Notes on Nucleophilic Acyl Substitutions in Organic Chemistry II

Nucleophilic Acyl Substitutions

  • Overview
    • Nucleophilic acyl substitutions differ from typical nucleophilic substitutions at saturated carbon atoms.
    • Closely related to nucleophilic additions at carbonyl groups (as seen in aldehydes and ketones).
    • Involves carboxylic acid derivatives (excluding nitriles which have different mechanisms).

Reaction Mechanism

  • Initial Step
    • Involves the nucleophilic attack on the carbonyl carbon.
  • Reformation of Carbonyl
    • Different from aldehyde and ketone reactions; relies on the removal of the leaving group attached to the carboxyl group.
    • Facilitates a more straightforward process compared to carbonyl reactions.

Carboxylic Acid Derivatives

  • Include:
    • Carboxylic acids
    • Esters
    • Amides
    • Anhydrides
    • Acid chlorides
  • Nitriles
    • Unusual due to carbonyl's sp-hybridization but shares oxidation number (+III).

Reactivity of Carboxylic Acid Derivatives

  • Determined by the effectiveness of the leaving group.
  • Leaving group activity in these derivatives contrasts with that of saturated carbon.
    • Electronegativity dominates over polarizability.
  • Reactivity Hierarchy
    • Acid chlorides > Anhydrides > Esters > Amides > Carboxylic acids.

Esters

  • Commonly found in nature; major role in plant oils and animal fats.
  • > Biodiesel production via transesterification from plant oils using methanol/ethanol.
  • Also significant in personal care products and food fragrances.

Amides

  • Vital in natural compounds, forming the basis of peptides and proteins.
  • Synthesis Challenges
    • Direct reaction of carboxylic acids with amines is hindered by competing acid-base reactions.
    • Common synthesis method = activation of carboxylic acid (using acid chlorides/anhydrides) followed by reaction with amines.

Hydrolysis of Amides

  • Irreversible hydrolysis products: carboxylic acids and amines.
  • More commonly performed in acidic conditions.
    • Mechanism involves protonation of carbonyl oxygen, nucleophilic attack by water, followed by rearrangement and eventual formation of the carboxylic acid product.

Acid Chlorides and Anhydrides

  1. Formation of Acid Chlorides
    • Reaction mechanism similar to dehydrative anhydride formation, using reagents like phosphoryl chloride or thionyl chloride.
  2. Reactivity and Economic Considerations
    • Acid chlorides are more reactive leading to easier acyl substitutions.
    • Anhydrides typically require dehydrating conditions, which may not be economical.