07048 IUPAC Gold Book - homodesmotic reaction 10.1351/goldbook.HT07048 HT07048 current 1 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.$\ce{c-(CH2)3 + 3CH3–CH3 -> 3CH3CH2CH3}$\(\Delta H_{\exp}^{\circ} = \pu{-26.5 kcal/mol}\) (\(\pu{110.9 kJ/mol}\)).$\ce{C6H6 + 3CH2=CH2 -> 3CH2=CH–CH=CH2}$(trans)\(\Delta H_{\rm{calc}}^{\circ} \rm{(MP2/6{-}31G^{**})} = \pu{23.9 kcal/mol}\) (\(\pu{100.0 kJ/mol}\)) 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. delocalization https://goldbook.iupac.org//terms/view/D01583 hybridization https://goldbook.iupac.org//terms/view/H02874 hydrocarbons https://goldbook.iupac.org//terms/view/H02889 strain https://goldbook.iupac.org//terms/view/S06037 strain energy https://goldbook.iupac.org//terms/view/ST07108 PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1943 (https://doi.org/10.1351/pac199971101919) https://goldbook.iupac.org/terms/view/HT07048/html https://goldbook.iupac.org/terms/view/HT07048/json https://goldbook.iupac.org/terms/view/HT07048/plain Citation: 'homodesmotic reaction' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.HT07048 The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms. If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org . The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using. 2026-06-27T22:15:21+00:00