On the interaction of morphine with Lugol’s iodine in alkaline medium

Abstract

The purpose of this communication is to clear up what is happening in the test tube during the interaction of an alkaline solution of morphine with Lugol’s iodine (Kippenberger test for morphine). The formation mode and reactivity of potassium triiodide, the salt present in Lugol’s solution, are commented. Reaction of iodine molecule (formed in a reversible manner) with sodium hydroxide gives rise to hypo-iodous acid. This reacts with the morphine phenolate formed in situ yielding an organic hypoiodite. This labile intermediate loses iodide ion, producing a ketone and Umpolung at the ortho-position which enhances reaction with hypoiodous acid (nucleophilic reaction). A new hypoiodite and a dienone are formed (first oxidation step). Enolization recovers aromatization and the phenoxide ion produces a synchronic mechanism involving three electron-shifts. This way an ortho-benzoquinone is obtained (second redox reaction). The observed colour in the test is due to halochromism.