Carboxylic Acids and Acid-Base Reactions

Structure and Properties of Carboxylic Acids

Carboxylic acids are organic compounds containing the functional group R–C(=O)–OH. They are characterized by their ability to donate a proton (H+), making them acids in Brønsted-Lowry terms.

• General formula: R–C(=O)–OH

• Example: Acetic acid (CH3COOH), pKa ≈ 4.76

• Acidity: Carboxylic acids have relatively low pKa values, indicating they are stronger acids than alcohols and amines.

Acid-Base Reaction Mechanism

When a carboxylic acid reacts with a base (e.g., NaOH), it donates a proton to the base, forming its conjugate base and the conjugate acid of the base.

• Reaction:

• Conjugate base: CH3COO- (strong)

• Conjugate acid: H2O (weak)

Comparing Acid and Base Strengths

The strength of acids and bases is often compared using their pKa values. The lower the pKa, the stronger the acid.

• Acid: Lower pKa

• Base: Higher pKa

Conjugate Acids and Bases

Definitions

• Conjugate acid: The species formed when a base gains a proton.

• Conjugate base: The species formed when an acid loses a proton.

Examples

• Acetic acid (CH3COOH) loses a proton to form acetate (CH3COO-).

• Ammonia (NH3) gains a proton to form ammonium (NH4+).

pKa and Equilibrium in Acid-Base Reactions

Determining Acid and Base in a Reaction

In an acid-base reaction, the reactant with the lower pKa acts as the acid. The equilibrium favors the formation of the weaker acid (higher pKa).

• Rule: Compare the pKa values of the acid reactant and the acid product.

• Equilibrium: Favors the side with the weaker acid (higher pKa).

Example Reaction

• CH3COOH (pKa = 4.76) + NH3 (pKa ≈ 34)

• Products: CH3COO- + NH4+ (pKa ≈ 9.1)

• Equilibrium favors formation of CH3COO- and NH4+

Henderson-Hasselbalch Equation

Equation and Application

The Henderson-Hasselbalch equation relates the pH of a solution to the pKa of the acid and the ratio of the concentrations of the conjugate base and acid.

• Equation:

• Application: Used to calculate pH in buffer solutions and to determine the ratio of acid/base at a given pH.

Worked Example

• For aspirin (pKa = 3), if pH = 3:

• Therefore,

• If pH < pKa,

• If pH > pKa,

Relative Strengths of Acids and Bases

Acid and Conjugate Base Strengths

Acid strength decreases as pKa increases. The strength of the conjugate base increases as the acid becomes weaker.

Lewis Acids and Bases

Definitions

• Lewis acid: A species that accepts an electron pair.

• Lewis base: A species that donates an electron pair.

Examples

• BH3 and AlCl3 are Lewis acids due to their ability to accept electron pairs.

• CH3O- is a Lewis base, donating an electron pair to form a bond.

Hybridization and Empty Orbitals

Aluminum in AlCl3 has an empty orbital after hybridization, allowing it to act as a Lewis acid.

• Electron configuration: 1s2 2s2 2p6 3s2 3p1

• Hybridization: Promotion of electrons leads to empty orbitals for electron pair acceptance.

Applications to Pharmaceuticals

Acetaminophen and Aspirin

• Acetaminophen: pKa ≈ 9.5

• Aspirin: pKa ≈ 3

• Acid-base properties: Influence solubility and absorption in biological systems.

Le Châtelier's Principle in Acid-Base Equilibria

Principle

If a system at equilibrium is disturbed, it will shift to counteract the disturbance and restore equilibrium.

• Application: Addition of acid or base shifts the equilibrium position in acid-base reactions.

Summary Table: Acid and Base Strengths

Key Equations

• Henderson-Hasselbalch equation:

• Formal charge calculation:

• Example for oxygen in acetate:

• Example for nitrogen in methylamine:

Additional info: The notes include inferred context on acid-base equilibria, pharmaceutical applications, and Lewis theory, expanding on brief handwritten points for clarity and completeness.