Organic Chemistry of Explosives

132 Synthetic Routes to AromaticC-Nitro Compounds

moderate the nitration step as well as protect the substrate from oxidation. In these reactions

a nitro group substitutes a sulfonic acid group and the now less reactive nitrophenol interme-

diates are nitrated much more smoothly. Such a procedure largely avoids side-reactions due

to oxidation. On a laboratory scale, picric acid (4) is readily synthesized from phenol via this

route. In a typical procedure, phenol (18) is heated on a steam bath with four equivalents of

concentrated sulfuric acid to form a mixture containing botho- andp-phenolsulfonic acids

(19 and 20) in addition to some 2,4-phenoldisulfonic acid (21); the latter predominates at

higher temperatures. This mixture is then cooled and treated with concentrated nitric acid.^21 ,^22

The same procedure using more dilute solutions of nitric acid (44–65 %) has been reported.^23

However, the direct nitration of 2,4-dinitrophenol, obtained from 2,4-dinitrochlorobenzene,

provides a more practical route to picric acid on an industrial scale (Section 4.8.1.3).^23

The reactivity of phenols to electrophilic nitration is illustrated further by the facile con-

version ofm-nitrophenol to 2,3,4,6-tetranitrophenol with anhydrous mixed acid.^24 The latter

is a powerful explosive, but chemically unstable, like all polynitroarylenes containing a nitro

group positionedo/p- to other nitro groups.

OH

OH

OH

NO

23

OH

NO

OH

NO 2

24

NO 2

OH

OH

OH

O 2 N NO 2

NO 2

5

22

H 2 SO 4 ,

NaNO 2

96–100 %

dil. HNO 3 ,

PhCH 3 , -5 °C

1. H 2 SO 4 , < 100 °C


2. HNO 3 , < 50 °C

H 2 SO 4 , HNO 3

Figure 4.6

The sulfonation–nitration strategy also provides a route to styphnic acid (5) (2,4,6-

trinitroresorcinol) from resorcinol (22) but the control of temperature in this reaction is very

important.^25 The synthesis of styphnic acid (5) from the nitration of 2,4-dinitroresorcinol (24)

with mixed acid or concentrated nitric acid is a higher yielding route. 2,4-Dinitroresorcinol (24)

is conveniently prepared from the nitrosation26a,bof resorcinol (22) followed by oxidation26cof

the resulting 2,4-dinitrosoresorcinol (23) with dilute nitric acid. 2,4-Dinitrosoresorcinol (23)

also generates styphnic acid (5) on treatment with concentrated nitric acid.^27

OH

OH

O 2 N NO 2

HO

NO 2

27

OAc

OAcAcOOH

OH

HO

NaOAc, Ac 2 O

130 °C, 95 %

Nitration

see text

25 26

Figure 4.7