On the enthalpy of formation of cyclohexa-2,4- and -2,5-dienone

Abstract

The generally accepted ∆ f HЊ 298 values for cyclohexa-2,4-and -2,5-dienone 1 and 2 respectively) were critically revised, based on three different approaches: (1) evaluation of available kinetic data; (2) group additivity; (3) density functional theory calculations at the B3LYP/6-31G(d,p) level with isodesmic reactions. The kinetics of the gas-phase Claisen rearrangement of allyl phenyl ether (3) sets ∆ f HЊ 298 for 1 at ≥ Ϫ9 kcal mol Ϫ1 , at least 8 kcal mol Ϫ1 above the current literature value of Ϫ17 ± 3 kcal mol Ϫ1 . Recent results on the gas-phase recombination of phenoxyl radicals also indicate a value for 1 15 kcal mol Ϫ1 above ∆ f HЊ 298 of phenol (40, Ϫ23 kcal mol Ϫ1 ). Our DFT calculations led to Ϫ7.3 and Ϫ8.0 kcal mol Ϫ1 for 1 and 2, respectively. Simple group additivity approaches yield values higher by ca. 6 kcal mol Ϫ1 -likely to be due to underestimating conjugation and specific structural effects. The hitherto accepted difference of 4 kcal mol Ϫ1 between the two isomers also appears to be too large. The literature values for the analogous hydrocarbons, 5-methylenecyclohexa-1,3-diene (32) and 3-methylenecyclohexa-1,4-diene (33) are critically reviewed as well on the basis of similar approaches; we prefer ∆ f HЊ 298 (32) = ϩ43 kcal mol Ϫ1 rather than the literature value of ϩ35 kcal mol Ϫ1 , and a value for 33 close to 43 kcal mol Ϫ1 .

General and Theoretical Aspects of Phenols

“…Both cyclohexa-2,4-dien-1-one 27 and cyclohexa-2,5-dien-1-one 28 Let us remember that the energy difference between phenol 26 and both keto isomers 27 and 28 amount to 73 and 69 kJ mol −1 , respectively ( Figure 31). The contribution of entropy is small, amounting to S = −9 and −1 J mol −1 K −1 , for both ketonization reactions, respectively, and this also leads to an estimate for the equilibrium constant of the enolization, pK E , ranging from −12 to −13, of the same order of magnitude as the experimental results in aqueous solution 186,187,469 . It should be stressed that such similarity of values in both gaseous and condensed phases should not be considered as an 'agreement' and need to be treated with much caution, due to the fact that the solvent effect on the equilibrium has not been taken into account.…”

“…Interestingly, the keto form possesses a total dipole moment of 5.0 D and thus it is more polar than the favourable enol form. The standard heats of formation of both cyclohexa-2,4-and -2,5-dienones have recently been re-evaluated as −31 and −34 kJ mol −1 , respectively, in better agreement with theoretical estimates187 .…”

Kinetics and Thermochemistry for the Gas-Phase Keto−Enol Tautomerism of Phenol ↔ 2,4-Cyclohexadienone

“…The error range of the calculated standard enthalpy of formation using the isodesmic reaction has been discussed by Zhu et al and is summarized here: The accumulative uncertainties from reference species are listed in Table (see also Supporting Information table 3), and the mean absolute deviation of the CBS-QB3 energy (using atomization reactions) on G2/97 set is tested to be 1.08 kcal/mol, so the final of 2 and 3 are −4.4 ± 2.4 and −6.0 ± 2.4 kcal/mol, respectively; These results are 3 and 2 kcal/mol higher than the values by Santoro and Louw, −7.3 and −8.0 kcal/mol, who used B3LYP/6-31G(d,p) calculation with isodesmic reactions. Le et al also report relative computation values (B3LYP/6-311++G(d,p) + ZPE) for 2 and 3 relative to phenol which, when used with the of −23 kcal/mol for phenol with neglect of the small (0.2 kcal/mol) differences in thermal energy, result in −5.55 and −6.49 kcal/mol, respectively.…”

“…27,[31][32][33][34][35][36][37] Here the CBS-Q//B3LYP/6-31G(d,p) results are 2.1 and 2.8 kcal/mol lower than the corresponding CBS-QB3 for 2 and 3, and are within 1 kcal/mol of the B3LYP/6-31G-(d,p) values of Santoro and Louw. 19 The CBS-QB3 results are recommended in this work because the method is considered to represent a small improvement over CBS-Q in accuracy, 20 it also offers improvement in the geometry optimization and frequency calculation steps.…”

“…The relative energies between 2,4-cyclohexadienone and 2,5-cyclohexadienone versus phenol were reported as 17.33 (MP2) and 16.88 (MP2) kcal/mol, respectively. The equilibrium constant for phenol ↔ 2,4-cyclohexadienone is then estimated as 1.98 × 10 -13 at 298 K. Santoro and Louw recently report the of 2,4- and 2,5-cyclohexadienones calculated at the B3LYP/6-31G(d,p) level using isodesmic reactions, which lead to enthalpy values of −7.3 and −8.0 kcal/mol for these two dienones. They also use simple group additivity but the values yielded are 6 kcal/mol higher due to the underestimation of conjugation and specific structure effects.…”