GEMINAL SUBSTITUENT EFFECTS .15. ENTHALPIES OF FORMATION OF A SERIES OF FLUORINATED HYDROCARBONS AND STRAIN-FREE GROUP INCREMENTS TO ASSESSPOLAR AND ANOMERIC STABILIZATION AND STRAIN

Enthalpies of combustion, Delta H-c(0), and enthalpies of vapourizatio n, Delta H-vap(0), for nineteen fluorohydrocarbons containing the grou ps CF, CF2, and CF3 were measured calorimetrically. From these data, e nthalpies of formation, Delta H-g(0)(g), were calculated and were part itioned into new strain-free group increments of the Benson and v. Sch leyer 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] = -6 7.3; CH[3 F] = -110.8; C[3 F, C] = -131.4; C[4 F] = -150.4; F[C] = -19 5.7 kJ mol(-1). These increments now allow the calculation of the enth alpies 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, ace tals, ketals, and orthoesters, for the analysis of the values of the i ncrements for CH3, CH2, CH, and C groups with their different neighbou ring atoms (given in brackets), is successfully applied to fluorohydro carbons. In this method, the number of pairwise geminal interactions X -C-Y between all attached atoms X and Y is counted. The anomeric stabi lization in geminal difluoromethylene groups taken from the F-C-F inte raction 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 literat ure.