A subclass of isodesmic reactions in which reactants and products contain equal numbers
of carbon atoms in corresponding states of
hybridization; moreover, there is matching of the carbon-hydrogen bonds in terms of the number
of hydrogen atoms joined to the individual carbon atoms. To achieve all this matching,
one should significantly extend the number and types of reference molecules. In the
aliphatic series of
hydrocarbons these are propane, isobutane and neopentanes as well as propene and isobutene; for
aromatics—buta-1,3-diene, 2-vinylbutadiene (3-methylidenehexa-1,5-diene) and 2,3-divinylbutadiene
(3,4-bismethylidenehexa-1,5-diene). Thus to assess
strain energy of cyclopropane and aromatic stabilization of benzene the following homodesmotic
reactions are to be respectively analysed.
c-(CH2)3 + 3 CH3–CH3
3 CH3CH2CH3
(
)
C6H6 + 3 CH2=CH23 CH2=CH–CH=CH2(trans)
(
)
Due to closer matching of the
hybridization states of the atoms of reactants and products as compared to isodesmic reactions,
the homodesmotic reactions give more accurate estimates of the intrinsic
strain and the cyclic
delocalization. The definition may be extended to molecules with heteroatoms.
Source:
Cite as:
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.
https://doi.org/10.1351/goldbook.
