5.1
Brønsted-Lowry Acids and Bases
In 1923, the Brønsted–Lowry definition of acids and bases was proposed by Johannes Brønsted and Thomas Lowry. According to this theory, a Brønsted aci…
From the sour taste of vinegar in salad dressings to the caustic liquids used as drain cleaners, acids and bases have many applications.
According to the Brønsted-Lowry theory, an acid is a proton donor, and a base is a proton acceptor; thus, an acid–base reaction is a proton-transfer reaction.
Examine the reaction between the Brønsted acid — acetic acid — and the Brønsted base — ammonia. The nitrogen atom in ammonia, owing to its lone pair, serves as the proton receptor site.
Proton transfer from the acid to the base is shown using curved arrows, which illustrate the movement of electrons.
The arrow from the base denotes the shift of nonbonding electrons to the proton of the acid, forming a new covalent bond.
The arrow on the acid shows that the O–H bond breaks to release a proton and that its electrons shift entirely to the oxygen atom.
The acetate ion that remains from the deprotonation of acetic acid is the conjugate base. The ammonium ion, resulting from the protonation of ammonia, is the conjugate acid.
Notice that acetic acid has two potential proton receptor sites — the carbonyl oxygen and the hydroxyl oxygen — and that it functions as a Brønsted base in its reaction with sulfuric acid.
Proton transfer from the acid to the carbonyl oxygen of the base gives cation A, whereas transfer to the hydroxyl oxygen of the base results in cation B.
The positive charge in cation A can delocalize over three atoms, resulting in three resonance structures of cation A. In cation B, however, the delocalization occurs over only two atoms, limiting the number of contributing resonance structures to two.
The greater charge delocalization in cation A makes the carbonyl oxygen of acetic acid the preferred protonation site in an acid–base reaction.
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Q1: What is the Brønsted-Lowry definition of an acid and a base?
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.
Q2: How do curved arrows represent proton transfer in acid-base reactions?
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.
Q3: What are conjugate acids and conjugate bases?
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.
Q4: Why does acetic acid preferentially protonate at the carbonyl oxygen rather than the hydroxyl oxygen?
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.
Q5: What role does the lone pair on nitrogen play in ammonia's basicity?
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.
Q6: How does charge delocalization affect the stability of protonated acetic acid?
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.
Q7: What determines the outcome of an acid-base reaction between two species?
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.