Acid-Base Chemistry in Organic Chemistry

Definitions of Acids and Bases

Understanding acids and bases is fundamental in organic chemistry, as these properties influence molecular reactivity and stability. There are three major definitions:

• Arrhenius Definition: An acid is a substance that dissociates in water to give H3O+ (hydronium ion), and a base is a substance that dissociates in water to give OH- (hydroxide ion).

• Bronsted-Lowry Definition: An acid is a species that can donate a proton (H+), and a base is a species that can accept a proton.

• Lewis Definition: An acid is an electron pair acceptor, and a base is an electron pair donor.

Conjugate Acids and Bases

When an acid reacts, it forms a conjugate base. When a base reacts, it forms a conjugate acid. This relationship is crucial for understanding acid-base equilibria.

• Conjugate base: The species formed after an acid donates a proton.

• Conjugate acid: The species formed after a base accepts a proton.

Acid Strength and Dissociation Constants

The strength of Bronsted-Lowry and Arrhenius acids is measured by their extent of ionization in water. The general ionization of an acid (HA) in water is:

The acid dissociation constant () quantifies this equilibrium:

The magnitude of indicates acid strength: the larger the , the stronger the acid.

pKa and Acid Strength

Acid strength is often expressed on a logarithmic scale:

• Strong acids:

• Weak acids:

• The weaker the acid, the larger the .

• The weaker the acid, the smaller the .

Predicting Acid/Base Equilibria Direction

Acid-base reactions favor the formation of the weaker acid and weaker base. Key points:

• The stronger the acid, the weaker its conjugate base.

• The weaker the acid, the stronger its conjugate base.

Example:

• Stronger acid (), stronger base weaker base, weaker acid ()

• Weaker acid (), weaker base stronger base, stronger acid ()

Weak is favored!

Factors Affecting Conjugate Base Stability

The more stable a conjugate base, the more acidic the parent acid. Three main factors affect stability:

• Electronegativity: More electronegative atoms stabilize negative charge better, leading to stronger acids.

• Size: Larger atoms can better disperse negative charge, increasing stability down a group in the periodic table.

• Resonance: Delocalization of charge through resonance structures stabilizes the conjugate base.

Electronegativity Trends

• Electronegativity: C < N < O

• Stability: CH4 < NH3 < OH-

• Acidity: CH4 < NH3 < H2O

• Basicity: CH3- > NH2- > OH-

Size Effects

• Negative charge is more stable when spread over a larger region of space. Acidity increases down a group as size increases.

Resonance Stabilization

• Conjugate bases stabilized by resonance are more likely to form, making the parent acid stronger.

Example: Carboxylate anion is stabilized by resonance, making carboxylic acids stronger than alcohols.

Additional info: These notes are based on lecture slides from the University of Santo Tomas, Department of Chemical Engineering, and are suitable for introductory college-level Organic Chemistry.