Statistical Thermodynamics Unveils How Ions Influence an Aqueous Diels-Alder Reaction.

The kinetics of Diels-Alder (DA) reactions in water has been known to be altered by salts for a long time. Yet the question how salts influence the reaction rate, either as rate-enhancing or rate-reducing additives, has so far remained unresolved. Conflicting hypotheses involve (i) indirect salt contributions through the modulation of internal pressure and (ii) making (or breaking) of the so-called "water-structure" by salts that strengthen (or weaken) the hydrophobic effect.