Overview

Deprotonation is the chemical process in which a molecule loses a proton (a hydrogen cation, H+). In Brønsted–Lowry terms, an acid donates a proton and a base accepts it. The species that loses the proton becomes its conjugate base; the species that gains it becomes protonated. For clarity, a proton is not the same as a neutral hydrogen atom: a hydrogen atom contains an electron and a proton. See hydrogen ion and hydrogen atom for related concepts.

Mechanism and equilibrium

In a typical deprotonation the proton departs, and the electron pair that originally bound the hydrogen often remains on the conjugate base as a lone pair or is redistributed in the bonding framework. Whether deprotonation proceeds depends on acid strength and the basicity of the acceptor; these tendencies are quantified by pKa values and equilibrium constants. A proton transfer reaction will favor the side with the weaker acid (higher pKa). It is important to distinguish proton loss from oxidation: deprotonation removes an H+ but does not necessarily change the formal oxidation state of the associated atom. For background on the proton itself see proton and on the role of electrons see electron.

Common bases and examples

  • In water, hydroxide (OH−) deprotonates many acids (for example, acetic acid → acetate).
  • Stronger, non-nucleophilic bases such as lithium diisopropylamide (LDA) or sodium hydride (NaH) are used to deprotonate weakly acidic C–H or O–H bonds to give enolates or alkoxides.
  • Organometallic reagents (e.g., BuLi) can deprotonate very weak acids and are common in synthetic organic chemistry.

The base becomes protonated in the process; this conjugate acid often determines the position of equilibrium and must be considered when choosing reagents or solvents. See an explanation of protonation states here.

Applications and significance

Deprotonation is a fundamental step in many reactions: formation of enolates for C–C bond formation, elimination reactions (E2 mechanisms), tautomerizations, acid–base titrations, and enzyme-catalyzed proton transfers in biology. Control of which proton is removed—regioselectivity and stereoselectivity—can be achieved by base strength, temperature, solvent, and steric effects.

Distinctions and practical notes

Do not confuse deprotonation with dehydrogenation (which removes H• or H2) or formal oxidation (which involves electron transfer). Solvent polarity and hydrogen-bonding capacity strongly affect deprotonation equilibria and rates: protic solvents stabilize ions and often facilitate proton transfer, while aprotic solvents influence base strength differently. Kinetics vary widely: some proton transfers are essentially barrierless, others require strong, non-nucleophilic bases or catalytic assistance.