Linear Correlation of the Basicity of Aromatic Compounds with Σσ⁺ Constants in the Hydrogen Chloride‐Aluminum Chloride System

In an automatic hydrochlorinator, twenty‐four aromatic compounds including halogen and methoxy substituted compounds were complexed with hydrogen chloride and aluminum chloride. Relative basicities of these compounds to toluene were measured. Except those methoxy substituted compounds and few weakly basic compounds, the relative basicities of nineteen compounds were found to correlate linearly with the summation of σ₊ constants. Thus, this technique provide a simple approach to the analysis of aromatic basicity. While hexamethylbenzene was found to be 15 × 10⁰ time more basic than was toluene, benzene was found to be only FIVE time less basic than was toluene. From the rate of chlorination, benzene was estimated to be 1300 time less basic than was toluene according to the literature report. Moreover, the resultant complexes are composed of three types of structures. In the weakly basic region, a π‐complex is proposed. In the medium basic range, the σ‐complex is attributed to. the formation of a dimeric aluminum chloride‐hydrogen chloride complex. In the strongly basic region, the aromatic compound complexes with a monomeric aluminum chloride‐hydrogen chloride to form a stable σ‐complex.