Mechanism of reactions of some N1-substituted biguanides with chromium(VI) in aqueous sulphuric acid media

Abstract

N ¹ -Phenylbiguanide RHN–C(X)NH–C(NH)NH₂(R = Ph, X = NH) reacts quantitatively in aqueous sulphuric acid media with Cr^VI and Ce^IV to produce 1,4-benzoquinone, carbamimidoylurea (R = H, X = O), and ammonium ion. N¹-(4-Methoxyphenyl)biguanide (R = 4-MeOC₆H₄, X = NH) reacts similarly except that, instead of a quinone, more extensively oxidized products are produced. Phenylbiguanide (pbg) itself, in acid solution, undergoes slow hydrolysis, and anilinium ion is one of the products. The hydrolyzed solution reacts rapidly with Cr^VI and Ce^IV. None of the other six biguanides investigated and three other structurally related compounds reacts with Cr^VI under the experimental conditions. The kinetics of the reactions is in accord with the rate law (I). A typical –d[Cr^VI]//_dt=–4//₃d[pbg]//_dt=k[pbg](i) value of k is (0.25 ± 0.03)× 10^–5 s^–1 at 20 °C, [H⁺]= 1.0 mol dm^–3, and I= 2.0 mol dm^–3, and when determined from experiments with an excess of Cr^VI(first-order kinetics observed) agrees closely with that determined in the presence of an excess of pbg (zero-order kinetics) and also that determined for reactions with Ce^IV. These results have been interpreted in terms of a mechanism involving the acid-promoted hydrolysis of phenylbiguanide [and (4-methoxyphenyl)biguanide], the oxidant functioning as an analytical assay of the anilinium product.