Amine hydrochloride salts : a problem in polyurethane synthesis

A major problem encountered during the industrial synthesis of isocyanates, is the loss of amine starting material through reaction with the hydrogen chloride (HC1) by-product. HC1 is formed by the phosgenation of polymeric amines and also by the subsequent decomposition of the carbamyl chloride, Equation (1). HC1 readily reacts with the polymeric amine to form an unwanted and highly insoluble amine hydrochloride salt. (Fig. 3620) Methylene dianiline (MDA) and 4-benzylaniline (4-BA) were used as models for the industrial amine starting material with their hydrochloride counterparts methylene dianiline dihydrochloride (MDA.2HC1) and 4-benzylaniline hydrochloride (4-BA.HC1) as models for the industrial waste material. To understand the forces controlling the structure and stability of these solid amine hydrochloride salts the sold state structures of MDA, MDA.2HCl, methylene dianiline monohydrochloride (MDA.HCl), 4-BA and 4-BA.HCl were investigated using single crystal X-ray diffraction which led to the determination of their lattice energies. The XRD studies were also used as a basis for Density Functional Theory (DFT) calculations. The information obtained from the solid state structures, combined with an investigation of the solution phase behaviour of 4-BA.HCl(s), resulted in the determination of a kinetic model for the recovery of amine starting material. The proposed reaction scheme describes the conditions under which the dissociation of 4-BA.HCl(s) can occur. Reaction in a closed system shows no amine production while reaction in an open system permits, within solubility limits, the complete consumption of solid waste to produce free amine. From these two extremes the conversion of waste hydrochloride salt in the industrial reactor can be rationalised.