Acids and Bases in Organic Chemistry

Brønsted-Lowry Theory of Acids and Bases

The Brønsted-Lowry theory is fundamental to understanding acid-base behavior in organic chemistry. It defines acids and bases based on their ability to donate or accept protons (H+).

• Acid: A substance that can donate a proton (H+).

• Base: A substance that can accept a proton (H+).

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

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

General Equations:

• Acid dissociation:

• Example:

Key Points:

• Any species with a hydrogen atom can act as a potential acid.

• Any species with a lone pair of electrons can act as a potential base.

Acid-Base Reactions and Equilibrium

Acid-base reactions are typically reversible and reach equilibrium. The position of equilibrium depends on the relative strengths of the acids and bases involved.

• Amphoteric substances: Compounds that can act as either an acid or a base, depending on the reaction partner (e.g., water).

• Acidity: The tendency of a compound to lose a proton.

• Basicity: The tendency of a compound to accept a proton.

• Strength relationships:

  • A strong acid has a weak conjugate base.

  • A strong base has a weak conjugate acid.

Quantifying Acid and Base Strength

The strength of acids and bases is measured using equilibrium constants and their logarithmic forms.

• Acid dissociation constant (): Indicates the strength of an acid in water.

• Base dissociation constant (): Indicates the strength of a base in water.

• pKa:

• pKb:

• Relationship: and (at 25°C)

• Interpretation: A small pKa value indicates a strong acid; a large pKa indicates a weak acid.

Equilibrium Expression for Acid Dissociation:

Equilibrium Expression for Base Dissociation:

pH and Solution Acidity

The pH scale is used to describe the acidity or basicity of a solution, based on the concentration of protons (H+).

• pH:

• Low pH: Indicates an acidic solution.

• High pH: Indicates a basic solution.

• pKa: An intrinsic property of a compound, describing its tendency to lose a proton.

Relative Acid Strengths: Examples and Applications

Organic and inorganic acids and bases vary widely in strength. The following table summarizes key examples and their pKa values.

Resonance and Acid Strength

Resonance stabilization of the conjugate base increases acid strength. For example, carboxylic acids are more acidic than alcohols because their conjugate bases (carboxylates) are stabilized by resonance.

• Carboxylic acid dissociation:

• The negative charge on the carboxylate ion is delocalized over two oxygen atoms, stabilizing the conjugate base.

• Alcohols lack this resonance stabilization, making them weaker acids.

Example:

• Acetic acid: , pKa = 4.76

• Methanol: , pKa = 15.5

Acid-Base Equilibria and Favorability

Acid-base reactions favor the formation of the weaker acid and the weaker base. The equilibrium lies toward the side with the less reactive (more stable) species.

• For a reaction: , equilibrium favors the side with the higher pKa (weaker acid).

Summary Table: Key Acid-Base Properties

Additional info:

• Some content was expanded for clarity, including definitions and the explanation of resonance stabilization.

• Tables were constructed to summarize acid strengths and key properties, as implied by the notes.