Organic Chemistry of Explosives

Nitration 129

synthesis of aromatic nitro compounds. In these reactions the nitration of aromatic substrates

proceeds via electrophilic aromatic substitution and uses reagents that generate the electrophilic

nitronium cation or other active electrophilic nitrating species. The aromatic substrate uses the

electron density of the aromatic ring to attack the nitronium ion and, consequently, substituents

on the aromatic ring which withdraw or release electron density have an enormous effect on

substrate reactivity. The nitro group is usually introduced by substitution of hydrogen. Sub-

stitution of other atoms such as halogen is usually only useful in saturated aliphatic systems,

although there are important exceptions. Detailed mechanistic studies on aromatic nitration

can be found in several important works.^1 ,^3 ,^4 ,^7 –^11 ,^14 ,^15

When an aromatic substrate is nitrated there is also the issue of selectivity to be considered.

Substituents which activate the aromatic ring towards electrophilic nitration direct substitution

to theo- andp-positions, whereas deactivating substituents usually direct tom-positions. In

short, this arises from partial charge localization at theo- andp-positions, so any increase or

decrease in electron density is most significant at these positions. Mechanistic reasoning for

these effects can be found in any standard organic chemistry textbook and so further discussion

is not given. There are other selectivity issues which are more specific and strongly dependent

on the nature of the nitrating agent used and the reaction conditions. Some of these effects are

typical of certain classes of aromatic substrate and so these are discussed.

Nitrations giving a complex mixture of products are not useful in organic chemistry or for

the synthesis of explosives, and so, another route to the required product should be considered

which is more selective. Although it is acceptable for commercial explosives to contain a

mixture of aromatic nitro compounds, military explosives are almost always single compounds

with well-defined physical properties.

4.3.1 Nitration with mixed acid

Acid-catalyzed electrophilic nitration is the most common and important route to polynitroary-

lene explosives. These reactions usually employ nitric acid in the presence of a Brønsted or

Lewis acid. While many nitrating agents have been reported^1 and these are discussed later, the

most widely used reagent for aromatic nitration on both industrial and laboratory scales is a

mixture of sulfuric and nitric acids known as ‘mixed acid’. This mixture contains many nitrating

species including the powerful nitronium electrophile (NO 2 +). 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 nitron-

ium ion (Equations 4.1, 4.2 and 4.3); nitric acid acts as a base in this respect. In concentrated

sulfuric acid, nitric acid exists almost entirely as nitronium ions.^15 ,^16 Nitric acid can protonate

itself, but even anhydrous nitric acid contains only 3–4 % of the nitrogen present as nitron-

ium ion.^15 ,^16 Consequently, the nitracidium cation is probably the active nitrating agent when

concentrated nitric acid is used alone for the nitration of some of the more activated substrates.

H 2 SO 4

H 2 SO 4

+ HONO 2

2 H 2 SO 4 + HONO 2

H 2 ONO 2 + HSO 4

+ H 2 ONO 2 NO 2 + HSO 4 + H 3 O

NO 2 + 2 HSO 4 + H 3 O

(Eq. 4.1)

(Eq. 4.2)

(Eq. 4.3)

Figure 4.4