3.1 Voltammetric experiments with a platinum electrode

The selected acetophenones and benzophenone were examined by cyclic voltammetry between 0 and 4 V with 0.1 V/s scan rate in acetonitrile. The results for the platinum electrode are presented in Fig. 1. The voltammograms of the different compounds are very similar, and they have a small peak around 3 V except for 4′-nitroacetophenone and benzophenone. It can be attributed to the oxidation of methyl group present in the acetyl group. Formation of the intermediate methyl radicals is followed by their dimerization resulting in the final product 1,4-diphenyl-1,4-butanedione containing the corresponding substituent in the para position in the benzene ring. The absence of this anodic peak in case of 4′-nitroacetophenone is because of the strong electron-withdrawing effect of the nitro group. Consequently, it reacts at higher potentials. Significant currents flow only at potentials close to 4 V as a result of the oxidation of solvent and subsequent oxidation of the substrate on the carbonyl oxygen. Benzophenone does not have this peak because it contains an additional aryl moiety instead of the methyl group. The voltammograms are very different in case of 4′-hydroxyacetophenone as compared with other derivatives, which have an additional anodic peak at 2 V, and the heights of the subsequent scans are almost the same indicating the lack of passivation. It has a phenol moiety and the acetyl group is present in the para position. It suggests that this para-substituted phenol derivative does not foul the surface of the platinum electrode in acetonitrile. This is because radical formed due to the electrochemical oxidation contained the unpaired electron in the para position, which is the most stable form. The final products can dissolve and diffuse into the bulk.

3.2 Investigation of acetophenones and benzophenone with a glassy carbon electrode

The electrochemical behaviour of the outlined compounds was also investigated with a glassy carbon electrode under the same conditions as in the case of the platinum electrode (Fig. 2). The remarkable difference as compared with the platinum electrode is the appearance of a more enhanced oxidation peak between 2.5 and 3 V during the first scans of each compounds except for 4′-nitroacetophenone. The curves have similar shape to that recorded with the platinum electrode, indicating also that this derivative does not show redox character in this voltage range. The small peak around 3 V can be attributed to the solvent, which was verified in a separate experiment (see below). The voltammograms of 4′-hydroxyacetophenone differ also significantly from the other ones as it contains also the oxidation peak of phenolic moiety at 1.8 V in the first scan. This acetophenone derivative can be oxidized through its two functional groups (hydroxy and methyl) scanning between 0 and 4 V; therefore, chain polymers can form and then adsorb on the surface. The subsequent voltammograms contain smaller peaks but after the second scan they do not decrease as it can be observed generally in the surface fouling reactions. It suggests that the polymer formed has a loose structure elevating the diffusion of the monomers to the electrode surface. Because the hydroxy group is an activating substituent the oxidation peak attributable to the methyl group appeared around 2.6 V. This is also characteristic for 4′-methylacetophenone-containing methyl group being also an activating substituent. This oxidation peak showed up close to 3 V by the halogen derivatives because they have electron-withdrawing character.

Further experiments were performed with 10 mM acetone in acetonitrile using 0–4 V potential range with the aim to get information whether the methyl group is oxidized. An oxidation peak appeared around 2.75 V, which is similar to that of acetophenones reinforcing the earlier hypothesis.

Interestingly, the oxidation peak above 2.5 V appeared also approximately in the same potential interval by benzophenone as in the case of acetophenones. In spite of that this compound does not have methyl group, only two phenyl groups. It suggests that benzophenone reacts on its benzene ring while an aryl radical forms. This radical can dimerize and then oxidize on the other phenyl group building up polymer chains. It is also clearly seen from the figure that the currents measured in the next scans are significantly lower indicating the deactivation of the glassy carbon surface. Moreover, this finding suggests that the examined acetophenones can be oxidized also on their benzoyl moieties on the glassy carbon electrode, resulting in polymeric products.

The voltammograms of acetophenones and benzophenone suggest that these compounds foul the surface of the glassy carbon electrode. However, looking at the ranges between 3 and 4 V where oxidation of solvent molecules occur, the currents do not decrease by repeating the scans indicating the lack of serious passivation. To get more insight into the processes on its surface a cycle was taken between 0 and 4 V with acetonitrile and then, a cyclic voltammogram in the solutions of the selected compounds. These scans showed high similarity to the second and subsequent cycles of acetophenones and benzophenone obtained previously (see Fig. 2). Two reasons can be responsible for these observations. Acetonitrile electrooxidation at higher potentials deactivates the glassy carbon electrode making the surface unsuitable for the further electrochemical reaction of acetophenones and benzophenone. This observation is supported by an earlier study of Staley et al. [9] with quinones in acetonitrile on the glassy carbon electrode due to the presence of surface oxides. These surface sites provide appropriate possibilities to build hydrogen bonds between substrate, products and the surface. This deactivation process could also be observed by repeating the scans only to 3 V where the first oxidation peak appeared in case of each compound.

As could be seen in the voltammograms of 4′-hydroxyacetophenone, the oxidation peak of phenolic hydroxy group appeared around 2 V. For obtaining more information concerning the electrochemistry of the phenolic moiety, the cyclic voltammetric experiments were repeated between 0 and 2.2 V with the glassy carbon electrode (Fig. 3). The peak currents, which are attributed to this phenol moiety, decreased slowly by repeating the scans. Previously, when the switching potential was 4 V the decreasing tendency of this peak was more significant. It indicates that the electrode becomes deactivated only at higher potentials because of the adsorption of products and solvent electrooxidation.

3.3 Studies using 1,4-dihydroxybenzene as redox probe

Complementary studies were carried out with the glassy carbon electrode using a redox probe 1,4-dihydroxybenzene between 0 and 1 V to get more information about the deactivation process. Previously, this electrode showed passivation and more insights were needed to understand the surface processes. The 5 mM concentration aqueous solution of the redox probe was used and the obtained data are collected in Table 1. Five cyclic voltammograms were recorded in acetonitrile solutions of the studied compounds between 0 and 4 V and finally a scan with the redox probe in its aqueous solution, respectively. For comparison, the data obtained after taking five cycles in the acetonitrile solvent are also presented. It can be seen in Table 1 that the anodic peak currents diminished to ∼90% of the value measured with the glassy carbon electrode without cycling in any acetonitrile solution. Remarkably, the anodic peak potentials were smaller and cathodic peak potentials were higher. Two voltammograms are depicted in Fig. 4 for 4′-chloroacetophenone in aqueous solution of 1,4-dihydroxybenzene. The curves clearly show that the anodic peaks are shifted to less positive and the cathodic peaks shifted to less negative potentials as compared with bare carbon. These findings suggest that substrate adsorbs readily on the surface of pretreated glassy carbon. The more oxidized surface and the presence of adsorbed products of acetophenone derivatives and benzophenone electrooxidation offer appropriate surface sites for adsorption of the substrate and the product 1,4-benzoquinone because of hydrogen bonds. The higher cathodic currents indicate that the increased number of adsorbed 1,4-benzoquinone is capable to reduce in less negative potential regions. This increase in the current could also be observed in case of the electrode with modified surface in acetonitrile but pretreatment in solutions of the acetophenones and benzophenone contributed to an additional small enhancement.