Diazonium cations stability

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). 

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 stabilities of pyridine-2- and -4-diazonium ions resemble those of aliphatic rather than benzenoid diazonium cations. Benzenediazonium ions are stabilized by mesomerism (726) which... 

The stabilities of pyridine-2- and -4-diazonium ions resemble those of aliphatic rather than benzenoid diazonium cations. Benzenediazonium ions are stabilized by mesomerism which involves electron donation from the ring, but such electron donation is unfavorable in 2- and 4-substituted pyridines. On formation, pyridine diazonium cations normally immediately react with the aqueous solvent to form pyridones. However, by carrying out the diazotization in concentrated HC1 or HBr, useful yields of chloro- and bromopyridines 752 can be obtained. Iodinated pyridines can be obtained in good yield using the Sandmeyer reaction. Aminopyridazines and -pyrazines, 2- and 4-aminopyrimidines, and amino-1,2,4-triazines behave similarly. Nucleophilic fluorination via the BalzSchiemann reaction of diazonium fluoroborates yields fluoropyridines, including 2-fluoropyridines. Fluoroborates can also be converted into fluoro compounds by ultraviolet irradiation. 

In contrast, tetrazadienes, RN=N—N=NR, which are unknown in the free state, are found in a growing number of transition metal complexes. The ligands can be generated in situ from organic azides or diazonium cations and, following the discovery of Fe(MeNNNNMeXCO)3 by Dekker and Knox in 1967 (59), an extensive range of transition metal tetrazadiene complexes has been synthesized. Finally, several papers report theoretical calculations on the stability and structure of N4 ligands bound to transition metals (196-198). 

TGA and DTA curves of BTf and ITf are shown in Fig. 2. The degradation temperature Td of BP was the highest of the diazo compounds studied as shown in Table 1. In the case of common diazonium cation, Td of BP is higher than that of BTf The thermal stability of diazo compounds depended on the counter anion. Td of IP was higher than that of ITf, which is in the same correlation to BP and BTf Td of the diazo compounds was lower than that of the iodonium salts with the same counter anion. 

In this context, [Ga4Lg] coordination cage was used for encapsulation and stabilization of aromatic diazonium cations and tropylium cations (Fig. 9.7) [19], The results indicated that only 4-(diethylamino) benzenediazonium cation, 1, and aromatic tropylium cation, 4, could be encapsulated within a [Ga4L5] coordination cage. Encapsulation protected reactive cations and resulted in deceleration of their decomposition [19]. 

This statement does not mean, however, that the mechanism of diazotization was completely elucidated with that breakthrough. More recently it was possible to test the hypothesis that, in the reaction between the nitrosyl ion and an aromatic amine, a radical cation and the nitric oxide radical (NO ) are first formed by a one-electron transfer from the amine to NO+. Stability considerations imply that such a primary step is feasible, because NO is a stable radical and an aromatic amine will form a radical cation relatively easily, especially if electron-donating substituents are present. As discussed briefly in Section 2.6, Morkovnik et al. (1988) found that the radical cations of 4-dimethylamino- and 4-7V-morpholinoaniline form the corresponding diazonium ions with the nitric oxide radical (Scheme 2-39). 

These last examples illustrate the effecte of heavy cations on anionic stability. The opposite case of an anionic effect is also possible. Thus diazonium salts are hardly stable but not dangerous when the anion is a chloride ion, whereas they become dangerous when the anion is a sulphide or carboxylate. 

The cation 18 did not show any DNA cleavage, due probably to its inherent stability (18 would be more stable than a benzyl cation [66], relatively stable ions that do not alkylate the heterocyclic bases in DNA). The 9-diazofluorene 19 would not undergo reduction like the corresponding diazonium salts (17) because of the presence of a negative charge on the fluorenyl carbon. 

Because of the small concentration of the 2 1 complex the last term can be ignored. From the extreme rate values in the absence of zinc and with an excess of zinc, 2i and 22 are determined as 2.4 X 104 min.-1 and 1.57 min.-1 respectively. These values can be combined with the trend in the rate constants to give the stability constant of the reactive complex, presumably Zn(OR)(OAc), as 3 X 107. For the simple zinc complex in water the literature values of the stability constant for the 1 1 complex vary from 2.5 X 108 to 6.3 X 108. The diazo coupling reaction of the complex indicates the smaller effect of coordination vis a vis protonation since this reaction is very sensitive to such effects and does not proceed with phenols. Unfortunately the choice of cations for such a reaction is restricted since the cation should not interfere with the analytical methods used to obtain the kinetic data nor should it introduce additional reactions such as occur with transition metal cations which can catalyze the decomposition of the diazonium salt via a redox process. 

Pyridine-2- and -4-diazonium ions are far less stable than benzenediazonium cations. Azole-diazonium salts generally show intermediate stability provided diazotization is carried out in concentrated acid, many of the usual diazonium reactions succeed. Indeed, azolediazonium salts are often very reactive in coupling reactions. 

With propanamine, loss of nitrogen from the diazonium ion gives the very poorly stabilized propyl cation, which then undergoes a variety of reactions that are consistent with the carbocation reactions discussed previously (see Sections 8-9B and 15-5E) ... 

The phenyl cation (134) firstpostulated by Waters335 is a highly reactive species oflow stability and plays a fundamental role in organic chemistry—for example, in the chemistry of diazonium ions. According to gas-phase studies and calculations, its stability is between that of the ethyl cation and the vinyl cation.336 Since it is an extremely electrophilic and short-lived species, it could not be isolated or observed directly in the condensed phase. For example, solvolytic and dediazoniation studies under superacidic conditions by Faali et al.337,338 failed to find evidence of the intermediacy of the phenyl cation. Hyperconjugative stabilization via orf/zo-Me3Si or... 

Shortly after the discovery of this reaction, a number of observers questioned whether radicals were intermediates or whether the reaction proceeded by purely cationic chemistry by the aryl cation mechanism (Scheme 2). If this were the case, then the formation of products such as the alcohols 4 would proceed equally well in the absence of TTF. The vigor of the effervescence seen immediately on addition of TTF, together with the long-term stability of solutions of the diazonium salts in solution in moist acetone and other solvents in the absence of TTF, immediately disproves this, but additional experiments were performed to show the special character of TTF in the reactions. 

Clearly DDM and diazoacetic ester are relatively stable diazoalkanes, and quite probably they differ little in stability from each other. The difference in rate-determining step in their reactions with acids can, therefore, be ascribed mainly to differences in the stability of the car-bonium ion intermediates. In the DDM reaction, the benzhydryl cation is of sufficient stability for loss of nitrogen from the diazonium ion to be more rapid than loss of a proton. For diazoacetic ester, on the other hand, 12... 

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