosteroids, which assumedly arise from the

dehydration ofβ-nitroalcohols or the elimination of nitric acid fromβ-nitro-nitrate esters.^25 ,^27

Temperature control in these reactions is important if an excess of oxidation by-products is to

be avoided.

Mixed acid has been reported to react with some alkenes to giveβ-nitro-nitrate esters

amongst other products.^26

Solutions of acetyl nitrate, prepared from fuming nitric acid and acetic anhydride, can react

with alkenes to yield a mixture of nitro and nitrate ester products, but theβ-nitroacetate is

usually the major product.^28 –^30 Treatment of cyclohexene with this reagent is reported to yield

a mixture of 2-nitrocyclohexanol nitrate, 2-nitrocyclohexanol acetate, 2-nitrocyclohexene and

3-nitrocyclohexene.^29 ,^30 β-Nitroacetates readily undergo elimination to theα-nitroalkenes on

heating with potassium bicarbonate.^5 β-Nitroacetates are also reduced to the nitroalkane on

treatment with sodium borohydride in DMSO.^31

Solutions of acetyl nitrate have also been used for the synthesis ofα-nitroketones from enol

esters and ethers.^30 ,^32

The reaction of alkynes with nitric acid or mixed acid is generally not synthetically use-

ful. An exception is the reaction of acetylene with mixed acid or fuming nitric acid which

leads to the formation of tetranitromethane.33aA modification to this reaction uses a mix-

ture of anhydrous nitric acid and mercuric nitrate to form trinitromethane (nitroform) from

acetylene.^34 Nitroform is produced industrially via this method in a continuous process in

74 % yield.^34 The reaction of ethylene with 95–100 % nitric acid is also reported to yield ni-

troform (and 2-nitroethanol).33b,cThe nitration of ketene with fuming nitric acid is reported to

yield tetranitromethane.^35 Tetranitromethane is conveniently synthesized in the laboratory by

leaving a mixture of fuming nitric acid and acetic anhydride to stand at room temperature for

several days.33d

1.4.2 Nitrogen dioxide

N 2 O 4 , O 2

5a 5b 5c

+ +

R

R

R

R

CC

R

R

R

R

O 2 NO 2 2 NNO 2 NO

R

R

R

R

ONO 2 CC

R

R

R

R

ONO

Et 2 O, 0 °C

CC

Figure 1.4

The addition of nitrogen oxides and other sources of NO 2 across the double bonds of alkenes is

an important route to nitro compounds. Alkenes react with dinitrogen tetroxide in the presence

of oxygen to form a mixture ofvic-dinitro (5a),β-nitro-nitrate ester (5b) andβ-nitro-nitrite

ester (5c) compounds; the nitrite ester being oxidized to the nitrate ester in the presence

of excess dinitrogen tetroxide.^36 A stream of oxygen gas is normally bubbled through the

reaction mixture to expel nitrous oxide formed during the reaction and so prevent more com-

plex mixtures being formed. These reactions can be synthetically useful for the synthesis

ofvic-dinitroalkanes because nitrate and nitrous ester by-products are chemically unstable

and are readily hydrolyzed to the correspondingβ-nitroalcohol on treatment with methanol.