Effect of solvent on the free energy of activation of S_N2 reaction between phenacyl bromide and amines

The kinetics of S_N2 reaction between phenacyl bromide and various amines in 12 different solvents were studied. Solvent effects on the rate of this reaction and free energy of activation, ΔG^#, were interpreted by applying the Abraham-Kamlet-Taft (AKT) equation. The solvent polarity (π₁*), solvent hydrogen-bond basicity (β₁) and Hildebrand cohesive density energy (δ²_H) are those parameters which increase the rate constant and decrease ΔG^#, while solvent hydrogen-bond acidity (α₁) will have the compensatory effect. A comparison among obtained values of second rate constants, k₂ for different amines in a given solvent indicates that the amine reactivities are highly dependent on their structures. The consequent decrease of the rate constant for different amines in any given solvent was found to be: primary > secondary > tertiary. This order results from steric effects of amines.