What is Isodesmic reaction?
An '''isodesmic reaction''' is a [[chemical reaction]] where the type of [[chemical bond]]s broken in the [[reactant]] are the same as the type of bonds formed in the reaction product. This type of reaction is often used as a [[hypothetical]] reaction in [[thermochemistry]]. An example of an isodesmic reaction is :CH<sub>3</sub><sup>-</sup> + CH<sub>3</sub>X → CH<sub>4</sub> + CH<sub>2</sub>X<sup>-</sup> ('''1''') :X = F, Cl, Br, I Equation 1 describes the [[deprotonation]] of a methyl [[halide]] by a [[methyl]] [[anion]]. The [[energy]] change associated with this [[exothermic]] reaction can be calculated using a [[computer]]. The energy change increases going from [[fluorine]] to [[chlorine]] to [[bromine]] and [[iodine]]. This shows that the CH<sub>2</sub>I<sup>-</sup> anion is the most [[wikt:stability|stable]] and least [[base (chemistry)|basic]] of all the halides. Each of these reactions are isodesmic. The energy change in each example also depends on the difference in [[bond energy]] of the C-X bond in the base and [[conjugate acid]]. In other cases, the difference may be due to [[steric strain]] (the blocking of atoms by other atoms). The steric strain is small in fluorine (which is a small diameter atom) but large in iodine (in favor of the anion). So, the energy change grows between each example halide despite the fact that C-F bonds are [[bond strength|stronger]] than C-I bonds.<ref>''Stabilization of Carbanions: Fluorine Is More Effective than the Heavier Halogens '' F. Matthias Bickelhaupt, Holger L. Hermann, Gernot Boche [[Angewandte Chemie International Edition]] Volume 45, Issue 5, Pages 823 - 826 '''2006''' [[DOI:10.1002/anie.200501633|DOI abstract]]</ref> The related term '''homodesmotic reaction''' also takes into account [[orbital hybridization]]. Homodesmotic reactions also have the same number of carbon to hydrogen (C-H) bonds.<ref>{{cite journal|url=http://goldbook.iupac.org/HT07048.html|title=IUPAC Gold Book - homodesmotic reaction|date=2010-12-22|doi=10.1351/goldbook.HT0704