Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
Enthalpies of combustion, ΔHc0, and enthalpies of vapourization, ΔHvap0, for nineteen fluorohydrocarbons containing the groups CF, CF2, and CF3 were measured calorimetrically. From these data, enthalpies of formation, ΔHf(g), were calculated and were partitioned into new strain-free group increments of the Benson and v. Schleyer type: CH3[F] = -42.1; CH2[F, C] = -37.1; CH[F, 2 C] = -35.7; C[F, 3 C] = -39.6; CH2[2 F] = -61.0; CH[2 F, C] = -71.7; C[2 F, 2 C] = -67.3; CH[3 F] = -110.8; C[3 F, C] = -131.4; C[4 F] = -150.4; F[C] = -195.7 kJ mol-1. These increments now allow the calculation of the enthalpies of formation of a large family of homologous fluorohydrocarbons. Fluorine substitution, in general, stabilizes saturated hydrocarbon structures. A new concept, which was proposed recently for ethers, acetals, ketals, and orthoesters, for the analysis of the values of the increments for CH3, CH2, CH, and C groups with their different neighbouring atoms (given in brackets), is successfully applied to fluorohydrocarbons. In this method, the number of pairwise geminal interactions X—C—Y between all attached atoms X and Y is counted. The anomeric stabilization in geminal difluoromethylene groups taken from the F—C—F interaction is found to be 8 kJ mol-1 smaller than the O—C—O interaction for acetals and ketals, in agreement with calculations in the literature.
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