Effect of reaction pH and C uSO 4 addition on the formation of catechinone due to oxidation of (+)‐catechin

Synopsis A novel hair dyeing technique being milder and safer for a human body is desired. The oxidation product of (+)‐catechin, catechinone, was invented as a safer dyestuff for hair colouring under such the situation. The preparation of catechinone by a chemical oxidation is a practical way and the objective of the study is clarify the effect of the solution p H and in the presence or absence of C u 2+ on the formation rate and yield of catechinone in order to improve the efficiency of the dye formation. The catechinone formation was monitored by ultraviolet‐visible spectroscopy. Catechinone was prepared chemically from (+)‐catechin in aqueous solution with O 2 gas introduced over a p H range of 7.1–11.7. The rate and amount of the dye formation increase with increasing p H . Dissociation of the hydroxyl group of the catechol part of (+)‐catechin is significant for the oxidation of (+)‐catechin and promotes the dye production. This is because the deprotonated (+)‐catechin has a higher reactivity with O 2 . The production of catechinone is accelerated by the addition of C u SO 4 and the production rate reaches the maximum at p H  = 8.8. (+)‐Catechin ‐ C u 2+ complexes are formed and the formation promotes the oxidation of the catechol part of (+)‐catechin at p H  ≤ 8.8. On the other hand, the complex becomes too stable to proceed for the oxidation reaction at p H  > 8.8.