Compressed carbon dioxide, as a solvent medium, offers unique benefits for the synthesis of ionic liquids (ILs) especially for the ability to combine reaction and separation. The second-order reaction rate constants were determined for the synthesis of 1-hexyl-3-methylimidazolium bromide ([HMIm][Br]) at 313.15 K and 333.15 K and 30-150 bar from the reactants, 1-methylimidazole and 1-bromohexane. Two different scenarios were encountered based upon the pressure: a one-phase supercritical mixture of the reactants becoming a vapor-liquid IL/CO2 system at complete conversion; or a two-phase CO2-expanded liquid (CXL) that yielded a three-phase vapor-liquid-liquid equilibrium (VLLE) at intermediate conversions. Despite multiphase phenomena, the vast majority of the reaction occurs in the reactant-rich liquid phase. The kinetic rate constants were comparable with traditional organic solvents. The reaction rates were higher in the CXL mode than in the initially supercritical mixtures. The changes in the kinetic rate constant with pressure (CO2 composition) could be explained and quantified using Kamlet-Taft (KT) polarity parameters that were measured in the mixtures using solvatochromic probes. (C) 2014 Elsevier B.V. All rights reserved.