Kolbe Electrolysis — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
Kolbe's Electrolysis Kolbe's electrolysis is the synthesis of symmetrical alkanes by electrolysis of a concentrated aqueous solution of sodium or potassium salt of a carboxylic acid. At the anode, carboxylate ions are oxidized, lose CO₂, and the resulting radicals couple to form an alkane with twice the carbon chain (minus the carboxyl carbons). Effervescence of CO₂ gas is observed at the anode during electrolysis. The product alkane may be collected as a gas (for small chains like ethane) or as an oily layer on the solution surface (for longer chains). The reaction is endergonic — it requires electrical energy input to drive the oxidation. The decarboxylation step (RCOO• → R• + CO₂) is thermodynamically favorable due to the stability of CO₂, and the radical coupling (R• + R• → R-R) is highly exothermic. At cathode: Na⁺ + e⁻ → Na (or 2H₂O + 2e⁻ → H₂ + 2OH⁻ in aqueous solution) At anode: RCOO⁻ → RCOO• + e⁻ (one-electron oxidation of carboxylate) Decarboxylation: RCOO• → R• + CO₂ (loss of carbon dioxide from the radical) Radical coupling: R• + R• → R-R (two alkyl radicals combine to form the alkane) Confusing Kolbe electrolysis with Kolbe-Schmitt reaction — Kolbe electrolysis makes alkanes from carboxylate salts. Kolbe-Schmitt makes salicylic acid from phenol + CO₂. Forgetting that the product has (2n) carbons when starting acid is CₙH₂ₙ₊₁COOH. E.g., acetic acid (C1) → ethane (C2), propionic acid (C2) → butane (C4). Thinking this works for aromatic acids — benzoic acid undergoes Kolbe electrolysis poorly due to instability of phenyl radical. Not recognizing that only symmetrical alkanes are formed — you cannot make unsymmetrical alkanes by simple Kolbe electrolysis.