Heats of vaporization of hydrogen‐bonded substances

Abstract

Abstract A new method is proposed for the calculation of the heat of vaporization of hydroxylated compounds to an accuracy of about ±0.5 kcal./mole from no more data than the molecular structure and a boiling point. The older methods, by comparison, achieved an accuracy of about ±1 kcal./mole with a far greater computational effort, since they required the (usually hypothetical) critical temperature and critical pressure in addition to a boiling point. The method is here applied to aliphatic and aromatic alcohols, to ether‐alcohols (e.g., the cellosolves), and to alcohols conaining keto or aldehyde groups (e.g., salicylaldehyde) and supersedes previous correlations covering the heats of vaporization of these compounds. The method can also be used to assess the quality of vapor‐pressure data of the compounds covered by it. The method is based on the assumption that the heat of vaporization consists of two terms, the dispersion energy and the hydrogen‐bond increment (close but not equal to the hydrogen‐bond strength). The first term is calculated from a knowledge of the heat of vaporization of the equistructural hydrocarbon, now easily available from the Tables of A.P.I. Research Project 44. The hydrogen‐bond term is calculated from a set of rules given in the report. The application of the increment method of this report to other properties and other functional groups is the subject of a continuing investigation.