A study of the kinetic and thermodynamic properties of carbon acids in methanol

• Main Author: Filger, Dennis L.
• Format: Text
• Language: unknown
• Published: Scholars' Mine 1974
• Subjects: Iodoform Carbonic acid Thermochemistry; Chemistry; Triton; Carbonic acid
• Online Access: https://scholarsmine.mst.edu/doctoral_dissertations/297; https://scholarsmine.mst.edu/cgi/viewcontent.cgi?article=1299&context=doctoral_dissertations

The thermodynamic acidity (pKa) of iodoform in methanol was determined at 25°C and 35°C; the values are 18.74 and 18.05, respectively . These acidity constants were determined by a modification of the equilibrium quenching technique which involves the conversion of the conjugate base of iodoform, present at equilibrium, to the radioactively labelled acid. The rate constants for the methoxide-catalyzed hydrogentritium exchange of chloroform, iodoform, 9-phenylfluorene and indene were determined in methanol at various temperatures. The second-order rate constants obtained from these studies were shown to be the rate constants for removal of a triton. The activation parameters (ΔH‡ and ΔS‡) for the exchange reactions were also determined for these hydrocarbons from the results of the exchange studies at various temperatures. The hydrogen-deuterium isotope effects for the reaction of chloroform, iodoform or 9-phenylfluorene with methoxide were determined in methanol at various temperatures. These isotope effects were found to be small for chloroform and iodoform (1.25 and 1.02 at 0°C, respectively), and large for 9-phenylfluorene (5 .3 at 10°C). The low isotope effects for the haloforms are suggested as evidence of a highly asymmetric transition state for the proton transfer. A method was developed to determine the hydrogen-tritium isotope effects for the reaction of methanol with the conjugate bases of chloroform, iodoform and 9-phenylfluorene in methanol at bases of chloroform, iodoform and 9-phenylfluorene in methanol at various temperatures. The values for these isotope effects at 0°C are 1.50 and 1.27 for the chloroform and iodoform conjugate bases, respectively and 12.9 for the conjugate base of 9-phenylfluorene at 10°C. From the results of the kinetic exchange studies on iodoform and chloroform, along with the thermodynamic pKa's determined for iodoform, and assuming a Bronsted exponent of unity, the pKa of chloroform in methanol at 25°C was calculated to be 21.1"--Abstract, pages iii-iv.