Nitrous acidium ion

The rate constants for the reaction remained relatively constant over the pH range 1-3.5, supporting the nitrous acidium ion mechanism. 

Secondary amines. The reaction of secondary amine type compounds with nitrous acid (HO NO) has been reviewed extensively by Turney and Wright (17), Ridd (18), Scanlan (19) and Mirvish (20). In a system containing (HO NO) as the nitrosating agent, the possible nitrosyl carriers are (H2O NO), (NO2 NO) and (NO+). The reactivity of (NO+) is very low and it is not considered an effective nitrosating form. Nitrous acidium ion (H2O NO+) plays a significant role only at concentrated acidic condi tions. Therefore, it seems likely that at the dilute acidic conditions that are encountered in the environment, it is nitrous anhydride (N2O3 = NO2 NO) which nitrosates secondary amines. 

The main nitrosating agent is probably nitrous acidium ion (NO OH2+). There is no simple rule, as there is for secondary amines, relating the ease of nitrosation to the properties of the amide. 

Unfortunately, in spite of the importance of this kinetic form, the identity of the electrophile is not yet clearly established. The kinetic form (19) is consistent with nitrosation by either the nitrous acidium ion [eqn (20)] or the nitrosonium ion [eqn (21)]. 

The nitrosonium ion is a well-known chemical species and the equilibrium constant for its formation has been determined spectrophotometrically as 3 x 10-7 mol-1 dm3 (Bayliss et al., 1963).10 There is no spectroscopic evidence for the existence of the nitrous acidium ion. 

Thus the present evidence does not exclude the nitrosonium ion as the electrophile responsible for the kinetic form of equation (19) provided it is recognized that the half-life of the electrophile must then be extremely short. The nitrous acidium ion remains a possible electrophile but the complete absence of spectroscopic evidence for this ion weakens the case for its consideration (cf. Bayliss et al., 1963). 

In strong acid solution there is much physical evidence for the existence of NO (Bayliss et al., 1963) and its salts are well known. It has been argued for lower acidities [where reaction occurs according to (6)] that the concentration of NO is too low to effect reaction, so the nitrous acidium ion has been proposed as the effective reagent. Although there is no physical evidence for the existence of HaNOJ, it may well be involved. The detailed case for and against involvement in reactions in dilute acid has been presented... 

A number of the reactions of nitrous acid, and of nitric acid with more or less extensive contamination by nitrous acid, have been clarified by the recognition that nitrosyl derivatives are involved. Table 9 lists some examples of the intermediates formed with several reactants, together with the concentration terms which appear in the relevant rate equations and some rate coefficients, for the cases in which the nitrous acidium ion is believed to be involved. 

RATES OF REACTION WITH THE NITROUS ACIDIUM ION TO FORM NITROSYL... 

At 25 °C, is 0.473 l.mole sec and the values of and AS are 14.2 kcal.mole" and 17.1 cal.mole. deg , respectively. The authors reject nucleophilic attack of HN2OJ on the nitrous acidium ion on the ground that the rates are some 400 times greater than those of other reactions of this species which... 

Rates which can be interpreted in terms of the formation of nitrosyl acetate and N2O3 have also been found . Where known , the corresponding coefficients for the O-methyl-, N, O-dimethyl-, O-isopentyl- and 0-/i-heptyl-hydroxylamines are lower only by a factor of 2-3. Thus, although the rates are all well below the encounter limit, there is little difference in intrinsic reactivity. Also the intrinsic reactivity is lower than that for the nitrous acidium ion H2NO2, so that the extensive conversion to ONX, which may occur at higher acidities, causes a resultant decrease in rate of reaction of hydroxylamine. 

The generally accepted mechanism for the formation of N2O3 in the reverse of reaction 42 involves a nucleophilic attack of NO2 on nitrous acidium ion H2NOJ (or NO" ), Eq. (49) (206,207). The reaction of the conjugate acid, HNO2, with H2NO, which would exhibit a first order dependence on [H" "], has been considered earlier, but was difficult to detect (208) at unfavorable pH conditions required by other reactants (209,210) or in competition with other mechanistic routes (211). The work with L (H20)RhOO (205) now provides credible evidence for nitrous acid acting as a nucleophile, Eq. (50). 

Acid-catalysed nitrosation in the absence of halide ions may involve the nitrous acidium ion (H20N0 ). The diazotisation rate of aniline in aqueous perchloric or sulphuric acid at first decreases and then increases with rising acidity . This implies a change in mechanism, because the reaction rate for nitrous anhydride itself should steadily decrease owing to increasing protonation of the amine. 

Ingold and Ridd were the first to interpret this result as evidence for rapid formation of the nitrous acidium ion, which then reacts with the unprotonated amine in a rate-determining step (Scheme 14). 

The nitrous acidium ion must be a very reactive species, for it readily reacts with nitroanilines whereas nitrous anhydride does not. Larkworthy has determined the rate coefficients from equation (54) for a few substituted anilines and these are listed in Tabk 7. 

Actual rate coefficients for the reaction of the nitrous acidium ion with the unprotonated amine cannot be computed, because the required equilibrium constants are unknown. However, nitrosyl chloride must... 

The exact nature of the nitrosating agent in these strongly acidic solutions is also not clear either the nitrous acidium ion or the nitros-onium ion (or even both) may be involved. In this connection, it is interesting to note that the nitrous acid deamination of benzamide in concentrated sulphuric acid has been interpreted as a reaction of the nitrosonium ion although precisely the opposite conclusion has been reached for the nitrosation of phenol in concentrated perchloric acid... 

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