Notes on Carboxylic Anhydrides, Esters, and Amides

Carboxyl Derivatives

  • Involves three classes of compounds derived from carboxylic acids: anhydrides, esters, and amides.
  • Each compound is related to the carboxyl group through the loss of water (H2O).

Anhydrides

  • Definition: Functional group consists of two carbonyl groups bonded to the same oxygen atom.
  • Types:
    • Symmetrical: Derived from two identical acyl groups.
    • Mixed: Derived from two different acyl groups.
  • Nomenclature: Remove "acid" from carboxylic acid name and add "anhydride" (e.g., acetic acid → acetic anhydride).

Esters

  • Definition: Functional group consists of a carbonyl group bonded to an -OR group, where R is an alkyl or aryl group.
  • Nomenclature: Name the alkyl or aryl group first, then the acid name replacing -ic acid with -ate (e.g., ethanoic acid → ethanoate).
  • Special Types: Cyclic esters are known as lactones.

Triglycerides

  • Definition: A triester of glycerol and three fatty acids.
  • Character: Often contains different fatty acid components and exhibits hydrophobic characteristics due to long nonpolar hydrocarbon chains.
  • Structure: All three hydroxyl groups of glycerol are esterified.
  • Saponification: Base-promoted hydrolysis of triglycerides leads to glycerol and fatty acid salts (soaps).

Fatty Acids

  • Abundance: Present in animal fats, vegetable oils, and biological membranes.
  • Structure: Unbranched carboxylic acids typically containing even numbers of carbon atoms, ranging from 10 to 20.
  • Characteristics:
    • Saturated fatty acids: Found in solid fats (high melting points).
    • Unsaturated fatty acids: Found in oils (lower melting points).
  • Short and Medium Chain Fatty Acids: Promote microbiota health; medium chain fatty acids are sources for energy.

Amides

  • Definition: Functional group consists of a carbonyl group bonded to a nitrogen atom.
  • Nomenclature: Remove -oic acid or -ic acid from the IUPAC name and add -amide. If nitrogen is bonded to groups, indicate with N- or N,N-di-.
  • Cyclic Amides: Known as lactams, such as penicillins and cephalosporins (most are b-lactam antibiotics).

Fischer Esterification

  • Process: Carboxylic acid reacts with an alcohol in the presence of an acid catalyst to form esters.

Preparation of Amides

  • Amides can form by treating a carboxylic acid with an amine, preferably through established conditions for removing water.
  • Alternatively, anhydrides can react with amines to efficiently form amides.

Hydrolysis Processes

  • Anhydrides: Hydrolysis produces two carboxylic acids in the presence of water.
  • Esters: Hydrolyze slower, especially without acid/base; can be sped up by heating in aqueous acid/base, reverting to carboxylic acid and alcohol.
  • Amides: Require vigorous hydrolysis conditions; treated similarly to esters but under harsher conditions yielding ammonium ions and carboxylic acids.

Reactions with Alcohols and Amines

  • Anhydrides: React with alcohols to form esters and carboxylic acids (e.g., aspirin synthesis).
  • Amines: Anhydrides and esters react with ammonia or amines to form amides, often using amine stoichiometry.

Phosphoric Esters

  • Formed from phosphoric acid, naming involves the word "phosphate" indicating presence of the ester.

Polymeric Reactions

  • Step-Growth Polymerization: Forms polymers by reacting molecules with multiple functional groups.
    • Polyamides: Examples include Nylon-66 and Kevlar.
    • Polyesters: Synthesized by reacting diesters and diols (e.g., Dacron, Mylar).
    • Polycarbonates: Formed via reactions involving bisphenol A and phosgene (e.g., Lexan).