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: #color(blue)("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

#"CH"_3"CH"_2"CH"_2"CH"_3 + 2"CH"_4 → "3CH"_3"CH"_3#

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 (#sp^3-sp^3, sp^3-sp^2, "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 " sp^3# bonds, 2 #"CH"_4 (sp^3)#, 2 #"CH"_3 (sp^3)#, and 2 #"CH"_2 (sp^3)# groups, while the products have 6 #"CH"_3(sp^3)# groups.

The following reaction is both isodesmic and homodesmotic:

#"CH"_3"CH"_2"CH"_2"CH"_3 + "CH"_3"CH"_3 → 2"CH"_3"CH"_2"CH"_3#

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 " sp^3# bonds, 4 #"CH"_3 (sp^3)#, and 2 #"CH"_2 (sp^3)# groups.

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