Diazoacetaldehyde: completion of the rate profile for hydration of its photo-Wolff rearrangement product, ketene, in aqueous solution and kinetics and mechanism of acid-catalyzed hydrolysis of the diazoaldehyde†

Abstract

Ketene was generated flash photolytically by the photo-Wolff reaction of diazoacetaldehyde and rates of its hydration to acetic acid were measured in dilute aqueous sodium hydroxide solution. The results give the isotope effect, k_HO/k_DO = 0.87, on the hydroxide-ion catalytic coefficient for this reaction, and, together with literature information, they provide a complete rate law for the hydration of ketene in aqueous solution promoted by solvent related species: k_obs/s^–1 = 1.01 × 10⁴[H⁺] + 3.65 × 10¹ + 5.26 × 10⁴[HO^–]. Rates of hydrolysis of diazoacetaldehyde in dilute perchloric acid solution were also measured and an inverse hydronium ion isotope effect was found: k_H⁺/k_D⁺ = 0.55. That, plus the fact that hydrogen exchange of the diazo hydrogen atom of diazoacetaldehyde takes place more rapidly than its hydrolysis, shows that hydrolysis occurs through a mechanism involving pre-equilibrium protonation of diazo carbon followed by rate-determining loss of nitrogen.