Illustrated Glossary of Organic Chemistry

Proton transfer: A reaction or reaction mechanism step in which a proton ("H+") is removed from one species (the acid) and accepted by another species (the base).

Dissolving sulfuric acid (H2SO4) in water causes a proton transfer. A proton (shown in red) from sulfuric acid (a strong acid and the proton source) is transferred to water (a weak base and the proton acceptor). This proton transfer reaction converts water into hydronium ion (H3O+; the conjugate acid of water) and sulfuric acid into bisulfate ion (HOSO3-; the conjugate base of sulfuric acid).