A structural and spectroscopic investigation of the hydrochlorination of 4,4′-methylenedianiline

Abstract

The hydrochlorination of 4,4′-methylenedianiline, NH₂C₆H₄CH₂C₆H₄NH₂ (MDA), in chlorobenzene to produce 4,4′-methylenedianiline dihydrochloride, [H₃NC₆H₄CH₂C₆H₄NH₃]Cl₂ (MDA·2HCl) is an important reaction for the production of isocyanates, which are used to manufacture polyurethanes. This reaction is examined here. MDA is moderately soluble in chlorobenzene, whereas MDA·2HCl is effectively insoluble. Controlled addition of anhydrous HCl to MDA in chlorobenzene led to the isolation of a solid whose stoichiometry is MDA·HCl. Crystals obtained from solutions of MDA·HCl in methanol were found by X-ray analysis to consist of the basic hydrochloride salt, [MDAH₂][Cl]₂[MDA]₂H₂O, which is stabilised by complex hydrogen-bonding. The starting material MDA has an H-bonded structure in which the molecules are linked in a one-dimensional chain. Hydrogen-bonding is extensive in MDA·2HCl which contains ladders of [H₃NC₆H₄CH₂C₆H₄NH₃]²⁺ dications stabilised by N–H⋯Cl linkages. Energy calculations on the crystalline systems allow an identification of the main factors in intermolecular cohesion; these are related to melting temperature and solubility data. Such improvements in understanding of solute–solute interactions are prerequisites for improving the atom economy of this important stage within the polyurethane manufacture process chain. The solid phase IR spectrum of MDA·2HCl is diagnostic, principally as a result of a Fermi resonance process.