7.5.5 Amide Chemistry

• Amides are organic compounds with an -CONR₂ functional group
• They can be prepared from the condensation reaction between an acyl chloride and ammonia or amine
• In a condensation reaction, two organic molecules join together and in the process eliminate a small molecule
• In this case, the acyl chlorides and ammonia or amine join together to form an amide and eliminate an HCl molecule

Condensation reaction

• The chlorine atom in acyl chlorides is electronegative and draws electron density from the carbonyl carbon
• The carbonyl carbon is therefore electron-deficient and can be attacked by nucleophiles
• The nitrogen atom in ammonia and amines has a lone pair of electrons which can act as a nucleophile and attack the carbonyl carbon
• As a result, the C-Cl bond is broken and an amide is formed
• Whether the product is a substituted amide or not, depends on the nature of the nucleophile
  • Primary and secondary amines will give a substituted amide
  • The reaction of acyl chlorides with ammonia will produce a non-substituted amide

Acyl chlorides undergo condensation reactions with ammonia and amines to form amides

• Note that ammonia is basic and the inorganic product is acidic, so there will be a reaction between the two molecules

NH₃ + HCl → NH₄Cl

• We can therefore write the overall equation for the reaction of propanoyl chloride and ammonia as

CH₃CH₂COCl + 2NH₃ → CH₃CH₂CONH₂ + NH₄Cl

Amide link

• Polyamides are also formed using condensation polymerisation

An amide link - also known as a peptide link - is the key functional group in a polyamide

Monomers

• A diamine and a dicarboxylic acid are required to form a polyamide
  • A diamine contains 2 -NH₂ groups
  • A dicarboxylic acid contains 2 -COOH groups

The monomers for making polyamides

Formation of polyamides

This shows the expulsion of a small molecule as the amide link forms

• Nylon 6,6 is a synthetic polyamide
• Its monomers are 1,6-diaminohexane and hexane-1,6-dioic acid
  • The ‘6,6’ part of its name arises from the 6 carbon atoms in each of Nylon 6,6 monomers

Nylon 6,6 is a synthetic polyamide made using diamine and dicarboxylic acid monomers

Kevlar

• Kevlar is another example of a polymer formed through condensation polymerisation
• The polymer chains are neatly arranged with many hydrogen bonds between them
• This results in a strong and flexible polymer material with fire resistance properties
• These properties also lend Kevlar to a vital application in bullet-proof vests
• The monomers used to make Kevlar
  • 1,4-diaminobenzene
  • Benzene-1,4-dicarboxylic acid

Kevlar is made using a diamine and dicarboxylic acid monomers