Amides and condensation polymers

What you’ll learn

• Recognise an amide linkage and its formation.
• Predict acidic and alkaline amide hydrolysis products.
• Connect monomer functionality to polyamide structure.

Understand the idea

An amide contains nitrogen attached directly to a carbonyl carbon. It can form by reaction of an acyl derivative with ammonia or an amine. Delocalisation of the nitrogen lone pair towards the carbonyl makes amides much less basic than amines. In naming an N-substituted amide, the N-prefix identifies a group bonded to nitrogen rather than the carbon chain.

Hydrolysis breaks the acyl–nitrogen link. In acidic conditions, an unsubstituted amide gives a carboxylic acid and an ammonium ion; in alkaline conditions it gives a carboxylate ion and ammonia. A substituted amide gives the corresponding amine or protonated amine. Condensation of a diamine with a dicarboxylic acid or suitable acyl derivative gives a polyamide such as nylon. Hydrolysis of its links can return smaller fragments or the original monomer-derived compounds.

A polymer repeat unit is a structural fragment, not a molecule with arbitrary capped ends. Trace every atom from the two monomers and show the continuation bonds through brackets. Biodegradability depends on accessible hydrolysable bonds and environmental conditions; the presence of an amide or ester linkage alone does not guarantee rapid degradation in every environment.

Key formulae

$$\mathrm{RCONH_2+H_2O+H^+\longrightarrow RCOOH+NH_4^+}$$

$$\mathrm{RCONH_2+OH^-\longrightarrow RCOO^-+NH_3}$$

$$\mathrm{-CO-NH-}\quad\text{(amide linkage)}$$

$$\mathrm{HOOC-R-COOH+H_2N-R'-NH_2}\quad\text{(bifunctional monomer pair)}$$

Worked example

Identify the monomer types needed for a polyamide containing the repeat fragment –NH–(CH₂)₆–NH–CO–(CH₂)₄–CO–.

1. Locate the two amide links and separate the repeat fragment at the acyl–nitrogen connections.
2. The nitrogen-containing portion corresponds to H₂N–(CH₂)₆–NH₂.
3. The carbonyl-containing portion corresponds to HOOC–(CH₂)₄–COOH; count the two carboxyl carbons as part of the acid chain.

Answer: Hexane-1,6-diamine and hexanedioic acid: the monomer types used for nylon-6,6.