Do homodesmotic reactions have a closer matching of the hybridization states of the atoms of reactants and products as compared to isodesmic reactions?

1 Answer
Ernest Z.
Oct 4, 2015

Quick answer: yes

Explanation:

An isodesmic reaction is a reaction that has the same number and type of bonds in the reactants and in the products.

An example of an isodesmic reaction is

CH3CH2CH2CH3+2CH4→3CH3CH3

Each side has 3 C-C bonds and 18 C-H bonds.

Homodesmotic reactions are a subset of isodesmic reactions.

Here, the reactants and products must share even larger similarity.

Homodesmotic reactions are reactions in which reactants and products have

  • equal numbers of each type of carbon-carbon bond (sp3−sp3,sp3−sp2,etc.)
  • equal numbers of carbon atoms with the same hybridization and the same number of hydrogen atoms

The above reaction is not homodesmotic because the reactants have 3 C-C sp3 bonds, 2 CH4(sp3), 2 CH3(sp3), and 2 CH2(sp3) groups, while the products have 6 CH3(sp3) groups.

The following reaction is both isodesmic and homodesmotic:

CH3CH2CH2CH3+CH3CH3→2CH3CH2CH3

It is isodesmic because both reactants and products have 4 C-C bonds and 16 C-H bonds.

It is homodesmotic because both reactants and products have 4 C-C sp3 bonds, 4 CH3(sp3), and 2 CH2(sp3) groups.

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