The Effect of Water and Additives on Chemical Reactivity

The influence of water as solvent on the rate of Diels-Alder reactions is likely the best example of the close relationship between the unique properties of liquid water and its strongly accelerative effect. This is probably due to the large negative value of the activation voliune of Diels-Alder reactions this value (ca. 30 cm mol ) is even somewhat more negative in water versus organic solvents [28]. 

The implication of hydrophobic effects rather than polar effects was further demonstrated by (i) a deviation from linearity when correlating the Gibbs 

In the retro-Diels-Alder reaction of anthracenedione [42], the volume of activation is small. Acceleration in water cannot come from a change in the hydration shell of the molecule. Hydrophobic interactions are negligible and aqueous acceleration is caused by the hydrogen-bond donating ability of water, which stabilizes the polarized activated complex. The Gibbs energy of activation displays a fair linear correlation with the Ej parameter. Hexafluoroisopropanol with an Ej value of 65.3 is even more efficient as a solvent than water Ej= 63.1) which appears to be less polar [41]. 

As for surfactants, they have uncertain, sometimes contradictory, consequences on reaction rates [45], but the main advantage of using surfactants as additives lies in their solubilizing effect. Special attention has been paid to the rate-accelerating effect of Lewis acid catalysts. The first study deals with the Diels-Alder reaction between cyclopentadiene and a bidentate dienophile a large acceleration can be achieved by the combined use of copper(II) nitrate as catalyst and water as solvent. The rate enhancement imposed on the catalyzed Diels-Alder reaction is much less pronounced than that for the uncatalyzed reaction 

Methylrhenium trioxide (CH3Re03) has proved to be an excellent catalyst in organic solvents, and in water when the dienophile is an a, -unsaturated ketone (or aldehyde). Nearly exclusively one product isomer was formed, the same one that usually predominates [47]. Likewise, scandium triflate [48] and indium trichloride [49] were found to catalyze the Diels-Alder reaction in a tetrahydro-furan/water mixture and in pure water, respectively. 

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