Nj. Pienta et Rj. Kessler, THERMODYNAMIC BASIS OF HYDROGEN-BONDING EFFECTS ON REACTIVITY OF NUCLEOPHILES WITH RETINYL AND TRIARYLMETHYL CARBENIUM IONS, Journal of the American Chemical Society, 115(18), 1993, pp. 8330-8339
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.