Thermochemistry and Vapor Pressure of Aliphatic Fluorocarbons. A Comparison of the C–F and C–H Thermochemical Bond Energies

The heat of formation of four organic fluorine compounds, including three completely fluorinated aliphatic compounds, was determined by a rotating-bomb method of combustion calorimetry. An earlier technique that used fused quartz ampoules for containing samples of volatile fluorine compounds was replaced by an improved technique that used sealed bags of a polyester film. For three of the compounds vapor pressure measurements were made by an improved method of comparative ebulliometry. The following values, in kcal. mole/sup -1/, are reported for the standard heat of formation, ..delta..Hf/sup 0//sub 296.16/, of these compounds from graphite and gaseous fluorine and hydrogen; m-fluorobenzotrifluoride(g), -184.17; perfluoromethylcyclohexane(g), -675.3; perfluoroethylcyclohexane(g), -769.7; and perfluoro-n-heptane(g), -789.0. These values were used with other thermochemical data to obtain a direct comparison of the C-F and C-H thermochemical bond energies in aliphatic compounds.