A study of the polarographic reduction of methaqualone

Abstract

Methaqualone [2-methyl-3- o-tolyl-4(3H)-quinazolinone] is reduced at pH 1.5–5 in a single two-electron step. At pH 1.5–3, wave i 1 appears and is gradually replaced by wave i 2 which predomiantes in the solution between pH 4.5 and 6.5. At pH > 5, the height of wave i 2 decreases in the shape of a dissociaton curve with increasing pH. For both waves, the diprotonated form of methaqualone (I a) is reduced to 1,2,-dihydromethaqualone as the final product. In wave i 1, the monoprotonated form I b is further protonated at the electrode surface before it accepts the first electron; in wave i 2, the free base form I accepts two protons at the electrode surface before the first electron uptake. Polarographic curves are complicated by the presence of three waves of catalytic hydrogen evolution. Wave i 1, cata appears at pH < 5, waves i 2,cata at −1.5 V and i 3,cata at −1.7 V at pH > 5. Citrate buffers pH 1.5–3 or Britton-Robinson buffers at pH 2.6–3.6 are most suitable for quantitative work with eitehr d.c. polarography or differential pulse polarography.