Kinetic studies on the reaction of formaldehyde with amines in the presence of sulfite

Abstract

The reaction of formaldehyde with amines with and without sulfite has been studied using anilines (RC(_6)H(_4)NH(_2)) and benzylamines (RC(_6)H(_4)CH(_2)NH(_2)). Reaction with anilines is known to produce aminomethanesulfonates, RC(_6)H(_4)NHCH(_2)SO(_3)(^-), which are industrially important in the azo dye industry. The kinetics and mechanism of formation of RC(_6)H(_4)NHCH(_2)SO(_3)" and RC(_6)H(_4)CH(_2)NHCH(_2)SO(_3) have been studied: rate constants are quoted for the individual reaction steps in addition to the overall reaction. The reaction of formaldehyde, HCHO, with the amine, RNH(_2), gives an N-(hydroxymethyl)amine, RNHCH(_2)OH, via a zwitterionic intermediate. Rate constants of 8.0 x 10(_3) to 4.3 x 10(^6) dm(^3) mol(^-1) have been obtained. RNHCH(_2)OH then dehydrates in acidic conditions or loses hydroxyl ion to form an iminium ion, [RNH=CH(_2)](^+). This then reacts rapidly with sulfite ions to yield the product. The rate determining step was found to depend on the pH of the reaction. At low and neutral pH the reaction of HCHO with RNH(_2) is the rate determining step. At high pH the rate determining step becomes dehydration of RNHCH(_2)OH to give [RNH=CH(_2)](^+). Hydroxymethanesulfonate, CH(_2)(OH)(S0(_3)Na), was used to introduce HCHO and sulfite, SO(_3)(^2-) , into the system. This must undergo decomposition initially to yield reactive free HCHO. Above pH 3 decomposition occurs mainly through the dianion, CH(_2)(0')(S0(_3)'(^-)) Below pH 3, decomposition through the monoanion, CH(_2)(OH)(S0(_3)') forms the major pathway: this may become the rate determining step in the overall reaction at low pH. Rate constants for decomposition equal to 24 ± 5 s(^-1) and 2.3 x 10(^-8) s(^-1) have been obtained for the dianion and monoanion respectively. pK(_a) values in the range 4.9 to 5.6 have been measured for protonated adducts, RN(^1)H(_2)S0(_3)(^-), formed from benzylamines. With benzylamines, reaction with another molecule of CH(_2)(OH)(S0(_3)Na) can occur to produce RN(CH(_2)S0(_3)(^-))(_2) in addition to RNHCH(_2)SO(_3)(^-). Polymerisation of imines has also been studied: cyclic trimers and 1 : 2 HCHO : RNH(_2) adducts have been synthesised.