Alcohol Dimer is Requisite to Form an Alkyl Oxonium Ion in the Proton Transfer of a Strong (Photo)Acid to Alcohol

Abstract

Alcohols, the simplest amphiprotic organic compounds, can exhibit either acidic or basic behavior by donating or accepting a proton. In this study, proton dissociation of a model photoacid in solution is explored by using time-resolved spectroscopy, revealing quantitatively for the first time that alcohol acts as a Brønsted base because of H-bonded cluster formation to enhance the reactivity. The protonated alcohol cluster, the alkyl oxonium ion, can be regarded as a key reaction intermediate in the well-established alcohol dehydration reaction. This finding signifies, as in water, the cooperativity of protic solvent molecules to facilitate nonaqueous acid-base reactions. Dimer delights: The formation of an alkyl oxonium ion, a long-proposed intermediate in alcohol dehydration, is studied by time-resolved fluorescence quenching of a strong Brønsted photoacid. The collaboration of two alcohol molecules is critical to enhance the reactivity and extract the proton from the acid. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim