Metamorphosis of a transition state into a stable species

Medium variations usually affect the shape of the bimolecular nucleophilic reaction profile at the reactants' and products' ends and, to a much lesser extent, the shape around the transition state. In water, the reactions of extended allylic systems such as have been computationally shown (for n = 2) to have a single transition state. As the polarity is decreased the transition state is gradually transformed into a double-humped profile that then changes smoothly through a triple-well profile into a single-well profile where the symmetric structure of the transition state is retained. The depth of the well is ca. 16 kcal/mol for n = 2 and reaches 40 kcal/mol for n = 7, resembling the stability of a weak chemical bond. This is traced to electrostatic effects as well as to the effect of an intermediate VB configuration. In the analogous polyynes, a stable adduct is already formed at n = 1. This is attributed to the formation of the relatively stable vinylic carbanion. As the number of acetylene units increases, the vinylic geometry (a CCC angle of 123) is gradually lost until at n = 5 the adduct attains a linear geometry.