Reaction of Chloride Ion with Isopropyl Bromide at Atmospheric Pressure by Ion Mobility Spectrometry

The S N 2 nucleophilic displacement reaction of chloride ion with isopropyl bromide ( i -PrBr) has been studied in nitrogen buffer gas at a pressure of 640 Torr over the temperature range 20−175 °C by ion mobility spectrometry (IMS). It is concluded that, under these conditions of relatively high buffer gas pressure, this nucleophilic displacement reaction occurs primarily by the distinctly two-step mechanism Cl - + i -PrBr ⇌ Cl - ( i -PrBr) → Br - + i -PrCl in which a thermal energy ion complex, Cl - ( i -PrBr), is maintained in a state of chemical equilibrium with the reactants. An S N 2 displacement reaction then occurs within the thermal energy cluster ion by its unimolecular conversion to products. Equilibrium constants, K 1, and rate constants, k 1, for Cl - + i -PrBr ⇌ Cl - ( i -PrBr) and Cl - ( i -PrBr) → Br - + i -PrCl, respectively, are determined from the IMS spectra as a function of temperature. In addition, second-order clustering to form the ion complex Cl - ( i -PrBr) 2 is also observed, and equilibrium constants, K 2, for this process are also obtained from IMS spectra. By these measurements, the major features of the potential energy surface for this reaction are characterized and place its S N 2 transition state at 1.6 kcal mol -1 above the energy of the reactants.