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

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,,, and C p °( 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. ( 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. ( 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, ( 1 ↔ 2 ) = −18.6 and ( 1 ↔ 3 ) = −17.0 kcal/mol, respectively. The rate constant through transition state 4 for the tautomerization of phenol to 2,4-cyclohexadienone is obtained as 8.06 × 10 12 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. Δ H rxn of these paths are estimated. of cis - and trans -1,3-butadienyl-4-ketenes are determined as 24.5 and 19.2 kcal/mol, respectively. of species 2 and 3 are also calculated at the CBS-Q//B3LYP/6-31G(d,p) level and compared with the CBS-QB3 results.