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

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, , , 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 × 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. ΔH 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.

Computational study of keto–enol equilibria of tropolone in gas and aqueous solution phase

“…Differences in bond length are rarely greater than Table 1 Gas phase absolute electronic energy in atomic units (hartrees), and relative energies (kcal mol À1 ) for tropolone (species (1) and (2)) and 3,5-and 3,6-cycloheptadiene-1,2-dione (molecules (3) and (4), respectively), evaluated at HF and B3LYP levels of theory, with 6-311++G ** and aug-cc-pVDZ basis sets, as well as with the G3 method Table 2 Absolute and relative electronic energy in atomic units and kcal mol À1 , respectively, at HF and B3LYP levels of theory for the four molecules in aqueous solution using the Onsager dipole and polarizable continuum solvent reaction field models in combination with 6-311++G ** and augcc-pVDZ basis sets 0.02 Å while bond angles, for the most part, agreed to within a degree. The largest difference between aqueous and gas phase bond angles was found for: A (3,4,5) in (1) with value +3.3°, A (12,14,15) in (2) is À0.9°, A (2,13,11) in (3) is +1.2°, and A (5,7,9) in (4) is +1.5°. The B3LYP gas phase optimized geometry of tropolone agrees very well with previous B3LYP/6-31+G * [15] and MP2/6-31G ** [14] calculations.…”

“…In a number of chemical systems, however, this trend is not followed. Examples of recently studied [1][2][3][4][5][6][7][8] molecules in which the enol structure is observed to predominate include hydroxycyclopropenone [8] and phenol [6,7]. Ab initio calculations of the tautomeric equilibrium constant on the latter are found to be in excellent agreement with experiment, and confirm the exclusive presence of phenol, while high-level computations on the former species in both gas and aqueous solution phase predict that equilibrium is directed toward the enol rather than to 1,2-cyclopropanedione.…”

Comprehensive DFT Study on Site‐, Regio‐, and Stereoselectivity of Diels–Alder Reactions Leading to 5‐Hydroxybenzofurans

“…The experimentally measured K T values for the tautomeric equilibrium phenol p cyclohexa-2,4-dienone are 5.4 ϫ 10 12 in aqueous solution [22] and 1.4 ϫ 10 13 in the gas phase. [23] As …”

Assessment of Gaussian-3X theory for chlorinated organic molecules. Enthalpies of formation of chlorobenzenes and predictions for polychlorinated aromatic compounds

“…As already noted, in the recent years the enthalpies of formation of different polychlorinated aromatic compounds were predicted from B3LYP calculations using isodesmic reactions [8][9][10][11][12][13][14][15]. To estimate the f H • 298 values of PCDDs, the following reactions may be considered as the most appropriate:…”

“…The thermodynamic properties of chlorinated aromatic species are important in the kinetic modeling their formation during the processes of pyrolysis and oxidation. A large number of studies on the estimation of thermochemical properties of polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans, naphthalenes, and biphenyls has been published last years (see [8][9][10][11][12][13][14][15], and references cited therein). In these works, the enthalpies of formation were calculated using the B3LYP total energies and isodesmic reactions.…”