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Preparation of dinitrogen pentoxide 351
nonprotic substrates it constitutes a nonacidic nitrating agent of high efficiency. These
reagents are well suited for the ring cleavage of strained oxygen and nitrogen heterocy-
cles and for selectiveO-nitration in the presence of other potentially reactive functionality.
These reagents can be regarded as environmentally friendly nitrating agents, largely avoid-
ing the need for hazardous acidic waste treatment and disposal.
The nitrating properties of the above two reagent types are related to the degree of dissoci-
ation in solution. In nitric acid, dinitrogen pentoxide is highly dissociated to give a high con-
centration of nitronium cation.^7 In contrast, dinitrogen pentoxide is unprotonated and largely
undissociated in chlorinated solvents and the concentration of nitronium cation is low.^8 These
two reagents are very much complementary in nature.
9.3 The chemistry of dinitrogen pentoxide
Dinitrogen pentoxide is a colorless crystalline solid which sublimes without melting at 32.5◦C
at atmospheric pressure.^4 Dinitrogen pentoxide is inherently unstable and readily decomposes
to oxygen and dinitrogen tetroxide at room temperature as shown in Equation (9.1). The rate
of decomposition is temperature dependent with a half-life of 10 days at 0◦C and 10 hours at
20 ◦C.^4 ,^9 It is stable for 2 weeks at− 20 ◦C and up to 1 year at temperatures below− 60 ◦C.
2 N 2 O 5 2 N 2 O 4 + O 2 (Eq. 9.1)
Figure 9.1
Dinitrogen pentoxide is readily soluble in absolute nitric acid and chlorinated solvents. The
polarity of the solvent has a significant effect on the rate of decomposition in solution. The rate
is fastest in nonpolar solvents like chloroform and slower in polar solvents like nitromethane.^10
The decomposition rate for solutions of dinitrogen pentoxide in nitric acid is very slow and
these solutions are moderately stable at subambient temperatures.^10
Being the ‘anhydride of nitric acid’, dinitrogen pentoxide readily reacts with moisture to
form nitric acid.
In the solid state, dinitrogen pentoxide is ionic, existing as NO 2 +NO 3 −and sometimes
called nitronium nitrate.^11 The same is true of dinitrogen pentoxide in polar solvents like nitric
acid where complete ionization to nitronium and nitrate ions is observed.^7 In the vapour phase,
and in nonpolar solvents, a covalent structure is observed.^11 This dichotomy of behavior^12
in both physical state and in solution means that no single nitrating agent is as diverse and
versatile as nitrogen pentoxide.
9.4 Preparation of dinitrogen pentoxide
Deville^6 first synthesized dinitrogen pentoxide in 1849 by reacting silver nitrate with chlorine
gas. This reaction probably involves the initial formation of nitryl chloride and, accordingly,
dinitrogen pentoxide can also be formed from the reaction of nitryl chloride or nitryl fluoride
with a metal nitrate.^13 These reactions are more of theoretical interest than of any practical
value.
