Spontaneous Generation of Aryl Carbocations from Phenols in Aqueous Microdroplets: Aromatic S_N1 Reactions at the Air–Water Interface

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 Csp2–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 Csp2–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 18O-water), yielding the ipso-substitution products of phenol through an aromatic SN1 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.

Identification of the Electrogenerated Hidden Nitrenium Ions by In Situ Mass Spectrometry

“…The partial solvation is thus helpful in alleviating nucleophilic attack by the solvents until they are eventually released into gaseous phase via desolvation. We also note the viable mechanism proposed by Banerjee et al, in which the survival of the cationic intermediates is ascribed to the formation of intimate or tight ion pair (R + X – , R + for the cationic intermediates and X – for their counteranions) in the vanishing droplets. − Similarly, such Coulomb interactions might have also played a role in hindering their reactions with nucleophiles.…”

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“…In this circumstance, the outer layer is rich in protons that can act as a superacid, forming stable diarylcarbenium ions. 54,55 Other studies 56 opine that aromatic compounds in microdroplets preferentially inhabit the air-water interface, positioning the aromatic ring towards the air side while the -OH group remains embedded in water. Such a high electric eld has the potential to break the C-O bond, thus leading to the production of the diarylcarbenium ions at the interface of the microdroplet.…”

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“…Furthermore, they found that the droplet surface electric fields of ∼0.16 V/Å yield enough power to lower the activation energy and accelerate the rates of chemical reactions by several orders of magnitude. Indeed, Nandi et al found that water droplets can generate aryl carbocations from phenols by an aromatic S N 1 reaction, which is presumably accelerated by the protonation capability of the droplet. More recently, Zhang, Xie et al reported that water droplets accelerate, by 7 orders of magnitude, the Menshutkin S N 2 reaction between pyridine and CH 3 I.…”