Diazonium cation

What evidence is there for the individual reaction steps The add-base reaction (Eq, 2) has the characteristics of a Broensted equilibrium, as has been shown in the case of diazomethane-benzoic acid (in toluene). Further evidence for this is provided by the reactions of diazoacetic ester and diazo ketones. The occurrence of free, mobile diazonium cations is also supported by the fact that solutions of diazomethane in methanol show greater conductivity than solutions of pure solvent. ... 

Whereas reaction of hydrazones disubstituted in the 4,5-position of the selenazole rings with diazotized arylamines only gives formazans, compounds unsubstituted in the 5-position can be attacked there by the diazonium cation. In fact, the azo coupling in this position is decidedly quicker reaction of 5-benzylidenehydrazino-4-phenyl-... 

Salts of diazonium ions with certain arenesulfonate ions also have a relatively high stability in the solid state. They are also used for inhibiting the decomposition of diazonium ions in solution. The most recent experimental data (Roller and Zollinger, 1970 Kampar et al., 1977) point to the formation of molecular complexes of the diazonium ions with the arenesulfonates rather than to diazosulfonates (ArN2 —0S02Ar ) as previously thought. For a diazonium ion in acetic acid/water (4 1) solutions of naphthalene derivatives, the complex equilibrium constants are found to increase in the order naphthalene < 1-methylnaphthalene < naphthalene-1-sulfonic acid < 1-naphthylmethanesulfonic acid. The sequence reflects the combined effects of the electron donor properties of these compounds and the Coulomb attraction between the diazonium cation and the sulfonate anions (where present). Arenediazonium salt solutions are also stabilized by crown ethers (see Sec. 11.2). 

Recently, stable salts of vinyl diazonium cations such as 144 have in fact been prepared (119,120), but their decomposition has not so far been investigated. 

Corke CT, NJ Bunce, A-L Beaumont, RL Merrick (1979) Diazonium cations as intermediates in the microbial transformations of chloroanilines to chlorinated biphenyls, azo compounds and triazenes. J Agric Eood Chem 27 644-646. 

Carbocations may be obtained from the decomposition of other cations, e.g. diazonium cations from the action of NaN02/HCl on RNH2 (cf. p. 119),... 

The first two reaction types often lead to the formation of stable end-products, but (c) and (d) lead to the formation of new carbocations to which the whole spectrum of reaction types is still open. Most of these possibilities are neatly illustrated in the reaction of 1-amino-propane (11) with sodium nitrite and dilute hydrochloric acid [the behaviour of diazonium cations, e.g. (12), will be discussed further below, p. 119] ... 

With simple aliphatic amines, the initial diazonium cations (56) will break down extremely readily to yield carbocations (cf. p. 107) which are, for reasons that are not wholly clear, markedly more reactive than those obtained from other fission processes, e.g. RBr- R Bre. Where the prime purpose is the formation of carbocations, the nitrosation is better carried out on a derivative of the amine (to avoid formation of H20) under anhydrous conditions ... 

The instability of aliphatic diazonium cations, in the absence of any stabilising structural feature, is due very largely to the effectiveness of N2 as a leaving group in aromatic diazonium cations, however, such a stabilising feature is provided by the n orbital system of the... 

Another classical electrophilic aromatic substitution reaction is diazo coupling, in which the effective electrophile has been shown to be the diazonium cation (cf. p. 120) ... 

This is, however, a weak electrophile compared with species such as N02 and will normally only attack highly reactive aromatic compounds such as phenols and amines it is thus without effect on the otherwise highly reactive PhOMe. Introduction of electron-withdrawing groups into the o- or p-positions of the diazonium cation enhances its electrophilic character, however, by increasing the positive charge on the diazo group ... 

The rearrangement has been shown under these conditions to be an infermolecular process, i.e. the diazonium cation becomes free, for the latter may be transferred to phenols, aromatic amines or other suitable species added to the solution. It is indeed found that the rearrangement proceeds most readily with an acid catalyst plus an excess of the amine that initially underwent coupling to yield the diazoamino compound (33). It may then be that this amine attacks the protonated diazoamino compound (39) directly with expulsion of PhNH2 and loss of a proton ... 

As the range of components available for use in the azoic dyeing process expanded, research was simultaneously targeted on improvements designed to make the process more attractive to the commercial dyer. The necessity for the dyer to diazotise the Fast Base was removed with the introduction of stabilised diazonium salts [111], known as Fast Salts. Stabilisation was achieved by a judicious selection of the counter-ion to the diazonium cation various anions have found use in commercial Fast Salts and some examples are listed in Table 4-4. Particularly effective is the diazonium tetrachlorozincate, which can be readily prepared by adding an excess of zinc chloride solution to a solution of the diazonium salt. The precipitated complex diazonium salt is usually admixed with an inert diluent, which enhances its stability, and in use the dyer only needs to dissolve the powder in water to prepare the necessary diazonium salt solution. 

The same strategy has recently been used in the quite challenging determination of the standard potentials for the reduction of phenyl and substituted phenyl radicals.48 In this case the radical is so easy to reduce that starting from iodides was not sufficient. One had to go to the very reducible phenyl diazonium cations to see the wave of the radical appear beyond the radical-producing wave. 

If the provoked or spontaneous acid-base reactions overcome the radical reactions of the primary radical, the secondary radical is easier to reduce, or to oxidize, than the substrate in most cases. Exceptions to this rule are scarce, but exist. They involve substrates that are particularly easy to reduce thanks to the presence of a strongly electron-withdrawing substituent (for reductions, electron-donating for oxidation), which is expelled upon electron transfer, thus producing a radical that lacks the same activation. Alkyl iodides and aryl diazonium cations are typical examples of such systems. 

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