Hydride transfer reactions of Michler's hydride with different π‐accetors

The hydride transfer reactions of 4,4′‐bis(dimethylaminophenyl)methane (Michler's hydride) with p‐benzoquinones were studied. The rate of formation of Michler's Hydrol Blue was followed spectrophotometrically. The second‐order rate constants and the activation parameters were estimated. The formation of a charge‐transfer complex was observed at low temperatures. Stable and unstable paramagnetic species formed during the reactions were assigned by ESR spectroscopy. The observed kinetic behaviour and the stoichiometry were in line with those previously obtained for the systems involving Leuco Crystal Violet, Leuco Malachite Green and Leuco Bindschedler's Green. Hence the reaction is considered to proceed according to the so‐far accepted multi‐step mechanism. However, when tetracyano‐p‐quinodimethane or tetracyanoethylene was used as a π‐acceptor, a comparatively stable radical ion pair was formed as a result of a one‐electron transfer, followed by the gradual formation of Michler's Hydrol Blue. The kinetic behaviour and the stoichiometry of the reaction were examined, together with the enhanced kinetic effects of added triethylamine. A modified mechanism for these systems is proposed. The role of cyanomethylenes was found to be essentially different from that of p‐benzoquinones, and therefore π‐acceptors are divided into two groups of cyanomethylenes and p‐benzoquinones.