Nitration 1414.3.4.4 Nitric acid–acid anhydrides
A solution of nitric acid in acetic anhydride is a versatile reagent for nitrolysis reactions and
for theN-nitration of amines (see Chapters 5 and 6). This reagent has also found considerable
use for theC-nitration of aromatic substrates.^98
HNO 3 + (CH 3 CO) 2 O
H 3 C ONO 2O
+ CH 3 CO 2 H (Eq. 4.4)Figure 4.13It is generally accepted that nitric acid–acetic anhydride mixtures contain acetyl nitrate
(Equation 4.4). Acetyl nitrate is a weak nitrating agent but in the presence of a strong acid, like
nitric acid, it can ionize to nitronium and acetate ions. Protonated acetyl nitrate could also be
an active nitrating agent under these conditions.
The reaction of fuming nitric acid with acetic anhydride can be violent, and therefore the
components are usually mixed between 0 to 5◦C. Acetic anhydride in these mixtures acts as
a dehydrating agent, reacting with water formed during the nitration to generate acetic acid.
Solutions of nitric acid–acetic anhydride are mild, nonoxidizing, and can be used for the
nitration of substrates prone to oxidation. Higho/p-ratios are observed for the nitration of
some activated substrates (Section 4.3.3).^69 –^71
Nitric acid–trifluoroacetic anhydride mixtures are used extensively for nitrolysis^99 andN-
nitration^100 reactions (see Chapters 5 and 6). The same is not true for aromatic nitrations. This
reagent contains trifluoroacetyl nitrate, which can ionize to nitronium and trifluoroacetate ions
in the presence of strong acid.
Nitric acid–triflic anhydride is a powerful nitrating agent and probably involves nitronium
triflate as the active nitrating agent.^1
4.3.4.5 Nitronium salts
Olah^1 ,^101 ,^102 has extensively reviewed the synthesis and use of nitronium salts in aromatic
nitration and so only a brief discussion is given here. A review by Gidaspov^103 brought to light
much important research conducted by Russian chemists in this area.
A convenient method for the preparation of nitronium tetrafluoroborate involves treating a
mixture of absolute nitric acid^92 or an alkyl nitrate^104 with anhydrous hydrogen fluoride and
an excess of boron trifluoride. Pure nitronium triflate can be synthesized by treating triflic
anhydride^105 or triflic acid^106 with dinitrogen pentoxide in an inert solvent. Other methods for
nitronium salt synthesis are discussed by Olah.^1
Nitronium salts are efficient and powerful nitrating agents. Nitronium tetrafluoroborate
is the most commonly used nitronium salt for nitration and is commercially available as a
solid or as a solution in sulfolane in which the ions are highly solvated and exist as an ion
pair. Nitronium tetrafluoroborate shows poor solubility in most organic solvents and so the
more soluble nitronium hexafluorophosphate is sometimes preferred for nitrations. Olah and
co-workers^1 ,^101 have studied the nitration of a vast array of aromatics with nitronium salts.
Solutions of nitronium salts in aprotic organic solvents are useful for the nitration of acid
sensitive or readily oxidized substrates. Nitronium tetrafluoroborate has been used for the