The influence of the solvent on organic reactivity. Part I. Kinetics and mechanism of the reactions of carboxylic acids with diazodiphenylmethane in donor aprotic solvents

Abstract

Apparent second-order rate coefficients have been determined for the reactions a t 37 "C of diazodiphenylmethane with benzoic acid in 1 9 donor aprotic solvents. Values determined for various initial concentrations of acid, c, are analysed in terms of the equation k(obs) = ko + k'c. Reactions of acetic acid and of deuteriated benzoic and acetic acid (PhC02D and MeC0,D) have also been studied. We conclude that k, corresponds to the reversible formation of a complex between monomeric acid and diazodiphenylmethane and its subsequent breakdown to products, while k' corresponds to the reaction of a small amount of highly reactive acid in the form of an open-chain dimer. The effect of a wide range of aprotic solvents on ko and k' is interpreted in terms of the following solvent parameters : a function of dielectric constant and one of refractive index, and a measure of solvent nucleophilicity and one of electrophilicity. On including results from an earlier study, multiple linear regression of log ko on the four parameters gives an excellent correlation for 24 solvents (R = 0.975, s = 0,209). A similar analysis for the log k'values for ten donor aprotic solvents is less satisfactory. Correlation analysis for log k, and log k'values of the reaction in 11 ketones is very unsatisfactory: the hoped-for advantages of considering a homogeneous set of aprotic solvents were thus not realised. Apparent second-order rate coefficients have also been determined for the reactions at 37 "C of diazodiphenylmethane with substituted benzoic acids in seven donor aprotic solvents. Values of ko and k', and of the Hammett reaction constant for the two types of rate coefficient are discussed.

Solvent and structural effects on the kinetics of the reactions of cycloalkenylcarboxylic and cycloalkenylacetic acids with diazodiphenylmethane

“…The results obtained show that the rate constants increase with solvent polarity. This is in accordance with the supposed mechanism of the reaction [6][7][8][9]. The exceptionally high value of the reaction rate constant in chloroform could be explained by the low polarity of this solvent (π * = 0.58) and a complete lack of proton acceptor effects, because of which the carboxylic acid dissolved in it exists in form of dimers [9].…”

Effects of solvent and structure on the reactivity of 6-substituted nicotinic acids with diazodiphenylmethane in aprotic solvents

“…The reaction rate constants (as log k 2 ) for the reaction of the examined acids with DDM in the employed solvents are given in Table I. The results from Table I show that the influence of a solvent on the reactivity is complex, due to the many types of solvent to solute interactions (dipolarity, HBD and HBA effects), acting not only at the electrophilic and nucleo-1362 DRMANIĆ, MARINKOVIĆ and JOVANOVIĆ philic acid sites (Fig. 24 As was stated in the literature, 19 carboxylic acids dissolved in chloroform exist in the form of dimers. Solvents of high dipolarity/polarizability and/or high proton-acceptor capability cause a significant decrease of the reaction rate.…”

A linear solvation energy relationship study for the reactivity of 2‐(4‐substituted phenyl)‐cyclohex‐1‐enecarboxylic, 2‐(4‐substituted phenyl)‐benzoic, and 2‐(4‐substituted phenyl)‐acrylic acids with diazodiphenylmethane in various solvents

“…The obtained second-order rate constants for the examined acids in 11 aprotic solvents, together with the previously determined rate constants for the same acids in the hydroxylic solvents, are given in Tables I-III. Comparison of the values of the reaction constants in protic and aprotic solvents indicates that the examined reaction is slower in aprotic than in protic solvents, which is in agreement with the supposed reaction mechanism [20][21][22].…”