Organic Chemistry of Explosives

Nitration of the parent alcohol 95

Commercially available nitronium tetrafluoroborate contains between 5 and 15 % nitroso-

nium tetrafluoroborate^44 and is difficult to purify due to its poor solubility in many organic

solvents. The synthesis of high purity nitronium tetrafluoroborate has been described.^45 −^47

Nitronium hexafluorophosphate is less widely available but can be purified by recrystalliza-

tion from nitromethane.^48

Allylic and homoallylic alcohols are particularly susceptible to oxidation and dehydration.

Hiskey and Oxley^49 successfully nitrated both allyl alcohol and 1-buten-4-ol with nitronium

tetrafluoroborate in diethyl ether at− 71 ◦C.

3.2.5 Transfer nitration

N

NO 2

22

NO 2

BF 4

N

NO 2

20

NO 2

21

BF 4

H 3 C N

BF 4

Figure 3.14

Olah and co-workers^50 −^53 conducted a comprehensive study into the use ofN-nitropyridinium

salts for nitration. Such salts are easily prepared from the slow addition of the appropriate

heterocyclic base to an equimolar suspension of nitronium tetrafluoroborate in acetonitrile.

Olah studied the effect on nitration of changing both the structure of the heterocyclic base and

the counter ion. Three of these salts (20, 21, 22) illustrated above have been synthesized and

used for theO-nitration of alcohols with success.^50 Transfer nitrations withN-nitropyridinium

salts are particularly useful for the preparation of nitrate esters from acid-sensitive alcohols

and polyols because conditions are essentially neutral.

N

CH 3

NO 2

24

H 3 C CH 3

BF 4

CH 3 CN

N

CH 3

H 3 C CH 3

BF 4

OH

OH

ONO 2

ONO 2

+ +

25

2 2

23 2

100 %

H

Figure 3.15

N-Nitrocollidinium tetrafluoroborate (24), generated from 2,4,6-collidine (2,4,6-trimethyl-

pyridine) and nitronium tetrafluoroborate, has been used with great success for the transfer

nitration of alcohols.^53 Olah and co-workers^53 used this reagent for theO-nitration of primary,

secondary and tertiary alcohols. This work extended to the synthesis of polynitrate esters;

nitroglycerine, ethylene glycol dinitrate (2) and 1,4-butanediol dinitrate have been formed from

glycerol, ethylene glycol (23) and 1,4-butanediol respectively. For safety reasons none of these

products were isolated from solution.N-Nitrocollidinium tetrafluoroborate is a less reactive

O-nitrating agent than nitronium tetrafluoroborate but offers better control of conditions and

yields are usual higher for difficult substrates –N-nitrocollidinium tetrafluoroborate^53 reacts

with 1-adamantanol to give an 82 % yield of the corresponding nitrate ester, whereas the same

reaction with nitronium tetrafluoroborate^54 yields only 2 % nitrate ester.