Kolbe’s electrolytic method

Kolbe’s electrolytic method

A concentrated solution of the sodium or potassium salt of a carboxylic acid or mixture of carboxylic acids is electrolysed, e.g.,

R¹CO₂K + R²CO₂K + 2H₂O → R¹ – R² + 2CO₂ + H₂ + 2KOH

If R1 and R2 are different, then hydrocarbons R1 – R2 are also obtained (cf. Wurtz reaction). Such mixtures can often be separated readily. Yields of 50-90 per cent have been obtained with straight-chain acids containing 2-18 carbon atoms. Alkyl groups in the a-position decrease the yield (usually below 10 per cent). The by-products are alkenes, alcohols (particularly in alkaline solution) and easters. It is also interesting to note that the yields of the alkanes are increased when dimethylformamide is used as solvent.

The Kolbe electrolytic method now has application in the synthesis of natural compounds, particularly lipids. Several mechanisms have been proposed for the Kolbe reaction. The free-radical theory is the one now favoured, and strong support for it has been provided by Lippincott et al. (1956), e.g., when sodium propionate is electrolysed, n-butane, ethane, ethylene and ethyl propionate are obtained. The propionate ion discharges at the anode to form a free radical :

C₂H₅CO²: ^-  →  C₂H₅CO₂^. + e

This propionate free radical then breaks up into the ethyl free radical and carbon dioxide :

C₂H₅CO₂^. → C₂H₅^. CO₂

Then :

(i)              2C₂H₅ → C₄H₁₀

(ii)           C₂H₅ + C₂H₅ → C₂H₆ + C₂H₄

(iii)        C₂H₅ +C₂H₅CO₂ → C₂H₅CO₂C₂H₅

Reaction (i) gives n-butane; (ii) gives ethane and ethylene by disproportionation (cf. Wurtz reaction) and (iii) gives ethyl propionate.