Benzenethiol: Thermodynamic Properties in the Solid, Liquid and Vapor States; Internal Rotation of the Thiol Group1

Abstract

Studies of benzenethiol by low temperature calorimetry, comparative ebulliometry, flow calorimetry and combustion calorimetry led to values of the standard entropy, heat capacity and heat of formation of the vapor. The calorimetric results were used with molecular structure information to evaluate the contributions of internal rotation and anharmonicity to the thermodynamic functions. Internal rotation of the thiol group is essentially free, and the contributions of anharmonicity are significant even at ordinary temperatures. Thermodynamic functions were computed by the methods of statistical mechanics for selected temperatures between 0 and 1000/sup 0/K. Values of the heat, free energy and logarithm of the equilibrium constant of formation were computed for the same temperatures. The experimental studies provided the following information: Values of the heat capacity for the solid (12 to 258/sup 0/K), the liquid (C/sub satd/ = 34.985 + 2.175 x 10/sup -3/ T + 6.500 x 10/sup -5/ T/sup 2/ cal deg./sup -1/ mole/sup -1/ (258 to 375/sup 0/K)) and the vapor (C/sup 0//sub p/ = -5.541 + 0.12212 T - 6.577 x 10/sup -5/ T/sup 2/ cal. deg./sup -1/ mole/sup -1/ (427 to 500/sup 0/K)); the heat of fusion (2736 cal. mole/sup -1/) at the triple point (258.27 +- 0.05/sup 0/K);more » the entropy of the liquid at 298.16/sup 0/K (S/sub satd/ = 53.25 cal. deg./sup -1/ mole/sup -1/); the heat of vaporization (..delta..Hv = 15808.5 - 13.620 T - 1.586 x 10/sup -3/T/sup 2/ cal. mole/sup -1/ (375 to 417/sup 0/K)); the second virial coefficient in the equation of state, PV = RT(1 + B/V), (B = -588 - 28.50 exp(1500/T) cc. mole/sup -1/ (375 to 500/sup 0/K)); the vapor pressure (log/sub 10/ p(mm) = 6.99019 - 1529.454/(t + 203.048) (114 to 212/sup 0/)); and the standard heat of formation (6C(graphite) + 3H/sub 2/(g) + S(rhombic) = C/sub 6/H/sub 5/SH (liq), ..delta..Hf/sup 0//sub 298.16/ = +15.02 +- 0.27 kcal. mole/sup -1/). The heat capacity of solid benzenethiol has a lambda-like anomaly near 128/sup 0/K.« less