Isodesmic reaction

An isodesmic reaction is a chemical reaction where the type of chemical bonds 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

CH₃^- + CH₃X → CH₄ + CH₂X^- (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 CH₂I^- anion is the most stable and least 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 stronger than C-I bonds.

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. An example of a homodesmotic reaction is:

c-(CH₂)₃ + 3 CH₃-CH₃ → 3 CH₃CH₂CH₃

Other websites

• Gold Book IUPAC definition Archived 2009-08-24 at the Wayback Machine

Further reading

• D. A. Ponomarev, V. V. Takhistov (February 1, 1997). "What are Isodesmic Reactions?". J. Chem. Educ. 74 (2): 201. Bibcode:1997JChEd..74..201P. doi:10.1021/ed074p201.{{cite journal}}: CS1 maint: uses authors parameter (link)