Solvolysis rates in aqueous – organic mixed solvents. V. Kinetics of the alkaline decarboxylation of trichloroacetate ion in water – ethanol solutions

Abstract

EL-HUSSIENY M. DIEFALLAH and A. M. EL-NADI. Can. J. Chem. 56,2053Chem. 56, (1978. The kinetics of the alkaline decarboxylation of trichloroacetate ion in ethanol-water solutions have been studied over the temperature range 35.0 t o 70.0nC. The rate of reaction is first order with respect to the trichloroacetate ion and is independent of the concentration of t h e hydroxide ion. The reactivity is enhanced by increasing the concentration of ethanol in t h e water-ethanol solutions and the rate of reaction varies with ethanol addition in a nonlinear manner. The rate of reaction increases with the reciprocal of the dielectric constant of t h e medium and the plot of log k vs. 1 / D is approximately linear for solvent mixtures with less t h a n about 0.7 water mole fraction but is strongly curved towards the pure water end. The activation parameters for the reaction show a regular increase in the solvent composition range 0.3 to 1.0 water mole fraction. The results are discussed in terms of the influence of solvent internal pressure and polarity on reactivity and of the increased amount of hydrogen-bonded structure i n the water-rich solutions. [Traduit par le journal]

Notizen: Kinetics of the Alkaline Decarboxylation of Trichloroacetate Ion in Water - Ethanol Solutions

“…This result is not in concordance with the postulated reaction mechanism, according to which we expect that the volume of the transition state to be greater than the reactant state, the volume of activation to be positive, and the internal pressure of the reactant ion to be higher than that of the activated complex [5,6].…”

“…Experimentelly, it is found that the rate of reaction decreases with increasing the dielectric constant of the medium; however, the plot of log k versus D _1 does not give rise to a straight line over the whole range of solvent composition [6]. The results may be approximated to the straight line relationship for solvent mixtures with less than about 0.7 water mole fraction, and over this linear portion of the curve it may be assumed that the reaction occurs in a structureless medium and that the simple electrostatic theory may be holding.…”

“…This result is in agreement with the postulated reaction mechanism and the calculated activation parameters. In the water rich solutions, a similar calculation shows that the simple electrostatic theory is not applicable [6]. The main reason for this are that the simple electrostatic theory is limited due to its neglect of solvent structural effects, and that the macroscopic dielectric constant is not a wholly adequate parameter for describing the molecular interactions.…”

“…Although my name has been included as a coauthor of this paper, it has been written and sent to press without my seeing it. However, the experimental data in this paper are a part of a series of papers [2][3][4][5][6][7] which I am sponsoring their publication on the subject.…”

“…This equation is derived and is generally applied for the situation in which the rate of formation of the activated complex is due to the reaction of an ion with charge Ze and a dipole molecule of moment /a [8][9][10], However, it is not understood why the dipole [1] for the radius of the activated complex (r*) lie within the range 2.67 to 3.1 Ä, and these are below the radius of the reactant ion which is about 3.4 Ä [6]. This result is not in concordance with the postulated reaction mechanism, according to which we expect that the volume of the transition state to be greater than the reactant state, the volume of activation to be positive, and the internal pressure of the reactant ion to be higher than that of the activated complex [5,6].…”