Brønsted-Lowry Acids and Bases

According to Brønsted-Lowry theory, an acid is a proton donor and a base is a proton acceptor. In an acid-base reaction, a proton transfers from the acid to the base. This proton-transfer reaction is fundamental to understanding how acids and bases interact in chemical systems.

Curved arrows illustrate electron movement during proton transfer. One arrow from the base shows nonbonding electrons shifting to the acid's proton, forming a new covalent bond. The other arrow on the acid shows the O-H bond breaking and its electrons shifting to the oxygen atom, releasing the proton.

A conjugate base forms when an acid donates a proton and loses a positive charge. A conjugate acid forms when a base accepts a proton and gains a positive charge. For example, acetate ion is the conjugate base of acetic acid, while ammonium ion is the conjugate acid of ammonia.

Acetic acid has two potential proton receptor sites: the carbonyl and hydroxyl oxygens. When protonated at the carbonyl oxygen, the resulting cation's positive charge delocalizes over three atoms through resonance structures. Protonation at the hydroxyl oxygen limits delocalization to two atoms. Greater charge delocalization stabilizes cation A, making the carbonyl oxygen the preferred protonation site.

The nitrogen atom in ammonia possesses a lone pair of electrons that serves as the proton receptor site. This lone pair enables ammonia to accept a proton from an acid, allowing it to function as a Brønsted base. The lone pair's availability is essential for ammonia's ability to participate in proton-transfer reactions.

When acetic acid accepts a proton, the resulting cation's stability depends on how the positive charge distributes across atoms. Cation A, formed by carbonyl protonation, exhibits three resonance structures allowing charge delocalization over three atoms. This greater delocalization makes cation A more stable than cation B, which has only two contributing resonance structures.

The outcome depends on the relative strengths of the acid and base involved and the stability of the resulting conjugate acid and conjugate base. Factors like charge delocalization, resonance stabilization, and molecular structure influence which proton-transfer direction is favored. Understanding position of equilibrium in acid base reactions helps predict reaction outcomes.