Thermal instability of 1-butyl-3-methylimidazolium chloride ionic liquid

[Display omitted] •Evaporation of [BuMIm+][Cl-](l) is accompanied by thermolysis and C2-methylation to form in situ new ion pairs.•At a low rate of thermolysis the pressures and the enthalpy of evaporation of [BuMIm+][Cl-] ion pairs are determined.•The suppression of [BuMIm+][Cl-] evaporation is a consequence of an increase involvement of ion pairs in side reactions.•The priority and possible directions of reactions involving ion pairs were revealed using quantum chemical calculations. The evaporation and thermal decomposition (thermolysis) of 1-butyl-3-methylimidazolium chloride ionic liquid, [BuMIm][Cl](l), were studied in the temperature range of 373–463 K by thermogravimetry–mass spectrometry (TG–MS) and Knudsen cell mass spectrometry (KCMS) methods. The saturated vapor pressures and the enthalpy of vaporization of [BuMIm][Cl](l) were determined within 373–398 K, where the thermolysis of this ionic liquid occurs via the SN2 mechanism at a low rate. With increasing temperature, the thermolysis becomes more extensive, which is accompanied by the C2-methylation of [BuMIm][Cl](l). The resulting ion pairs [BuMMIm+][Cl-] are thermally unstable and decomposed. A decrease in the vapor pressure of the ion pairs [BuMIm+][Cl-] at a constant temperature takes place due to side reactions, resulting in the suppression of the evaporation. Complex ionic compounds were found in the thermolysis/evaporation residues using atmospheric pressure chemical ionization (APCI) and matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) methods. The priority and possible pathways of reactions involving ion pairs and the thermolysis products were established by quantum chemical calculations.