Kolbe product

In aqueous solution an elevated pressure favors the Kolbe-coupling against non-Kolbe products [37]. A possible explanation is that high pressure aids the formation of a lipophilic medium at the electrode surface that prevents the adsorption of water and thus disfavours the formation of carbenium ions. 

Dimethylformamide is also a suitable solvent [50], it has, however, the disadvantage of being oxidized at fairly low potentials to A-acyloxy-iV-methyl formamide [51]. The influence of the composition of the ternary system water/methanol/dimethyl-formamide on the material and current yield has been systematically studied in the electrolysis of co-acetoxy or -acetamido substituted carboxylates [32]. Acetonitrile can also be used, when some water is.added [52]. The influence of various solvents on the ratio of Kolbe to non-Kolbe products is shown in Table 1 [53]. 

Some cyclopropylcarboxylic acids, namely 14 [125] and 15 [46] could be coupled to bicyclopropyl compounds, others led to allylic compounds via ring opening of an intermediate carbenium ion (see chap. 7). Tertiary alkanoates yield predominantly non-Kolbe products (see chap. 8). 

Substituted phenylacetic acids form Kolbe dimers when the phenyl substituents are hydrogen or are electron attracting (Table 2, Nos. 20-23) they yield methyl ethers (non-Kolbe products), when the substituents are electron donating (see also chap. 8). Benzoic acid does not decarboxylate to diphenyl. Here the aromatic nucleus is rather oxidized to a radical cation, that undergoes aromatic substitution with the solvent [145]. 

The coupling reactions are in general limited to primary carboxylic acids (RCH2CO2H). a-Branched carboxylic acids lead to non-Kolbe products. However, carboxylic acid (74) with the electron-attracting trifluoromethyl group in the a-position yields the Kolbe coupling product (75) (Scheme 26) [99,100]. 

In many cases both Kolbe and non-Kolbe products are isolated from a reaction. Carboxylic acids with an a-alkyl substituent show a pronounced dual behaviour. In these cases, an increase in the acid concentration improves the yield of the Kolbe product. An example of the effect of increased substrate concentration is given in Kolbe s classical paper [47] where 2-methylbutyric acid in high concentration affords mostly a dimethylbexane whereas more recent workers [64], using more dilute solutions, obtained both this hydrocarbon and butan-2-ol. Some quantitative data is available (Table 9.2) for the products from oxidation of cyclohexanecar-boxylic acids to show the extent of Kolbe versus non-Kolbe reactions. The range of products is here increased through hydrogen atom abstraction by radical intermediates in the Kolbe reaction, which leads to some of the monomer hydrocarbon... 

If this is true, then the oxidation of carboxylic acids should be the preferred process on SrTi03 photoanodes, in the absence of such defect surface states. We see from Figure 5 that the range of potentials reported for the normal Kolbe reaction (at platinum) actually crosses the valence band levels of both SrTiC>3 and Ti02 in the neutral pH region. It may well be that at high pH, the photo-Kolbe potential lies at or below the valence band edge for these semiconductors, consistent with the observation that photo-Kolbe products are not observed under these conditions. 

The yield of the cross Kolbe products A — B is generally unsatisfactory 232>. 

Ultrasound may influence the outcome of competing reactions thus, for example, on electrolyzing cyclohexanecarboxylate at a platinum electrode in methanol, a switch from one-electron Kolbe products to two-electron non-Kolbe products was observed. Reviews of the use of ultrasound in electrochemistry have recently appeared [18]. 

Kolbe products fatty acids (sodium Europe, Japan, India, P... 

The dependence of Kolbe products on anode potential and electrode material has been the subject of considerable study. Only a few examples from recent studies will be discussed here since the interest is in mechanism determination and not electrosynthesis. Dickinson and Wynne-Jones analyzed the composition of gases evolved from acetate solutions under various conditions and their results are presented in Table 5. They also observed that with a current density of 30mA/cm in citrate and phthalate solutions, oxygen was evolved at current efficiencies of not less than 90% on the metals they examined (Pt, Ir, Pd, Au, and Ni). The results of Conway and Dzieciuch with aqueous potassium formate are seen in Table 6. These results agree with those of Dickinson and Wynne-Jones, showing that the decarboxylation reaction is almost completely inhibited by the oxygen evolution process. The anodic reaction at Pd and Au in aqueous potassium trifluoro-acetate was... 

The formation of a film at a limiting current density in aqueous solutions, is generally regarded as a necessary occurrence before the Kolbe reaction proceeds at appreciable current efficiencies. In nonaqueous solutions film formation still occurs, but it does not seem to be a prerequisite, since some Kolbe products were found below the critical current density. 

A weakly acidic medium favors the Kolbe product. Therefore, the carboxylic acid is partially neutralized to the extent of 2-5 %. The concentration of the carboxylate anion remains constant during the whole electrolysis process since the base is regenerated at the cathode at the same rate as the carboxylate is consumed at the anode. While water has been used before, methanol or aqueous methanol is now the solvent of choice. Obviously, the selection of the best solvent rests mostly on experimental investigations. Temperature is usually not a critical... 

---