# Homodesmotic Reactions

## Key Questions

• States of hybridization in a homodesmotic reaction refer to whether the bonds are sp³, sp², or $s p$ hybridized.

The products and reactants in homodesmotic reactions must also have equal numbers of each type of carbon-carbon bond: ${C}_{s {p}^{3}} \text{-} {C}_{s {p}^{3}}$, ${C}_{s {p}^{3}} \text{-} {C}_{s {p}^{2}}$, ${C}_{s {p}^{3}} \text{-} {C}_{s p}$, ${C}_{s {p}^{2}} \text{-} {C}_{s {p}^{2}}$,${C}_{s {p}^{2}} \text{-} {C}_{s p}$, ${C}_{s p} \text{-} {C}_{s p}$, ${C}_{s {p}^{2}} \text{=} {C}_{s {p}^{2}}$, ${C}_{s {p}^{2}} \text{=} {C}_{s p}$, ${C}_{s p} \text{≡} {C}_{s p}$.

An example illustrating this definition is the reaction

The reactants and products contain equal numbers of bonds between carbons in the same state of hybridization and hydrogens bound to identical atom types.

${C}_{s {p}^{3}} \text{-} {C}_{s {p}^{2}}$ = 1

${C}_{s {p}^{2}} \text{-} {C}_{s {p}^{2}}$ = 1

${C}_{s {p}^{2}} \text{-} {C}_{s p}$ = 1

${C}_{s {p}^{2}} \text{=} {C}_{s {p}^{2}}$ = 3

${C}_{s p} \text{≡} {C}_{s p}$ = 1

${C}_{s {p}^{3}} {H}_{3}$ = 1

${C}_{s {p}^{2}} {H}_{2}$ = 3

${C}_{s {p}^{2}} H$ = 2

${C}_{s p} H$ = 1

• Yes, homodesmotic reactions are a subset of isodesmic reactions.

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

Homodesmotic reactions are a subclass of isodesmic reactions.

Here, the reactants and products are even more similar. They must contain equal numbers of

• atoms in the same state of hybridization
• groups containing the same number of H atoms.

An example of a homodesmotic reaction is

The reactants and products contain equal numbers of bonds between carbons in the same state of hybridization and hydrogens bound to identical atom types.

${C}_{s {p}^{3}} \text{-} {O}_{s {p}^{3}}$ = 1

${C}_{s {p}^{3}} \text{-} {C}_{s {p}^{2}}$ = 1

${C}_{s {p}^{2}} \text{-} {O}_{s {p}^{3}}$ = 1

${C}_{s {p}^{2}} \text{-} {C}_{s {p}^{2}}$ = 7

${C}_{s {p}^{2}} \text{=} {C}_{s {p}^{2}}$ = 6

${C}_{s {p}^{3}} {H}_{3}$ = 2

${C}_{s {p}^{2}} {H}_{2}$ = 2

${C}_{s {p}^{2}} H$ = 9

• 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.