Spontaneous Generation of Aryl Carbocations from Phenols in Aqueous Microdroplets: Aromatic S1 Reactions at the Air-Water Interface.

Although phenol is stable in bulk water, we report an exceptional phenomenon in which phenol is spontaneously transformed into a phenyl carbocation (Ph) in water microdroplets. The high electric field at the air-water interface is proposed to break the phenolic Csp-OH bond, forming Ph, which remains in equilibrium with phenol as deciphered by mass spectrometry. We detected up to 70% conversion of phenol to Ph in aqueous microdroplets, although catalyst-free activation of the phenolic Csp-OH bond is challenging. This transformation is well tolerated by a wide range of electron-donating and -withdrawing substituents in phenolic compounds. The Ph in water microdroplets could be reacted with various nucleophiles (amine, pyridine, azide, thiol, carboxylic acid, alcohol, and O-water), yielding the -substitution products of phenol through an aromatic S1 mechanism. Despite the fleeting life of Ph in the bulk, this study demonstrates its unusual stability at the aqueous microdroplet surface, enabling its detection and transformation.