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

Abstract

The SN2 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 SN2 displacement reaction then occurs within the thermal energy cluster ion by its unimolecular conversion to products. Equilibrium constants, K1, and rate constants, k1, 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, K2, for this...