Kinetics and thermochemistry for the gas-phase keto-enol tautomerism of phenol ↔ 2,4-cyclohexadienone

Document Type

Article

Publication Date

5-15-2003

Abstract

Isomerization of phenol is considered an important step in the metabolism of aromatic compounds and it may be the first step of phenol decomposition in thermal reaction systems. Ideal gas thermochemical properties, ΔH°f,298, S°298, and Cp°(T) (300 ≤ T/K ≤ 1500) for two keto forms of phenol (1), 2,4-cyclohexadienone (2), and 2,5-cyclohexadienone (3), are calculated in this study, ΔHf,298°(2) is computed as -4.4 ± 2.4 kcal/mol at CBS-QB3 level and isodesmic reactions are utilized to minimize the systematic calculation errors. ΔHf,298°(3) = -6.0 ± 2.4 kcal/mol is obtained from the total energy difference between 2 and 3. The two keto tautomers are less stable than their enol form, phenol, ΔΔH°f,298(1↔2) = 18.6 and ΔΔH°f,298 (1↔3) = -17.0 kcal/mol, respectively. The rate constant through transition state 4 for the tautomerization of phenol of 2,4-cyclohexadienone is obtained as 8.06 × 1012 exp(-69.4 kcal mol-1/Rt) s-1. The equilibrium constants are computed as 7.15 × 10-14 and 2.16 × 10-13 at 298 K for reactions 1 ↔ 2 and 1 ↔ 3, respectively. Kinetic parameters for the unimolecular isomerization and decomposition of 2 are also determined. IRC analysis on the transition state indicates that decomposition occurs by cleavage of the weak bond between allylic carbon and carbonyl carbon (73.1 kcal/mol, doubly allylic) to cis-1,3-butadienyl-4-ketene. ΔHrxn of these paths are estimated. ΔH°f,298 of cis- and trans-1,3-butadienyl-4-ketenes are determined as 24.5 and 19.2 kcal/mol, respectively. ΔH°f,298 of species 2 and 3 are also calculated a the CBS-Q//B3LYP/6-31G(d,p) level and compared with the CBS-QB3 results.

Identifier

0038010550 (Scopus)

Publication Title

Journal of Physical Chemistry A

External Full Text Location

https://doi.org/10.1021/jp0212545

ISSN

10895639

First Page

3696

Last Page

3703

Issue

19

Volume

107

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