Kinetics benzenediazonium

Luchkevich et al. (1986, Table 6) demonstrated that for the three isomeric nitro-benzenediazonium ions and their (Z)-diazohxydroxides the acidity constants can be determined by ultraviolet spectrophotometry, by potentiometry, from the kinetics of reaction with hydroxide ions, from the (Z) (E) isomerization kinetics, and from the kinetics of azo coupling reactions. These independent methods gave surprisingly consistent results. ... 

Shortly afterwards and independently, the group of Sterba in Pardubice (Czechoslovakia) published a series of papers on the kinetics of acid-base and isomerization equilibria of substituted benzenediazonium ions. Initially they used classical methods for rate measurements (Machackova and Sterba, 1972 a, 1972 b Jahelka et al., 1973a), then later stopped-flow techniques (Jahelka et al., 1973b). 

The fundamental understanding of the diazonio group in arenediazonium salts, and of its reactivity, electronic structure, and influence on the reactivity of other substituents attached to the arenediazonium system depends mainly on the application of quantitative structure-reactivity relationships to kinetic and equilibrium measurements. These were made with a series of 3- and 4-substituted benzenediazonium salts on the basis of the Hammett equation (Scheme 7-1). We need to discuss the mechanism of addition of a nucleophile to the P-nitrogen atom of an arenediazonium ion, and to answer the question, raised several times in Chapters 5 and 6, why the ratio of (Z)- to ( -additions is so different — from almost 100 1 to 1 100 — depending on the type of nucleophile involved and on the reaction conditions. However, before we do that in Section 7.4, it is necessary to give a short general review of the Hammett equation and to discuss the substituent constants of the diazonio group. 

The dediazoniation kinetics of 3- and 4-substituted benzenediazonium ions are probably the best known example of a failure of the classical Hammett equation (Scheme 7-1, see discussion in Sec. 7.2). 

Kuokkanen (1986, 1987 a, 1991) supported the proposal of Nakazumi et al. (1983) based on kinetic and spectrophotometric comparisons of arenediazonium salt solutions in the presence of 18-crown-6 and pentaglyme. He also extended the systematic work on complex formation of benzenediazonium salts, substituted in the 2-position, and in the presence of 15-crown-5 (Kuokkanen, 1990 Kuokkanen et al, 1991). He discovered a useful way to differentiate between the two types of complexes in Scheme 11-2. Increasing the relative concentration of the host compound shifts the ultraviolet absorption band of both types of complex hypsochromically, whereas the NN stretching frequencies are significantly increased only in the case of insertion complexes. ... 

Besides the azo coupling reactions of 1-methyl- and 2,5-dimethylpyrrole with benzenediazonium-4-sulfonate mentioned above, Butler et al. (1977) synthesized almost all possible combination products of the unsubstituted and four 4-substituted benzenediazonium ions with pyrrole itself, with most isomeric mono-, di-, and trimethyl-pyrroles, and with 3-ethyl-2,4-dimethylpyrrole. These authors also investigated the kinetics of all these combinations (see Sec. 12.7). 

More recently, Bagal and coworkers (Luchkevich et al., 1991) obtained similar results in a kinetic investigation of the coupling reactions of some substituted benzenediazonium ions with 1,4-naphtholsulfonic acid, and with 1,3,6-, 2,6,8-, and 2,3,6-naphtholdisulfonic acids. The kinetic results are consistent with the transient formation of an intermediate associative product. The maximum concentration of this product reaches up to 94% of the diazonium salt used in the case of the reaction of the 4-nitrobenzenediazonium ion with 1,4-naphtholsulfonic acid (pH 2-4, exact value not given). The authors assume that this intermediate is present in a side equilibrium, i. e., the mechanism of Scheme 12-77 mentioned above rather than that of Scheme 12-76, and that the intermediate is the O-azo ether. 

The same conclusion was reached in a kinetic study of solvent effects in reactions of benzenediazonium tetrafluoroborate with substituted phenols. As expected due to the difference in solvation, the effects of para substituents are smaller in protic than in dipolar aprotic solvents. Alkyl substitution of phenol in the 2-position was found to increase the coupling rate, again as would be expected for electron-releasing substituents. However, this rate increase was larger in protic than in dipolar aprotic solvents, since in the former case the anion solvation is much stronger to begin with, and therefore steric hindrance to solvation will have a larger effect (Hashida et al., 1975 c). 

Penton and Zollinger (1979, 1981 b) reported that this could indeed be the case. The coupling reactions of 3-methylaniline and A,7V-dimethylaniline with 4-methoxy-benzenediazonium tetrafluoroborate in dry acetonitrile showed a number of unusual characteristics, in particular an increase in the kinetic deuterium isotope effect with temperature. C-coupling occurs predominantly (>86% for 3-methylaniline), but on addition of tert-butylammonium chloride the rate became much faster, and triazenes were predominantly formed (with loss of a methyl group in the case of A V-di-methylaniline). Therefore, the initial attack of the diazonium ion is probably at the amine N-atom, and aminoazo formation occurs via rearrangement. 

Noelting and Michel, as well as A. R. Hantzsch, had observed around 1900 that benzenediazonium cations evolved dinitrogen on treatment with azide anions, but only R. Huisgen and I. Ugi s kinetic reinvestigation of this reaction at low temperature revealed in 1956—1957 the formation of 1-phenyl-pentazole, and allowed later the isolation of stable arylpentazoles when the phenyl group had electron-donating substituents.39... 

Kinetic data for the ortho position azo coupling of substituted benzenediazonium cations with three activated naphthalene derivatives give good correlations with Hammett a-values of the substituents66 and the data indicate a transition state near in energy to that of the Wheland intermediate. 

In the presence of iron(II) sulfate, /cH-butyl hydroperoxide decomposes to yield acetone and methyl radicals, For a discussion of the kinetics and mechanism of purine C8 methylation by methyl radicals see ref 50. Reaction of adenine with benzenediazonium ions yields ( )-6-(3-phenyl-2-triazen-l-yl)purinc (7, Ar = Ph), which undergoes a C-arylation reaction, probably involving an intermolecular free-radical substitution at C8, to give 8. " ... 

Arenediazonium ion reactions often are complex.61 Benzenediazonium fluoro-borate in 2,2,2-trifluoro ethanol (TFE), for example, decomposes by first-order kinetics to give a mixture consisting largely of the ether (62%) formed by solvolysis and fluorobenzene (34%) resulting from reaction with fluoroborate ion. Addition of pyridine changes the kinetic order and gives rise to additional products.62 The same diazonium ion solvolyzes faster in TFE than in water. Because water is a better nucleophile than TFE, it is likely that a nucleophile is not involved in the rate limiting step of the reaction.63 ... 

Using the decomposition of benzenediazonium chloride, Reed et al. (120) compared the kinetic results obtained by several different methods these comparisons are shown in Table 5.12. They concluded that the Borchardt and Daniels method can be used for the quantitative determination of kinetic parameters if the experimental conditions closely approximate the assumptions of the theory, namely, reaction order, n. with respect to only one component, and the absence of temperature gradients and overlapping peaks. 

Table 5.12. Kinetic Constants for the Decomposition of Benzenediazonium Chloride...

Fitzpatrick et al., 1984) investigated the kinetics of these two and seven other nitrous acid scavengers. The reaction kinetics of the addition of the nitrosyl cation to phenylurea have been studied by Casado s group (Meijide et al., 1987). There is evidence for an initial O-nitrosation step followed by two competitive rearrangements of the nitroso groups to the two N-atoms of the ureido part. One of the N-nitroso compounds formed may be converted into the benzenediazonium ion. Meijide et al. assume, however, that the O-nitroso intermediate leads directly to the benzenediazonium ion (4-1). 

Decamethylferrocene and ferrocyanide were used as one-electron reducing agents for substituted benzenediazonium compounds. The reaction is of second order kinetics, first order in each of the reacting species... 

Finally, the failure to detect an intermediate, and the observation that the rate constants for the reactions between the hydroquinone monoanion and various para-substituted benzenediazonium ions are predicted by the Marcus theory of kinetics, strongly suggest that the reduction occurs by an outer-sphere electron transfer mechanism between the hydroquinone monoanion and the diazonium ion, equation 52. 

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