Eschweiler–Clarke Reaction

Eschweiler–Clarke reaction also called the Eschweiler–Clarke methylation is a chemical reaction whereby a primary (or secondary) amine is methylated using excess formic acid and formaldehyde. This reductive amination reactions not produce quaternary ammonium salts, but instead stop at the tertiary amine stage.

Mechanism of Eschweiler–Clarke Reaction

From this mechanism it is clear that a quaternary ammonium salt will never form, because it is impossible for a tertiary amine to form another imine or iminium ion.