Dediazoniation in Alkaline Aqueous Solutions

The reason is soon discovered on making a serious attempt to investigate such a system on the one hand, numerous polymeric products (diazo tars) that are difficult to identify are formed at pH 6-11, and on the other hand these preparative and kinetic experiments are not readily reproducible. The material of the reaction vessel, light, and the atmosphere influence the product formation and the rate and order of the reaction to an extent rarely encountered in organic chemistry. 

The dediazoniation of 4-chlorobenzenediazonium tetrafluoroborate in HCO / CO2- buffers can be cited as an example (Schwarz and Zollinger, 1981 Besse and 

Zollinger, 1981). In the presence of less than 5 ppb of 02 it obeys first-order kinetics in glass vessels, but zero-order kinetics in Teflon vessels. With between 60 and 100 ppb of 02, a fast initial reaction slackens off after about 15% conversion autocatalysis is observed on exposure to air, but in 100% 02 there is again a first-order reaction. 

On the basis of the nucleophilicity parameters B, NBs, and fi (see Table 8-2) one expects less of the homolytic product in water than in methanol. This is, however, not the case. It has been known for many decades that a very complex mixture of products is formed in the decomposition of diazonium ions, including polymeric products, the so-called diazo tars. In alcohols this is quite different. The number of products exceeds three or four only in exceptional cases, diazo tars are hardly formed. For dediazoniation in weakly alkaline aqueous solutions, there has, to the best of our knowledge, been only one detailed study (Besse et al., 1981) on the products of decomposition of 4-chlorobenzenediazonium fluoroborate in aqueous HCOf/ CO]- buffers at pH 9.00-10.30. Depending on reaction conditions, up to ten compounds of low molecular mass were identified besides the diazo tar. 

The observation of a maximum rate of dediazoniation at pH = p m can therefore be explained just as well in terms of a mechanism involving a diazoanhydride. 

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The reaction with nitrite proceeds smoothly and with relatively high yields of the corresponding nitroarene (see Sec. 10.6). Obviously a major part of the driving force of this reaction is the formation of a stable, i. e., an energetically favorable, radical, nitrogen dioxide. With the hydroxide ion — a much stronger nucleophile than the nitrite ion — the reaction is expected to produce very unstable radicals, the hydroxy radical OH and the oxygen radical anion O, from the diazohydroxide (Ar - N2 — OH) and the diazoate (Ar-N20 ) respectively. Consequently, dediazoniation in alkaline aqueous solution does not follow the simple Scheme 8-41 with Yn = OH, but instead involves diazoanhydrides (Ar — N2 —O —N2 —Ar) as intermediates (see Sec. 8.8). 

The homolytic dediazoniations in alkaline aqueous solutions, in methanol and in highly nucleophilic solvents are extremely complex. CIDNP spectra were considered for many years as a useful source for the understanding of these processes, as shown by the relatively large number of such investigations before 1981 (16, see Zollinger7 ), but the potential of this method for these problems seems to be exhausted (5 publications 1982-1992). 

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