Homodesmotic reactions

Homodesmotic reactions describe chemical reactions of hydrocarbon compounds for which the following applies:

• the number of carbon-carbon bond types remains constant, the bond type being given by the hybridization stages involved in the carbon atoms.
• the number of carbon atom types remains constant, the carbon atom types being given by the hybridization stage and the number of hydrogen substituents. .

In the theoretical prediction of reaction energies, errors in the calculation methods are more significant, since reaction energies are small quantities in terms of magnitude compared to the energies, the difference between which they result. Systematic errors generally cancel each other out in the formation of differences. Thus, in theoretical chemistry it makes sense to characterize reactions according to the type of systematic error, the influence of which can be neglected when calculating the reaction energy.

definition

A homodesmotic reaction is a reaction of hydrocarbons for which the following applies:

• the number of carbon-carbon bond types remains constant, the bond type being given by the hybridization stages involved in the carbon atoms.
• the number of carbon atom types remains constant, the carbon atom types being given by the hybridization stage and the number of hydrogen substituents.

Homodesmotic reactions are always also hypohomodesmotic , isogyric and isodesmic reactions .

example

${\displaystyle \mathrm {2-Methyl-1,3-Pentadien+Ethen\ \longrightarrow \ Isopren+Propen} }$

Educts

• 2-methyl-1,3-pentadiene
  • 2 sp ³ -sp ² bonds
  • 3 sp ² -sp ² bonds
  • 2 sp ³ -C with 3 H substituents
  • 1 sp ² -C with 2 H substituents
  • 2 sp ² -C with 1 H substituent
  • 1 sp ² -C with 0 H substituents
• Ethene
  • 1 sp ² -sp ² binding
  • 2 sp ² -C with 2 H substituents

Products

• Isoprene
  • 1 sp ³ -sp ² binding
  • 3 sp ² -sp ² bonds
  • 1 sp ³ -C with 3 H substituents
  • 2 sp ² -C with 2 H substituents
  • 1 sp ² -C with 1 H substituent
  • 1 sp ² -C with 0 H substituents
• Propene
  • 1 sp ³ -sp ² binding
  • 1 sp ² -sp ² binding
  • 1 sp ³ -C with 3 H substituents
  • 1 sp ² -C with 2 H substituents
  • 1 sp ² -C with 1 H substituent

Web links

• LMU Munich: Isodesmic Reactions
• Yale University: Homodesmotic Reactions

Individual evidence

1. ↑ Steven E. Wheeler, Kendall N. Houk, Paul v. R. Schleyer, Wesley D. Allen: A Hierarchy of Homodesmotic Reactions for Thermochemistry . In: Journal of the American Chemical Society . tape 131 , no. 7 , February 25, 2009, p. 2547-2560 , doi : 10.1021 / ja805843n .
2. ↑ http://ursula.chem.yale.edu/~chem220/chem220js/STUDYAIDS/thermo/homodesmotic/homodesmotic.html