Initially, the triphenyl phosphine (2) makes a nucleophilic attack upon diethyl azodicarboxylate (1) producing a betaine intermediate 3, which deprotonates the carboxylic acid (4) to form the ion pair 5. DEAD itself deprotonates the alcohol (6) forming an alkoxide that can form the key oxyphosphonium ion 8. The ratio and interconversion of intermediates 8–11 depend on the carboxylic acid pKa and the solvent polarity.[7][8][9] Although several phosphorus intermediates are present, the attack of the carboxylate anion upon intermediate 8 is the only productive pathway forming the desired product 12 and triphenylphosphine oxide (13).