Chapter 2 – Acids and Bases

The Basics of Acids and Bases

This section introduces the fundamental concepts of acids and bases, focusing on the Brønsted-Lowry definitions and their importance in organic chemistry. Understanding these concepts is essential for predicting reaction outcomes and the behavior of organic molecules in different environments.

• Brønsted-Lowry Acid: A proton (H+) donor.

• Brønsted-Lowry Base: A proton (H+) acceptor.

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

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

• Strong acids form weak conjugate bases; strong bases form weak conjugate acids.

• Equilibrium in acid-base reactions favors the formation of the weaker acid .

Conjugate Acid-Base Pairs

Every acid-base reaction involves the transfer of a proton from an acid to a base, resulting in the formation of a conjugate base and a conjugate acid.

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

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

• Example: When ammonia (NH3) acts as a base and accepts a proton from water (H2O), it forms ammonium (NH4+) and hydroxide (OH-).

Acid and Base Strength: Ka and pKa

The strength of an acid is measured by its acid dissociation constant (Ka), but due to the wide range of values, the logarithmic pKa scale is commonly used.

• Acid Dissociation Constant (Ka):

• pKa:

• Lower pKa values indicate stronger acids.

• pKa values allow for easier comparison of acid strengths across different compounds.

Organic Acids and Bases: Functional Groups and Approximate pKa Values

Common organic functional groups have characteristic pKa values, which are important for predicting their behavior in acid-base reactions.

Additional info: These values are approximate and can vary depending on the molecular environment.

Mechanisms of Acid-Base Reactions

Acid-base reactions in organic chemistry are depicted using curved arrow notation to show the movement of electron pairs. Understanding these mechanisms is crucial for predicting reaction outcomes.

• Arrows start at a pair of electrons (lone pair or bond) and end at an atom or bond where electrons are moving.

• Always show the movement of electrons, not atoms.

• Do not violate the octet rule when drawing mechanisms.

• Example: The reaction of ammonia with water, where a lone pair on nitrogen attacks a proton on water, forming ammonium and hydroxide.

Predicting Acid-Base Equilibria

The direction of acid-base equilibria can be predicted by comparing the pKa values of the acids on both sides of the reaction.

• The equilibrium favors the side with the weaker acid (higher pKa).

• Equation for equilibrium constant:

• If , products are favored; if , reactants are favored.

Factors That Determine Acid Strength

Several factors influence the acidity of a compound by stabilizing or destabilizing the conjugate base.

• Electronegativity: Atoms with higher electronegativity stabilize negative charge better, increasing acidity.

• Hybridization: Greater s-character (sp > sp2 > sp3) increases electronegativity and acidity.

• Size: Larger atoms can better stabilize negative charge by spreading it over a larger volume.

• Substituents (Inductive Effect): Electron-withdrawing groups stabilize the conjugate base, increasing acidity. The effect decreases with distance from the acidic proton.

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

pH, pKa, and the Henderson-Hasselbalch Equation

The relationship between pH, pKa, and the ratio of acid to conjugate base is described by the Henderson-Hasselbalch equation, which is essential for understanding buffer systems and the ionization state of molecules.

• Henderson-Hasselbalch equation:

• When pH = pKa, [HA] = [A-]

• pH < pKa: same

• pH > pKa: change

Lewis Acids and Bases

The Lewis definition of acids and bases is broader than the Brønsted-Lowry definition and is useful for understanding a wider range of chemical reactions.

• Lewis Acid: Electron pair acceptor (often an electrophile).

• Lewis Base: Electron pair donor (often a nucleophile).

• Many organic reactions can be described as nucleophile (Lewis base) attacking an electrophile (Lewis acid).

Summary Table: Approximate pKa Values of Common Functional Groups

Additional info: These values are essential for predicting the direction of acid-base reactions in organic chemistry.