Acids and Bases

Definitions of Acid and Base

Organic chemistry uses several definitions to classify acids and bases, each with its own scope and application. The two most important theories are the Brønsted-Lowry Theory and the Lewis Theory.

• Brønsted-Lowry Theory: An acid is a proton (H+) donor, and a base is a proton acceptor.

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

Examples:

• Brønsted-Lowry:

• Brønsted-Lowry:

• Lewis:

Note: In organic chemistry, "acids and bases" typically refer to Brønsted-Lowry acids and bases.

Acid-Base Equilibria and pKa

Equilibrium Constants and pKa

Acid-base reactions are equilibrium processes. The strength of an acid is measured by its tendency to donate a proton, quantified by equilibrium constants and pKa values.

• General Acid Dissociation:

• Equilibrium Constant (Keq): Describes the ratio of products to reactants at equilibrium.

• Acid Dissociation Constant (Ka):

• pKa:

• pH:

Key Point: Organic chemists use pKa to describe acid strength, not pH, because pH depends on solution concentration, while pKa is an intrinsic property of the acid.

Identifying Acidic Protons

Acidic Proton and X-H Bond Strength

In molecules with multiple hydrogens, the acidic proton is the one most easily removed. This is typically the hydrogen attached to the most electronegative atom or the atom that stabilizes the negative charge best after deprotonation.

• Weakest X-H Bond: The most polar bond to hydrogen is usually the weakest and most acidic.

• Example: In a molecule with both alcohol and carboxylic acid groups, the carboxylic acid hydrogen is more acidic.

Practice: Identify the most acidic proton in molecules by considering bond polarity and atom electronegativity.

pKa Table and Acid Strength

pKa Values of Common Acids and Their Conjugate Bases

The following table lists pKa values for common acids and their conjugate bases. Lower pKa indicates a stronger acid.

Note: pKa refers to the acidic form, not the basic form.

Factors Affecting Acid Strength

Conjugate Base Stability

The strength of an acid is directly related to the stability of its conjugate base. A more stable conjugate base corresponds to a stronger acid.

• Electronegativity: The higher the electronegativity of the atom bearing the negative charge, the more acidic the compound.

• Size: The larger the atom bearing the negative charge, the more acidic the compound.

• Inductive Effects: Electron-withdrawing groups stabilize the conjugate base by dispersing negative charge through the molecule.

• Resonance: Delocalization of electrons through resonance stabilizes the conjugate base, increasing acidity.

Practice: Compare H2S and H2O for acidity. Sulfur is larger and less electronegative than oxygen, affecting acid strength.

Resonance and Acid Strength

Resonance Stabilization

Resonance is a key factor in acid strength. If the conjugate base can delocalize its negative charge through resonance, it is more stable and the parent acid is stronger.

• Resonance involves delocalized electrons.

• Resonance is represented by multiple resonance contributors and a single resonance hybrid.

• Only π bonds and lone pairs can move in resonance structures; atoms and σ bonds do not move.

Example: The nitro group (NO2) stabilizes the conjugate base through resonance.

Effect: Resonance stabilizes the conjugate base, increasing acid strength.

pKa Approximations for Functional Groups

Useful pKa Values

• Alcohols/Water: pKa ≈ 15-16

• Amines: pKa ≈ 35-38

• Carboxylic Acids: pKa ≈ 4-5

• Strong Acids (e.g., HCl, HBr, HI): pKa < 0

• Anything Protonated: pKa < 0

Relationship Between pH and pKa

Major Species in Solution

The ratio of acid to conjugate base in solution depends on the relationship between pH and pKa.

• If pH > pKa: The conjugate base predominates.

• If pH < pKa: The acid form predominates.

• If pH ≈ pKa: Both forms are present in roughly equal amounts.

Practice: For valine (an amino acid), the carboxylic acid group has pKa = 2.2 and the amine group has pKa = 9.5. At pH = 7, the carboxylic acid is deprotonated (COO-), and the amine is protonated (NH3+). At pH = 10, both groups are deprotonated.

Summary Table: Factors Affecting Acid Strength

Key Terms and Concepts

• Acidic Proton: The hydrogen atom most easily removed from a molecule.

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

• pKa: A logarithmic measure of acid strength; lower pKa means stronger acid.

• Inductive Effect: The transmission of charge through a chain of atoms in a molecule, affecting acidity.

• Resonance: Delocalization of electrons that stabilizes the conjugate base.

Additional info: Some practice problems and explanations were expanded for clarity and completeness. The summary table was inferred from the context and typical organic chemistry curriculum.