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

Oct 4, 2015

Quick answer: $\textcolor{b l u e}{\text{yes}}$

#### 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

${\text{CH"_3"CH"_2"CH"_2"CH"_3 + 2"CH"_4 → "3CH"_3"CH}}_{3}$

Each side has 3 $\text{C-C}$ bonds and 18 $\text{C-H}$ 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 ($s {p}^{3} - s {p}^{3} , s {p}^{3} - s {p}^{2} , \text{etc.}$)
• 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 $\text{C-C } s {p}^{3}$ bonds, 2 ${\text{CH}}_{4} \left(s {p}^{3}\right)$, 2 ${\text{CH}}_{3} \left(s {p}^{3}\right)$, and 2 ${\text{CH}}_{2} \left(s {p}^{3}\right)$ groups, while the products have 6 ${\text{CH}}_{3} \left(s {p}^{3}\right)$ groups.

The following reaction is both isodesmic and homodesmotic:

${\text{CH"_3"CH"_2"CH"_2"CH"_3 + "CH"_3"CH"_3 → 2"CH"_3"CH"_2"CH}}_{3}$

It is isodesmic because both reactants and products have 4 $\text{C-C}$ bonds and 16 $\text{C-H}$ bonds.

It is homodesmotic because both reactants and products have 4 $\text{C-C } s {p}^{3}$ bonds, 4 ${\text{CH}}_{3} \left(s {p}^{3}\right)$, and 2 ${\text{CH}}_{2} \left(s {p}^{3}\right)$ groups.