Question

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

Answer 1

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.