On the mechanism of the Tattersall test for morphine

Being morphine a restricted drug, a rapid colour test for its detection is very useful. Tattersall employed as reagent trisodium ortho-arsenate in sulphuric acid. However, the more common disodiumhydrogen ortho-arsenate heptahydrate can be used also because both salts yield the same reactive species, that is, protonated arsenic acid and a cation with arsenic(IV). An arsenate is formed with the phenolic group in morphine. Dehydration of this organometallic ester yields a cationic intermediate that induces reaction with arsenic acid at ortho-position (electromeric effect), and a concerted mechanism takes place. A dienone is formed and there is elimination of meta-arsenous acid. Aromatization followed by acidolysis of the new arsenate gives water, meta-arsenous acid, and morphine ortho-quinone, via a six-member concerted mechanism. The colours observed in the test are due to halochromism.