Spontaneous Generation of Aryl Carbocations from Phenols in Aqueous Microdroplets: Aromatic S<sub>N</sub>1 Reactions at the Air–Water Interface
Abhijit Nandy, Anubhav Kumar, Supratim Mondal, Debasish Koner, Shibdas Banerjee
Abstract
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 2 –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 2 –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 18 O-water), yielding the ipso -substitution products of phenol through an aromatic S N 1 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.