Kinetic and thermodynamic acidities of selected weakly acidic hydrocarbons

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Strait, Darrell R.
University of Missouri--Rolla
thesis / dissertation description
"The kinetic and thermodynamic acidities in methanol for bromoform and phenylacetylene have been determined at various temperatures. The thermodynamic acidities of phenylacetylene were obtained through a quench method which involved tritium tagging of the phenylacetylene anions present at equilibrium. With the use of this method, a pKa of 18.34 for phenylacetylene at 0ºC was obtained. Through the use of these acidities, it was shown that the kinetics for these proton exchange processes are not diffusion controlled.Isotope effects ( kH/ kD) were obtained for chloroform, bromoform and phenylacetylene at three temperatures (0ºC, -10ºC and -20ºC). These isotope effects at 0ºC are 1.25, 1.07 and 0.92 respectively. The inverse isotope effects for phenylacetylene were explained as being due to the formation of stronger bonds in the transition state than are being broken in the ground state of the reactants. These isotope effects were used to calculate a Bronsted α of 0.55 for both phenylacetylene and the haloforms. Through the use of the Bronsted relationship and these α's, a pKa of 19.73 was determined for bromoform at 0ºC.As a natural consequence of kinetic studies at various temperatures, activation energies of 18.91 Kcal/mol for phenylacetylene and 19.23 Kcal/mol for bromoform were obtained. These activation energies reinforced the conclusion that the kinetics in this study are not diffusion controlled"--Abstract, page iii.