Heat of formation group additivity methods in thermochemistry enable the calculation and prediction of heat of formation of organic compounds based on additivity.
[1] Starting with simple linear and branched alkanes and alkenes the method works by collecting a large number of experimental heat of formation data (see: Heat of Formation table) and then divide each molecule up into distinct groups each consisting of a central atom with multiple ligands: To each group is then assigned an empirical incremental value which is independent on its position inside the molecule and independent of the nature of its neighbors: The following example illustrates how these values can be derived.
Likewise propane (-25.02 kcal/mol) can be written as 2P+S, isobutane (-32.07) as 3P+T and neopentane (-40.18 kcal/mol) as 4P+Q.
An alternative model has been developed by S. Gronert based not on breaking molecules into fragments but based on 1,2 and 1,3 interactions [2][3] The Gronert equation reads:
The pentanes are now calculated as: Key in this treatment is the introduction of 1,3-repulsive and destabilizing interactions and this type of steric hindrance should exist considering the molecular geometry of simple alkanes.
In methane the distance between the hydrogen atoms is 1.8 angstrom but the combined van der Waals radii of hydrogen are 2.4 angstrom implying steric hindrance.
Also in propane the methyl to methyl distance is 2.5 angstrom whereas the combined van der Waals radii are much larger (4 angstrom).
In the Gronert model these repulsive 1,3 interactions account for trends in bond dissociation energies which for example decrease going from methane to ethane to isopropane to neopentane.
In this model the homolysis of a C-H bond releases strain energy in the alkane.
In traditional bonding models the driving force is the ability of alkyl groups to donate electrons to the newly formed free radical carbon.