Formation of Hydrates and Hemiacetals

Overview

• Focus on acid and base catalyzed formation of hydrates and hemiacetals.
• Comparison to uncatalyzed reactions.

Acid-Catalyzed Reactions

Hydration

• Starting Materials: Aldehyde or ketone.
• Process:
  1. Protonation: Carbonyl oxygen is protonated by hydronium (H3O+), increasing its electrophilicity.
  2. Nucleophilic Attack: Water functions as a nucleophile, attacking carbonyl carbon.
  3. Formation of Intermediate: Intermediate with two OH groups and a positive formal charge on oxygen.
  4. Deprotonation: Another water molecule acts as a base, removing a proton to form the hydrate (gem diol).

Hemiacetal Formation

• Starting Materials: Alcohol, acid-catalyzed (e.g., R"OH).
• Process:
  1. Protonate Alcohol: Similar initial steps as hydration but using alcohol instead of water.
  2. Nucleophilic Attack: Alcohol attacks the carbonyl carbon.
  3. Formation of Hemiacetal: Final product contains one O-R" group.
• Note: Hemiacetals often proceed to form full acetals in acid.

Base-Catalyzed Reactions

Hydration

• Starting Materials: Aldehyde or ketone with base.
• Process:
  1. Formation of Hydroxide Anion: Base deprotonates water.
  2. Nucleophilic Attack: Hydroxide ion attacks carbonyl carbon, forming an intermediate.
  3. Protonation: Water acts as an acid, protonating the intermediate to form a hydrate.

Hemiacetal Formation

• Starting Materials: Alcohol and base.
• Process:
  1. Formation of Alkoxide Anion: Base deprotonates alcohol.
  2. Nucleophilic Attack: Alkoxide attacks carbonyl carbon.
  3. Formation of Hemiacetal: Final product contains O-R" group.
• Stability: Hemiacetals do not proceed to form acetals under base conditions, making acetals stable in base.

Key Points

• Acid Catalysis: Makes carbonyl carbon more electrophilic, facilitating nucleophilic attack.
• Base Catalysis: Enhances nucleophile strength (hydroxide vs water, alkoxide vs alcohol).
• Acetals: More details on their formation and stability in future content.
• Comparison: Hemiacetals are intermediates and unstable in acid, but stable in base.