High-Pressure Kinetic Studies of Solvent and Substituent Effects on Diels-Alder Reactions

Abstract

The radiofrequency (1-20 MHz) ozonizer discharge or "chemical corona" has been compared in its operation in oxygen to the same device operated at 60 Hz. Comparable chemical, energetic, and electrical data were taken. In the high-frequency regime where the behavior of the discharge is dominated by a steady-state accumulation of excess positive ions in the gap, the best energy efficiency is several orders of magnitude worse than at low frequencies. A simple model, assuming minimization of the rate of production of entropy, relating the electrical parameters and the observed contraction of the discharge is seen to account for this difference in chemical efficiency. Regions of E/p sufficiently high to support the underlying ionization avalanche mechanism and to create ozone molecules are seen to be confined to layers comparable in thickness to the Debye length adjacent to the electrode (barrier) and thin compared to the aerodynamic boundary layer which probably accounts for the remarkable insensitivity of this discharge to flow rate.

Extreme solvent effects on reaction rate constants at supercritical fluid conditions

“…This result is similar to those of high-pressure kinetic studies in the liquid state in that a change in pressure of many hundreds of bars is required to achieve a significant change in the rate constant. For example, the rate constant for the DielsAlder reaction of chloroprene and maleic anhydride changes only by a factor of 5.8 in acetone at 65OC for a change in pressure of 1,400 bar (McCabe and Eckert, 1974).…”

Why can the activation volume of the cycloadduct decomposition in isopolar retro‐Diels–Alder reactions be negative?

“…Considering DARs as an isopolar process 1,2,9, it is difficult to explain why the transition state with a partial CC bonding should be more compact than that of the cycloadduct 10–16.…”

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].…”