Proton Transfer (Acid/Base)

The first and most fundamental mechanistic pattern in organic chemistry is proton transfer. You’ve already encountered it, perhaps without even realizing it. Proton transfers involve the movement of a proton (H⁺) from one atom or molecule to another, typically from an acid to a base. This type of reaction may seem simple, but it offers a critical introduction to the broader language of organic mechanisms: curved arrow notation, or "electron pushing."

Key Principles of Proton Transfer:

A complete proton transfer mechanism always requires two curved arrows:

One arrow originates from a lone pair on the base, indicating the formation of a new bond to the proton.
The second arrow originates from the bond between the proton and its original atom (the acid), showing the bond breaking and the electrons retreating back onto that atom (usually creating a lone pair on oxygen or nitrogen).

In this example, the oxygen of the ketone acts as the base, using a lone pair to attack the proton from the hydronium ion. Simultaneously, the O-H bond in the hydronium ion breaks, with the electrons returning to the oxygen, forming water.

Abbreviated Proton Transfer Notation:

In textbooks and articles, you'll sometimes see proton transfers abbreviated with a shortcut notation: a single arrow above the reaction arrow labeled “–H⁺”. This is common for deprotonation reactions, such as when a Brønsted acid loses a proton:

While this shorthand is convenient, especially once you're fluent in electron flow, it hides important mechanistic detail. Until you are confident with curved arrow mechanisms, it's best to avoid using the “–H⁺” abbreviation. These simplifications can obscure what's really happening with the electrons.

Proton Transfer Involving Resonance:

More complex proton transfers may also involve resonance in the conjugate base. When the base that forms after deprotonation has multiple resonance structures, it may take three or more curved arrows to show how the electrons rearrange.