Acid-Base Reactions between Amines and Carboxylic Acids in Hexane

The reactions of carboxylic acids (HX) and amine bases (B) were studied in the solvent hexane at 25° using the differential vapor pressure method and infrared spectroscopy. In general, the following equilibria are required to interpret the experimental results: 2HX rlhar2; (HX)²; B + HXrlhar2; (1/m)(BH⁺X⁻)_m; and (1/m)(BH⁺_- X⁻)_m + HX rahar2;(1/n)(BH+HX₂⁻)_n. Detectable association of cyclohexyl- and n-hexylamines occuss at the 0.01 M level, while trichloroacetic and benzoic acids appear to be quantitatively dimerized at the same concentration. Some evidence for the BH⁺H₂X₃⁻ species is given. The results in hexane are compared to a previous study by Bruckenstein and Saito¹ in benzene as solvent. Compared to hexane, in benzene values of m and n are smaller, and trichloroacetic acid is far less dimerized, clearly indicating that benzene is capable of solvating the species participating in the acid-base and concurrent equilibria.