Heteroaromatic azo-activated substitutions. Part 4. Kinetics and mechanism of the hydrolysis of 3-(4-methoxyphenylazo)-5-methylisoxazole in aqueous sulphuric acid media

Abstract

The kinetics of the hydrolysis of the title compound, (3), have been investigated in moderately concentrated aqueous sulphuric acid media at 30 "C. Activation parameters have also been determined. Rate correlation b y the Cox-Yates excess acidity method shows that hydrolysis occurs from the monoprotonated substrate by the A-S,2 mechanism of the S,Ar type. Nucleophilic attack b y water at the aryl carbon and a subsequent proton-transfer equilibrium are fast processes which precede the electrophilically catalysed separation of the leaving group, in which the functional catalysts are all general acids in solution. A n abnormal value of 1.4 is obtained for the slope parameter mS (-Kresge's a*); this is discussed in terms of differential solvation of the initial and transition states. The Bunnett-Olsen slope parameter (cpScp,) of -2.0 indicates that the transition state of the reaction is substantially less solvated than its initial state. The values of

Hydrolysis Mechanisms for the Acetylpyridinephenylhydrazone Ligand in Sulfuric Acid

“…In this case, the rate constant k 2 = 3.35 10 -6 M -1 s -1 was obtained for the rate-determining step. The protonation of a base normally is diffusion-controlled and, therefore, fast, with rate constants accessible by nonconventional techniques designed for fast reactions; however, among the wide variety of reactions that follow the A-S E 2 mechanism, there is in principle no upper limit in the k 2 value. ,−

5

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Acid–base behaviour of organopalladium complexes [Pd(CNN)R]BF4

“…Experimental and theoretical aspects of this issue have been extensively treated. [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Evaluation of the X function in 10% ethanol-water at temperatures different from 25 1C, required use of the X function in water 22,23 and the relationship X T = X 298.15 (298.15/T), 24 but the results provided (not shown) agreed only poorly with those from other acidity functions. Under I in Table 1 are listed the pK SH 2+ values provided in 10% ethanol-water at 25 1C; due to the limited capability of the program used, the results may vary with the particular range of wavelengths, hence the constant value was averaged.…”

“…The Cox-Yates method. This method, developed by Cox et al 16 from an earlier approach, 17,18 involves introduction of the excess acidity function X (or medium parameter); X represents the difference between the observed acidity and that of the system if it behaves ideally, 19 and is based on the free energy relationship:…”