What are homodesmotic reactions?

1 Answer
Dec 26, 2014

A homodesmotic reaction (from Greek homos "same" + desmos "bond") is a reaction in which the reactants and products contain equal numbers of

  • carbon atoms in the same state of hybridization
  • CH₃, CH₂, and CH groups

This matching of hybridization and groups makes it easier to evaluate strain energy in rings such as cyclopropane.

An example of a homodesmotic reaction is

cyclo-(CH₂)₃ + 3CH₃-CH₃ → 3CH₃CH₂CH₃; #ΔH# = -110.9 kJ/mol

All C atoms are sp² hybridized, and there are six CH₃ and three CH₂ groups on each side of the equation.

Since all bond types and groups are matched, the value of #ΔH# represents the strain energy in cyclopropane.