This paper reports a theoretical study, at the B3LYP/6-31 R G(d,p) and M05-2X/6-31G R (d,p) levels, on the thermal decomposition of menthyl benzoate (2-isopropyl-5-methylcyclohexyl benzoate). It undergoes a unimolecular first-order elimination to give 3-menthene (1-isopropyl-4-methylcyclohexene), 2-menthene (3-isopropyl-6-methylcyclohexene), and benzoic acid. We studied two possible mechanisms trying to explain the formation of 2-and 3-menthene, via six-membered or four-membered cyclic transition states. Rate constants were calculated at two temperatures, 587.1 and 598.6 K, and they agree well with the experimentally determined values. We verify that 3-menthene is the product mainly formed at both temperatures. The progress of the reactions has been followed by means of the Wiberg bond indices. Intrinsic reaction coordinate (IRC) calculations have been carried out to verify that the localized transition state structures connect with the reactants and products and also to verify that the parent compound, menthyl benzoate, is taking the cis-configuration needed in the reaction. a Values calculated using 6-31þG(d,p) basis set. b A scaling factor [26] of 0.9804 for ZPE has been used. c Values taken from ref [6] .

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The mechanism of thermal decomposition of 4-hydroxy-2-butanone in m-xylene solution was studied experimentally and theoretically at the M05-2X/6-31G(d , p) level of theory. It follows first-order kinetics and appears to be homogeneous and unimolecular. The proposed mechanism is via a six-membered cyclic transition state to give a mixture of formaldehyde and acetone. Rate constant values were experimentally determined at three temperatures: 483.15, 493.15, and 503.15 K. Calculated rate constants are of the same order of magnitude than the experimental ones. Calculated Gibbs energies of activation agree very well with the experimental values. Computationally, the progress of the reactions was followed by means of the Wiberg bond index. The results indicate that the transition state has an intermediate character between reactants and products, and the calculated synchronicity shows that the reaction is slightly asynchronous. The bond-breaking processes are more advanced than the bond-forming ones, indicating a bond deficiency in the transition state.

The products and kinetics of the thermal decomposition of several methyl-β-hydroxyesters in m-xylene solution have been studied. It has been shown that all β-hydroxyesters studied pyrolyze to form a mixture of methyl acetate and the corresponding aldehyde or ketone and that the decomposition follows first-order kinetics and appears to be homogeneous and unimolecular. The rate pyrolysis of methyl-3-hydroxypropanoate, methyl-3-hydroxybutanoate, and methyl-3-hydroxy-3-methylbutanoate has been measured between 250 and 320 • C. The relative rates of primary, secondary, and tertiary alcohols at 553 K are 1.0, 8.5 and 54.1, respectively. The absence of large substituent effects indicates that little charge separation occurs during the breaking of carbon-carbon single bond. The activation entropy is compatible with a semipolar six-membered cyclic transition state postulated for other β-hydroxy compounds. C 2006 Wiley Periodicals, Inc. Int J Chem Kinet 39: 92-96, 2007

An experimental study of the thermal decomposition of a β-hydroxy alkene, 3-methyl-3-buten-1-ol, in m-xylene solution, has been carried out at five different temperatures in the range of 513.15-563.15 K. The temperature dependence of the rate constants for the decomposition of this compound in the corresponding Arrhenius equation is given by ln k (s −1 ) = (25.65 ± 1.52) − (17,944 ± 814) (kJ·mol −1 )·T −1 . A computational study has been carried out at the M05-2X/6-31+G(d,p) level of theory to calculate the rate constants and the activation parameters by the classical transition state theory. There is a good agreement between the experimental and calculated rate constants and activation Gibbs energies. The bonding characteristics of reactant, transition state, and products have been investigated by the natural bond orbital analysis, which provides the natural atomic charges and the Wiberg bond indices. Based on the results obtained, the mechanism proposed is a one-step process proceeding through a six-membered cyclic transition state, being a concerted and slightly asynchronous process. The results have been compared with those obtained previously by us (Struct Chem 2013, 24, 1811-1816 for the thermal decomposition of 3-buten-1-ol, in m-xylene solution. We can conclude that in the compound studied in this work, 3-methyl-3-buten-1-ol, the effect of substitution at position 3 by a weakly activating CH 3 group is the stabilization of the transition state formed in the reaction and therefore a small increase in the rate of thermal decomposition. C