Acids and Bases in Organic Chemistry

Introduction

Acids and bases are fundamental to understanding organic chemistry, as they play a central role in many chemical reactions and mechanisms. Their behavior, strength, and interactions determine the outcome of numerous organic processes.

What Are Acids and Bases?

Definitions and Basic Concepts

• Acid: A species that can lose a proton (H+).

• Base: A species that can gain a proton (H+).

• General reaction:

• Example:

Key Point: An acid loses a proton, forming its conjugate base; a base gains a proton, forming its conjugate acid.

Acid-Base Reactions and Equilibrium

Reversibility of Reactions

• Most acid-base reactions are reversible.

• Reversible reaction: (both forward and reverse reactions occur).

• Irreversible reaction: (only forward reaction occurs).

Equilibrium Arrows

• The longer half-arrow in equilibrium notation indicates which side (products or reactants) predominates at equilibrium.

• If products predominate: (arrow toward products is longer).

• If reactants predominate: (arrow toward reactants is longer).

Conjugate Acid-Base Pairs

Formation of Conjugate Pairs

• When an acid loses a proton, it forms its conjugate base.

• When a base gains a proton, it forms its conjugate acid.

• Example (acid and conjugate base):

• Example (base and conjugate acid):

Acid Strength

Strong vs. Weak Acids

• Strong acid: Products are favored at equilibrium; the acid loses its proton readily.

• Weak acid: Reactants are favored at equilibrium; the acid loses its proton less readily.

• Example (strong acid):

• Example (weak acid):

Key Point: The stronger the acid, the more readily it loses a proton.

Acid Dissociation Constant () and

Quantifying Acid Strength

• The acid dissociation constant () measures the extent of acid dissociation in water.

• General equation:

• Since is constant,

Relationship Between and

• Lower means a stronger acid.

• Higher means a weaker acid.

Classification of Acids by

Key Point: The stronger the acid, the smaller its .

Example

• Hydrogen chloride (HCl): (very strong acid)

• Acetic acid (CH3COOH): (weak acid)

*Additional info: The scale is logarithmic, so each unit change represents a tenfold difference in acid strength.*