Mechanism Of The Aqueous Rate Acceleration

Several factors have been invoked to explain the aqueous rate acceleration: aggregation of the reactants leading to micellar catalysis, effects connected with the internal pressure of the solvent, polarity of the solvent, H-bonding interactions with the solvent, and hydrophobic interactions (D#V < 0). The initial literature was rather controversial, and there was a strong need for a systematic study using physical-organic techniques.

Several experiments gave evidence against homotactic or heterotactic association of diene and dienophile.8,9 Vapor-pressure measurement for cyclopentadiene (CPD) (Figure 7.2), in pure water and in 10% (w/w) n-PrOH/water, show that Henry's law is obeyed (vapor pressure varies linearly with solute concentration) until [CPD] is 0.03 M in pure water and 0.06 M in n-PrOH/water. In kinetic measurement the concentration of CPD was always below 0.002 M, indicating that association is highly unlikely. Similar results have been obtained with methyl vinyl ketone, ethyl vinyl ketone, and naphthoquinone.

It was also observed that the second-order rate constants are independent of the concentration of CPD, even in excess, supporting the notion that the

Peak areax 10-3

Peak areax 10-3

Figure 7.2 Plots of the peak area of cyclopentadiene, obtained after injection of a standard volume of vapor, withdrawn from the vapor above an aqueous solution of cyclopentadiene as a function of the molality of cyclopentadiene, at 25°C; solution of cyclopentadiene in pure water, b, and solution of cyclopentadiene in aqueous solution, containing 10% (w/w) of ethanol, •.

Figure 7.2 Plots of the peak area of cyclopentadiene, obtained after injection of a standard volume of vapor, withdrawn from the vapor above an aqueous solution of cyclopentadiene as a function of the molality of cyclopentadiene, at 25°C; solution of cyclopentadiene in pure water, b, and solution of cyclopentadiene in aqueous solution, containing 10% (w/w) of ethanol, •.

Figure 7.3 Standard Gibbs energies of transfer for reactants and activated complex for the Diels-Alder reaction of cyclopentadiene (1,B) with ethyl vinyl ketone (2,O) from 1-PrOH to 1-PrOH- water as a function of the mole fraction of water; initial state (1 + 2,«); activated complex (o).

Figure 7.3 Standard Gibbs energies of transfer for reactants and activated complex for the Diels-Alder reaction of cyclopentadiene (1,B) with ethyl vinyl ketone (2,O) from 1-PrOH to 1-PrOH- water as a function of the mole fraction of water; initial state (1 + 2,«); activated complex (o).

Gibbs energy

Cyclopentadiene Diels Alder Diene

Figure 7.4 The cause of the rate acceleration of (most) Diels-Alder reactions in water.

reaction coordinate

Figure 7.4 The cause of the rate acceleration of (most) Diels-Alder reactions in water.

acceleration of the Diels- Alder in water is not dependent on homotactic/hetero-tactic association. Intramolecular Diels-Alder reactions are also greatly accelerated in water, again indicating that association of diene and dienophile is not responsible for the aqueous rate acceleration.

The pseudothermodynamic analysis of solvent effects in 1-PrOH-water mixtures over the whole composition range (shown in Figure 7.3) depicts a combination of thermodynamic transfer parameters for diene and dienophile with isobaric activation parameters that allows for a distinction between solvent effects on reactants (initial state) and on the activated complex. The results clearly indicate that the aqueous rate accelerations are heavily dominated by initial-state solvation effects. It can be concluded that for Diels-Alder reactions in water the causes of the acceleration involve stabilization of the activated complex by enforced hydrophobic interactions and by hydrogen bonding to water (Table 7.1, Figure 7.4).10

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