Predictive Force-Field Calculations for the Equilibrium Dimerization of Isoprene

Abstract

The Diels-Alder dimerization of isoprene is an important reaction; dipentene (or racemic limonene), one of the products formed in this reaction, has major applications in the manufacture of polymers and adhesives.' Dipentene also has various uses in the food and pharmaceutical industries.' In the present work, the QCFF force field program2 was used to calculate gas-phase thermodynamic properties of the monomer (isoprene) and the dimers 1 -methyl-5-( 1 -methylethenyl)cyclohexene (diprene) and 1 -methyl-4-( 1 -methylethenyl)cyclohexene (dipentene) for the temperature range 298.15-1000 K. These QCFF-calculated thermodynamic values were compared, when possible, with corresponding values obtained experimentally or from other force field programs, and the agreement was found to be satisfactory. The QCFF values were further used to derive gas-phase equilibrium properties-AHO, ASo, AGO, and &-for the isoprene dimerization reaction at various temperatures.These computational results suggest that, based upon thermodynamics, diprene and dipentene are about equally favored dimerization products of isoprene and that reported experimental data reflect kinetic control of dipentene formation at lower temperatures. The equilibrium thermodynamics data for the isoprene dimerization reaction presented in this paper are felt to be sufficiently reliable that they can be utilized in the absence of any additional experimental data; the present work thus illustrates the power of predictive thermodynamic computational techniques in extending the thermochemical database. IntroductionIn the recent past, a large number of force-field methods have been developed to predict structures, vibrational frequencies, and energies for a wide range of isolated molecules. However, very few of these force-field methods have been demonstrated to accurately compute gas-phase So and Cop values. Previously, MOLDB3 (Boyd and co-workers)' and more recently QCFF (Warshel and Karplus)2 and MM3 (Allinger and co-~orkers)~

Tuning the product distribution during the catalytic pyrolysis of waste tires: The effect of the nature of metals and the reaction temperature

“…A computational study carried out by Kar et al [45] suggested that diprene and d,l-limonene are products that are equally favored by isoprene dimerization; therefore, since diprene was not observed as a product and limonene selectivity was always high in our reactions, this indicate that intramolecular cyclization was the favored reaction in this case. Menares et al [13] suggested this reaction route because when the pyrolysis temperature increased above 600 °C, a notable decrease in the amount of isoprene was observed with a simultaneous increase in the amount of limonene.…”

“…Another reaction route for the production of limonene that has been argued by some authors and also reported by us [13] is the dimerization of two isoprene monomers via a Diels-Alder addition reaction [25]. However, this mechanism is J o u r n a l P r e -p r o o f controversial, since it can also lead to significant amounts of 1-methyl-5-(1-methylethenyl) cyclohexene (diprene) [45], which was not observed within the reaction products (Table 3).…”

A review of dipentene (dl-limonene) production from waste tire pyrolysis

“…Indeed, Kar et al has modeled the dimerization of isoprene using predictive force-field calculations [26]. It was calculated that between 25 and 1000 • C the conformer diprene was the energetically more favorable product of the dimerization, not dipentene.…”

Dimerization and Polymerization of Isoprene at High Pressures

“…Among the different isomers that can be obtained by the dimerization of isoprene, the main products are those resulting from a [4π + 2π] cycloaddition: the 1-methyl-4-(1-methylethenyl)-cyclohexene, commonly called limonene or dipentene, and 1-methyl-5-(1-methylethenyl)-cyclohexene, also known as sylvestrene or diprene. Only the latter forms near room temperature, whereas a temperature increase results in a lower selectivity of the reaction and both dimers are obtained. , A pressure increase seems to have the same effect as a temperature decrease, favoring the formation of sylvestrene, but this result is still controversial. The final product of the dimerization is actually fundamental to understand the structure of the diradical intermediate.…”