Organic Chemistry of Explosives

90 Synthetic Routes to Nitrate Esters

3.2 Nitration of the parent alcohol

3.2.1O-Nitration with nitric acid and its mixtures

The direct action of nitric acid and its mixtures on the parent alcohol is by far the most important

method for the production of nitrate esters on both an industrial and laboratory scale.^4 While

such reactions are essentially esterifications they are commonly referred to asO-nitrations

because the reaction mechanism, involving substitution of hydrogen for a nitro group, is not

dissimilar to other nitrations and frequently involves the same nitrating species.

H 2 SO 4 + HONO 2

H 2 SO 4 + H 2 ONO 2

2 H 2 SO 4 + HONO 2

H 2 ONO 2 + HSO 4 (Eq. 3.1)

NO 2 + HSO 4 + H 3 O (Eq. 3.2)

NO 2 + 2 HSO 4 + H 3 O (Eq. 3.3)

Figure 3.4

Mixed acid generated from sulphuric and nitric acids still remains the most important reagent

for the industrial production of nitrate esters and explosives in general. The mixed acid contains

many nitrating species including the powerful nitronium electrophile (NO 2 +) (Equations 3.1,

3.2 and 3.3). For a long time it was thought that the sulfuric acid in mixed acid acted solely

as a dehydrating agent to mop up water formed during the nitration. It is now known that

sulfuric acid, a stronger acid than nitric acid, protonates the latter to form the nitracidium

cation (H 2 ONO 2 +) which can lose water to form the nitronium ion; nitric acid acts as a base in

this respect. Water is formed duringO-nitration with the effect of diluting the acid mixture and

hence shifting the equilibrium towards the starting material. In this respect the sulfuric acid does

serve as a dehydrating agent. It is believed that the heat of solution liberated from the dilution

of the mixed acid is responsible for the exothermic nature ofO-nitration with mixed acid.

Mixed acid has been used to synthesize many commercially important nitrate ester explo-

sives from the parent polyols including: nitroglycerine (NG), ethylene glycol dinitrate (EGDN),

diethylene glycol dinitrate (DEGDN), triethylene glycol dinitrate (TEGDN), metriol trinitrate

(MTN), 1,2,4-butanetriol trinitrate (BTTN), 1,2-propanediol dinitrate (PDDN) etc.^1 Glycerol

is a by-product of soap manufacture and is also synthesized from propylene procured from the

petrochemical industry. Ethylene glycol, which is also used to synthesize diethylene glycol

and triethylene glycol, is synthesized from ethylene gas, also from the petrochemical industry.

Higher polyols like erythritol and mannitol and other naturally occurring sugars are far less

commercially available in quantity and this has played a part in why their nitrate esters have

not found wide use as explosives. Cellulose is widely available in nature, and after suitable

processing, isO-nitrated with mixed acid for the manufacture of nitrocellulose. An enormous

amount of research has been focused on the relationship between mixed acid composition,

reaction time and temperature, and the nitrogen content of the nitrocellulose produced.^5

Mixed acid is by no means the perfect nitrating agent. This reagent is highly acidic, oxidizing

and unselective. Attempts at the selective nitration of glycerol with mixed acid inevitably lead

to a mixture of products, and substrates like glycidol cannot be nitrated with mixed acid due

to acid-catalyzed ring opening.^6 Nitrations with mixed acid and nitric acid are exothermic

and on a large scale there is always the problem of thermal runaway and potential explosion.

Consequently, on an industrial scale, the mixed acid nitration of polyols requires strict control,