Conductimetric determination of reaction rates: the solvolysis of benzhydryl chloride

Abstract

The aim or the thesis was to produce accurate rate measurements, from which an accurate value of ∆Cp* at constant atmospheric pressure could be calculated and so obtain more information concerning the activation process for the S (_n) l solvolysis of benzhydryl chloride. It was hoped that the data would be accurate enough to determine any possible temperature coefficient of ∆Cp*.The solvolysis of benzhydryl chloride can be followed accurately via the development of acidity, either by titrimetric analysis or conductance measurement of the reaction mixture. It was calculated assuming that conductance and concentration are linearly related, that conductance measurements were capable of producing rate coefficients with a fractional error better than 1 x 10 (^-4) Whilst such an assumption has been used in the past, it was found not to be applicable in this case. It was therefore decided to computer fit the rate data using equation I, a relationship between conductance (L) and concentration (c): c = AL + BL (^3/2) + C I together with the rate equation, equation II: ln (P - P) = A - kT II which when combined together yield equation III which was computer fitted: y = ln (l - X) = A(_l) T + A (_2) – ln (l – A (_3). X (_1+x(^2)) III where X = L/L A (_1) = -k A (_2) Involves all the Xo terms A(_3) = A 1/3 / 1 + A 1/3 A 1/ = B/A. L ½. The rate coefficients calculated from equation III had a fractional error in the range 0.001 to 0.0008, whilst being an order of magnitude better than any obtained in the past, were not as good as the 1 x 10 (^-4) that had been expected. This in turn meant that the activation parameters, particularly ACP*, had disappointing accuracies. One of the causes is without doubt the limited temperature range (now 25, normally 50), over which the rate coefficients were determined due to experimental difficulties. Also the introduction of the parameter B/A, is also partly responsible for the poor accuracy of the activation parameters. In the event, no conclusions as to the magnitude of d/dT (ACp*) were possible. It was also the aim of this thesis to study the same reaction under conditions of constant volume, since suggestions had been made that activation parameters at constant volume might be easier to interpret. However, even greater experimental difficulties were encountered, it was therefore decided to limit the study to obtain accurate rate data at constant atmospheric pressure.