Nitryl cation

This principle also applies to polyatomic cations corresponding to radicals with special names ending in -yl (Sec. 3.1.2.10) for example, PO+, phosphoryl cation NO+, nitrosyl cation NOj, nitryl cation O2+, oxygenyl cation. 

The unpaired electron in NO2 appears to be more localized on the N atom than it is in NO and this may explain the ready dimerization. NO2 is also readily ionized either by loss of an electron (9.91 eV) to give the nitryl cation N02 (iso electronic with CO2) or by gain of an electron to give the nitrite ion NO2 (isoeleelronic with O3). These changes are accompanied by a dramatic diminution in bond angle and an increase in N-O distance as the number of valence electrons increases from 16 to 18 (top diagram). 

There are many reports for nitration of alkenes using various nitrating agents, which proceeds via an ionic or radical addition process.49 Nitration of cyclohexene with acetyl nitrate gives a mixture of 3-and y-nitrocyclohexenes, 1,2-nitroacetate, and 1,2-nitronitrate. This reaction is not a simple ionic or radical process instead, [2+2] cycloaddition of nitryl cation is proposed.50... 

The simplest and most commonly used method of preparing nitric esters consists in the O-nitration reaction of alcohols with nitric acid, usually in the presence of sulphuric acid. The reaction is accompanied by reversible hydrolysis, which is typical of esterification reactions. It is very likely that the main nitrating agent is the nitronium ion NO (nitryl cation). It acts through electrophilic substitution. The presence of sulphuric and perchloric acids in the esterifying mixed acid favours the esterification, as it increases the concentration of NO ion. 

At physiological pH the protonated form of ONOO-, the peroxynitrous acid (ONOOH), is unstable and decomposes to nitrate (N03). ONOOH can also react directly with reductants or can decompose by homolytic dissociation to form nitrogen dioxide (N02) and hydroxyl radical (OH"), or can dissociate by a heterolytic mechanism to yield nitryl cation (NOj), which reacts with thiol, methionyl, tyro-syl, and tryptophanyl residues in proteins. 

Acetyl nitrate (or nitryl acetate, Ac0NO2), prepared from nitric acid and acetic anhydride, reacts with simple acyclic and cyclic alkenes to give complex mixtures of nitro acetates, nitro nitrates and nitroalkenes, which are often difficult to separate12 103, The reaction with unsubstituted cycloalkenes was recently reexamined and a complex mixture of products, including allylic and homoallylic nitroalkenes, 1,2-, 1,3- and 1,4-nitro acetates, and 1,4-nitro nitrates, was identified104. These experimental observations are best accommodated by the proposal, supported by theoretical calculations, that the initial reaction of acetyl nitrate with alkene substrates is a [2 + 2] addition of the nitryl cation to the C-C double bond to form a cyclic cationic intermediate. 

Fig. 20.8. Above the molecular structures of dinitrogen pentoxide and nitric acid in the gas phase. Below the structures of the nitryl cation and the nitrate anion found in crystalline N2O5.

The structure of nitric acid is retained in the crystalline phase. The anhydride, however, crystallizes as an ionic compound, nitryl nitrate or (N02)+(N03) . The nitryl cation, which is isoelectronic with carbon dioxide, is linear. The nitrate ion which, is valence shell isoelectronic with sulfur trioxide, is trigonal planar. 

Problem 20.6 The NO bond distance in the nitryl cation is 4 pm shorter than in the neutral NO2 molecule, while the bond distance in the nitrate anion is equal to that in the neutral molecule. Can these differences be explained in terms of the molecular orbital model ... 

Consider the nitryl cation, NO2. (a) Write one or more appropriate Lewis structures for this species, (b) Are resonance structures needed to describe the structure (c) With what familiar species is it isoelectronic ... 

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