Kinetics of the Decomposition of a Mannich Base

Abstract

The elimination reaction of 3-(4-o-methoxyphenyl-1-piperazinyl)-4′-morpholinopropiophenone dihydrochloride (Su 17595A) was studied over the pH range of 1–10.It appears that only the unprotonated form of the Mannich base undergoes elimination in the pH range of 3–10.The observed rate constant for elimination as a function of pH corresponds well with the dissociation curve for the compound.The value k1= 7.15 × 10−2min.−1(60°) was obtained for the specific rate constant for elimination.The elimination reaction is reversible, and the extent of reaction is also a function of pH because the reverse reaction, the addition of 1-(o-methoxyphenyl)-piperazine to 4′-morpholinoacrylophenone, proceeds only with the unprotonated amine.The reaction does not remain at equilibrium, because competitive (e.g., desaminomethylation) and/or consecutive (e.g., hydration and dimerization) reactions also can occur.In strong acid solution, the elimination of the protonated form is acid catalyzed.