are entirely consumed. In practice, many systems are well approximated as operating under kinetic control, due to negligibly slow equilibration. For example, many enantioselective catalytic systems provide nearly enantiopure product (> 99% ee), even though the enantiomeric products have the same Gibbs free energy and are equally favored thermodynamically. Under pure thermodynamic reaction control, when the equilibrium has been reached, the product distribution will be a function of the stabilities G°. After an infinite amount of reaction time, the ratio of product concentrations will equal the equilibrium constant Keq and therefore be a function of the difference in Gibbs free energies, (equation 2) In principle, "pure" thermodynamic control is also impossible, since equilibrium is only achieved after infinite reaction time. In practice, if A and B interconvert with overall rate constants kf and kr, then for most practical purposes, the change in composition becomes negligible after t ~ 3.5/(kf + kr), or approximately five half-lives, and the system product ratio can be regarded as the result of thermodynamic control. In general, short reaction times favour kinetic control, whereas longer reaction times favour thermodynamic reaction control. Low temperatures will enhance the selectivity under both sets of conditions, since T is in the denominator in both cases. The ideal temperature to optimise the yield of the fastest-forming product will be the lowest temperature that will ensure reaction completion in a reasonable amount of time. The ideal temperature for a reaction under thermodynamic control is the lowest temperature at which equilibrium will be reached in a reasonable amount of time. If needed, the selectivity can be increased by then slowly cooling the reaction mixture to shift the equilibrium further toward the most stable product. When the difference in product stability is very large, the thermodynamically controlled product can dominate even under relatively vigorous reaction conditions. If a reaction is under thermodynamic control at a given temperature, it will also be under thermodynamic control at a higher temperature for the same reaction time. In the same manner, if a reaction is under kinetic control at a given temperature, it will also be under kinetic control at any lower temperature for the same reaction time. If one presumes that a new reaction will be a priori under kinetic control, one can detect the presence of an equilibration mechanism (and therefore the possibility of thermodynamic control) if the product distribution: changes over time, shows one product