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The hydrochlorination of 4-benzylaniline in chlorobenzene to produce 4-benzylaniline hydrochloride has been examined. This has required spectroscopic and computational analysis of the solvation of gaseous HCl in the process solvent. The characterisation of the reagent and product of the hydrochlorination reaction by various techniques, including FTIR and (1)H NMR spectroscopy and X-ray diffraction, is described. The infrared spectrum of the hydrochloride salt contains a strong Fermi resonance interaction, readily distinguishing it from that of the starting material. Using the structural results as a basis, the lattice energies of reagent and product have been evaluated by the recently developed PIXEL method. This method allows the contributions of specific intermolecular interactions to the total lattice energy to be assessed and, in this case, tentatively correlated with solubility measurements.

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The hydrochlorination of 4,4'-methylenedianiline, NH(2)C(6)H(4)CH(2)C(6)H(4)NH(2) (MDA), in chlorobenzene to produce 4,4'-methylenedianiline dihydrochloride, [H(3)NC(6)H(4)CH(2)C(6)H(4)NH(3)]Cl(2) (MDA x 2 HCl) 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 x 2 HCl is effectively insoluble. Controlled addition of anhydrous HCl to MDA in chlorobenzene led to the isolation of a solid whose stoichiometry is MDA x HCl. Crystals obtained from solutions of MDA x HCl in methanol were found by X-ray analysis to consist of the basic hydrochloride salt, [MDAH(2)][Cl](2)[MDA](2)H(2)O, 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 x 2 HCl which contains ladders of [H(3)NC(6)H(4)CH(2)C(6)H(4)NH(3)](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 x 2 HCl is diagnostic, principally as a result of a Fermi resonance process.