Estimation of heats of formation of organic compounds by additivity methods

In the present manuscript describing traditional, macroscopic thermochemical properties, the authors` language will be that of molecular structure. Enthalpies (or heats of formation) are the subject of this article, and since the most important practical application of enthalpies is to explore reactivities and/or equilibria, they take a kineticist`s perspective in answering the question, what is a ``sufficiently accurate`` prediction of an enthalpy of formation. In a general reaction, A + B {yields} C + D, a shift in {Delta}{sub r}H (enthalpy of reaction) of 1 kcal/mol will generally result in a change in the equilibrium constant, K{sub eq}, of exp({minus}500/T) where T is the temperature in Kelvins. At room temperature, this means a factor of over 5 in K{sub eq}; the difference between, say, 90% and 64% reaction yield. Or, in terms of the time required for reaction completion, it could also mean an increase of a factor of 5. This factor of 5 is the same whether the total enthalpy of reaction is 5 kcal/mol or 500 kcal/mol. Thus, while theoreticians have struggled to attain the stage where they can with pride calculate enthalpy quantities with 2--4 kcal/mol uncertainty, they are not solving the practical problems at hand.