The role of intramolecular hydrogen bonding in the electrochemical behavior of hydroxy-quinones and in semiquinone stability

Abstract

The study of a selected group of α and β-hydroxyquinones ([α] 2-hydroxy-1,4-naphthoquinone 2HNQ, [β] 5-hydroxy-1,4- naphthoquinone 5HNQ and [β, β]5,8-dihydroxy-1,4-naphthoquinone DHNQ) showed that both type of functionalities differ considerably in terms of electrochemical and chemical reactivity, and these differences are also manifest even upon comparison between compounds of the same group. The results proved that the energy needed to electrochemically reduce the studied hydroxyquinones is related both to the stability of intramolecular hydrogen bonds (IHBs) and coupled chemical reactions occurring during their electrochemical reduction such as self-protonation sequences in 2HNQ. The analysis of the electron transfer kinetics associated with the monoelectronic reductions occurring for 5HNQ and DHNQ showed that IHB effects do not affect the kinetics of the first reduction process, but induce great changes in the second electron transfer kinetics. This suggests that semiquinone species are chemically different for the studied compounds, as was corroborated by the analysis of the electronic structure of semiquinone species by ESR spectroscopy. This analysis seems to be related to the loss of symmetry in the electrogenerated radical and induces a lower rate ks value for the second reduction transfer of 5HNQ, compared with DHNQ. ©2005 Sociedade Brasileira de Química.