Solvent effects on the activation volume of a Diels-Alder reaction

Abstract

The kinetics of the isoprene+maleic anhydride reaction have been studied in various solvents at pressures up to 1400 atm. The solvent effect on the activation volumes and energies at 35°C are reported. It is shown how the solvent effect on activation volumes is useful both for mechanistic interpretation as well as for determination of the dipole moment of the transition state.

Anomalies in the Change of Volumetric Parameters of the Diels–Alder Reaction in Solution

“…For this reaction in other solvents it was found that V = retro < 0 and V = forw / V r-n > 1 [11,31]. In the DAR with maleic anhydride, the "normal" relationship V = retro > 0 and V = forw / V r-n < 1 in acetonitrile and ethyl acetate, and the "abnormal" relationship in other solvents V = retro < 0 and V = forw / V r-n > 1, were obtained [18][19][20]. Different solvent accessibility is the reason for the large differences in the values of PMV of branched structures of adamantane (136.9 and 142.7) and congressane (160.2 and 177.4 cm 3 ·mol −1 ) in n-hexane and benzene [32].…”

“…There are only a few direct measurements of the pressure influence on the rate of adduct decomposition [8,[15][16][17]. Why the AC in isopolar DAR should be more compact than the cycloadduct is not yet understood [15][16][17][18][19][20].…”

“…It was assumed that these interactions could cause an additional volume contraction in the AC and as a result the volume ratio is V = forw / V r-n > 1 [1,18]. It is well established [5][6][7]20] that the value of the volume change due to secondary attractive interactions should be only about 1-2 cm 3 ·mol −1 and cannot explain this result.…”

Gas chromatography

“…Modification of the injection port of a commercial gas chromatograph enabled it to be used as a microreactor suitable for catalyst studies a t temperatures as high as 400 "C (666). Other kinetic studies include the direct determination of the kinetics of gas phase thermolysis reactions (669), measurement of the rate of reversible adsorption of hydrogen on a nickel catalyst (846), study of solvent effects on the activation volume of the Diels-Alder reaction of isoprene and maleic anhydride at pressures up to 1400 atmospheres (379), and evaluation of the rate constant and activation energy for decomposition of IBR in the Brz-IBr-12 system (429).…”

High-Pressure Influence on the Rate of Diels-Alder Cycloaddition Reactions of Maleic Anhydride with Some Dienes

“…Solvent electrostriction in this reaction can be neglected due to the small effect of solvent polarity on the enthalpy of activation (∆ H ≠ av = 52 ± 2 kJ mol −1 ), the volume of activation (∆ V ≠ av. = –38.5 ± 1.0 cm 3 mol −1 ), and the reaction volume (∆ V r‐n, av = –35.5 ± 1.0 cm 3 mol −1 ) . The “abnormal” ratio of ∆ V ≠ /∆ V r‐n > 1 corresponds to the higher volume compactness of the activated complex with incomplete bond formation as compared to the adduct where bond formation is entirely completed.…”