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 F(-) + H-(CH=CH)n-CH2-F → F-CH2-(CH=CH)n-H + F(-) 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.