THERMODYNAMIC BASIS OF HYDROGEN-BONDING EFFECTS ON REACTIVITY OF NUCLEOPHILES WITH RETINYL AND TRIARYLMETHYL CARBENIUM IONS

The enthalpies and free energies for hydrogen bonding have been determ ined between a series of acceptors (anionic and neutral nucleophiles) and donors (water, 2,2,2-trifluoroethanol, and 4-nitrophenol) in dipol ar aprotic solvents (acetonitrile, dimethylformamide, acetone, and dim ethyl sulfoxide). The calorimetric measurements correspond to conditio ns under which the rates of coordination between carbenium ions and th e nucleophiles were collected. The relationship between the free energ y of nucleophile hydrogen bonding, DELTAG(HB), and the change in free energy of activation in the absence and presence of nucleophile hydrog en bonding, DELTADELTAG(rxn), depends on the nature of the carbocation s. Poor electrophiles like those that adhere to Ritchie's N+ scale (e. g., crystal violet, malachite green, and trianisyl cation) show a DELT ADELTAG(rxn) that exactly matches the hydrogen bonding free energy. Ca tions like trityl or retinyl are less sensitive to the degree that the nucleophiles are complexed with the hydrolytic solvents. Plots of the rates of reaction between the cations and nucleophiles (as log k(Nu)) versus Swain-Scott/Pearson n values are linear and provide a selectiv ity measure for the group of cations more reactive than those that adh ere to the N+ scale and can be used to show that the two groups of car benium ions have different rate determining steps. These observations appear to be general for all carbocations and are discussed in terms o f S(N)1 mechanistic reactivity and selectivity.