Cyclization of glycol monoesters to give hemiorthoesters: a test of the thermochemical method for determining free energies of tetrahedral intermediates

Abstract

From calorimetric heat of formation data, free energies of cyclization (to the hemior-thoester) for the following glycol monoesters have been calculated: ethylene glycol monoformate, +7.54; ethylene glycol monoacetate, +10.72; pinacol monoformate, +3.21; ethylene glycol monotrifluoroacetate, +2.83; pinacol monotrifluoroacetate, −3.38. The latter two results are shown to be in satisfactory agreement with experimental results. This investigation represents a further test of the thermochemical method for calculating free energies of tetrahedral intermediates in acyl transfer reactions which was reported earlier.In the course of this work, various thermochemical quantities were determined. Heats of reaction for the process:[Formula: see text]have been measured for three compounds: 1, R = H, R′ = CH3, R″ = H; x = 0.92, ΔHobs = −7.13 ± 0.60 kcal/mol; 2, R = H, R′ = CH3, R" = CH3; x = 0.98, ΔHobs = −10.11 ± 0.40 kca1/mol; 9, R = CH3, R′ = C2H5, R″ = H; x = 0.98, ΔHobs = −4.76 ± 0.19 kcal/mol. From these may be obtained heats of formation of the liquids: 1, −126.01 ± 0.94; 2, −137.25 ± 0.87; 9, −173.87 ± 2.09 kcal/mol. For some related esters, heats of hydrolysis were measured, leading to the following heats of formation of the liquids: tert-butyl formate, −117.85 ± 0.80; tert-butyl trifluoroacetate, −271.28 ± 1.14; pinacol monoformate, −175.02 ± 2.12 kcal/mol. Heats of hydrolysis were measured for methoxyethyl formate and acetate, but in the absence of a reliable value for the heat of formation of methoxyethanol, do not lead to calorimetric heats of formation. Equilibrium constants for formation of the monoesters of ethylene glycol with formic, acetic, and trifluoroacetic acids were measured (nmr analysis of equilibrated solutions) and lead to free energies of hydrolysis of −2.34, −1.65, and −2.75 kcal/mol respectively.