What are Isodesmic Reactions?

Abstract

The heat of isodesmic (formal) reaction Q is systematically used as a measure of deviations from the additivity of bond energies. For molecules these Q values are applied to quantification of strain energies in cyclic compounds, to evaluation of aromaticity of benzene, to explanation of the reasons for deviation from additivity in the heats of formation for disubstituted benzenes and some alkenes. The isodesmic reactions are further used for quantification of structure-stability relationships for cations and free radicals as reactive intermediates in many organic reactions. The scales of relative thermodynamic stabilities comprising 31 cations and 33 free radicals are represented with the range of Q values ca. 225 and 90 kcal/mol respectively. The evaluated energies of aromaticity for benzene, tropylium cation, cyclopentadienyl cation are 30.5, 21.8, 20.3, and -26.8 kcal/mol, respectively.

Thermochemistry of benzoquinones

“…This energy of stabilization is associated with the change of enthalpy accompanying hypothetical reactions, known as isodesmic reactions, where the relationship between bonds change in the process, but keeping the number of each specific formal bond [13][14][15][16][17]. In the case of chemical structures with alternate double bonds, isodesmic reaction can be designed to determine the energy corresponding to the total stabilization energy due to p electron delocalization [15], as is shown in equation (9) for the benzene. The isodesmic stabilization energy corresponds to the enthalpy change associated to equations (10) and (11), respectively, for the p-benzoquinone and the duroquinone.…”

“…From results of enthalpy of formation in the gas phase derived from calorimetric measurements, and applying the scheme of Klages [12], the energy of resonance was deduced, both molecules featuring it. However, we have also made use of isodesmic and homodesmotic stabilization energy criteria [13][14][15][16][17] in order to clarify the aromatic character of both quinones. Results show that the p-benzoquinone is an antiaromatic compound, whereas the duroquinone is marginally aromatic.…”

Theoretical studies on thiabenzene and its fused derivatives: DFT and ab initio computations

“…The heat of isodesmic and/or homodesmic (formal) reaction is systematically used as a measure of aromaticity and to quantification of structure-stability relationships [22]. Subtracting the energies of starting materials (1-4 and C 4 H 6 ) from the corresponding homodesmic products (1 -4 ) are related to the relative stabilities and homoaromaticity character for 1-4 (Scheme 3 and Table 3).…”

Tests for aromaticity applied to the pentalenoquinones — A computational study

“…40a, see refs. [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] calculations were obtained with the program PC Spartan Pro. 2 The DFT calculations, designated in Spartan pBP/DN*, employed the perturbational procedure (41) with the Becke-Perdew B88 and P86 functionals (42) (gradient-corrected but nonhybrid) and the numerical DN* basis set (43).…”

“…This paper addresses primarily the question of the aromaticity of the pentalenoquinones 7 ( Fig. 1), using as probes geometries and bond orders (23), chemical hardness (24)(25)(26), NICS (nucleus-independent chemical shift) (27)(28)(29)(30), a variant of isodesmic reactions (31,32) similar to that used by Glukhovtsev et al (15), and Diels-Alder reactivity. We classify these quinones as fulvene-like (7a), monocyclopentadiene-like (7b), and dicyclopentadiene-like (7c and 7d), and designate them the 1,2-, 1,5-, 1,4-, and 1,6-quinones, respectively, using numbering based on an "aromatic" nucleus (cf.…”