SN2 versus E2 reactions in a complex microsolvated environment: theoretical analysis of the equilibrium and activation steps of a nucleophilic fluorination.

The reactivity of the fluoride ion towards alkyl halides is highly dependent on the solvating environment. In polar aprotic solvents with large counter-ions is highly reactive and produces substantial E2 product, whereas in polar protic solvents leads to slow kinetics and high selectivity for SN2 reactions. The use of a more complex environment with stoichiometric addition of tert-butanol to acetonitrile solvent is able to module the reactivity and selectivity of tetrabutylammonium fluoride (TBAF). In the present work, we have performed a detailed theoretical analysis of this complex reaction system by density functional theory, continuum solvation model, and including explicit tert-butanol molecules. A kinetic model based on the free energy profile was also used to predict the reactivity and selectivity. The results indicated that the TBAF(tert-butanol) complex plays the key role to increase the SN2 selectivity, whereas higher aggregates are not relevant. The E2 product is formed exclusively via free TBAF, because the solvating tert-butanol in the TBAF(tert-butanol) complex inhibits the E2 pathway. Our analysis suggests that diols or tetraols could produce an improved selectivity.

Infinite dilution activity coefficient from SMD calculations: accuracy and performance for predicting liquid-liquid equilibria.

Prediction of liquid-liquid phase equilibria is an important goal in the physical chemistry of solutions. Quantum chemistry methods, combined with a dielectric continuum description of the solvent, has received attention as a first principle approach. In this work, the performance of the continuum solvation model based on density (SMD) for prediction of γ∞ in binary liquid mixtures, using 46 values of γ∞, was evaluated. We found the mean uncertainty of RTln γ∞ to be 0.92 kcal mol-1. Based on the calculated γ∞ and the two parameters of the Redlich-Kister expansion, we calculated the liquid-liquid phase equilibria. Based on 26 values of solubility, an uncertainty of 0.66 was found in the logarithm of molar fraction of the smallest component in each phase. Our results suggest this approach can be used for fast and semi-quantitative prediction of phase behavior. More reliable predictions could be obtained with improvements in the SMD model. Graphical abstract Prediction of liquid-liquid phase equilibriaᅟ.