6.4.4Organic Chemistry and Analysis
Amides
5 min read 4 parts Lesson 15 of 33
Section 1 of 4Structure and Naming
Amides contain the –CONH– grouping, a carbonyl group joined to a nitrogen atom.
Primary amide
- Ethanamide, CH₃CONH₂, has the nitrogen bonded to two hydrogens and one acyl group.
N-substituted amide
- Replacing one N–H hydrogen by an alkyl group gives, for example, N-methylethanamide, CH₃CONHCH₃.
Naming
- The name uses the stem of the parent acid with the ending -amide, and N- shows an alkyl group on nitrogen.
Link in polymers
- The same amide link joins the units in proteins and in polyamides such as nylon.
Properties
- The N–H bonds allow hydrogen bonding, so primary amides are solids with relatively high melting points for their molar mass.
Section 2 of 4Making and Breaking Amides
Amides form from acyl chlorides and are broken down by hydrolysis.
Making and Breaking Amides
Preparation
- Ethanoyl chloride reacts with ammonia or with an amine to give an amide and a chloride salt.
Acid hydrolysis
- Heating with dilute acid gives the carboxylic acid and ammonium salt: CH₃CONH₂ + H₂O + HCl → CH₃COOH + NH₄Cl.
Alkaline hydrolysis
- Heating with aqueous NaOH gives the carboxylate and ammonia: CH₃CONH₂ + NaOH → CH₃COONa + NH₃.
Test for ammonia
- The gas turns damp red litmus paper blue, showing that the amide has been hydrolysed in alkali.
Conditions
- Hydrolysis needs prolonged heating under reflux, since amides are much less reactive than acyl chlorides.
Polymers
- Polyamides such as nylon are long-chain amides that can be hydrolysed back into their monomers under strongly acidic or alkaline conditions.
| Hydrolysis medium | Organic product | Nitrogen product |
|---|---|---|
| Dilute HCl, reflux | Ethanoic acid | Ammonium chloride |
| NaOH(aq), reflux | Sodium ethanoate | Ammonia |
Section 3 of 4Why Amides Are Not Basic
1
The nitrogen in an amide has a lone pair, as in an amine, so it might be expected to accept a proton.
2
That lone pair overlaps with the π system of the neighbouring C=O group and is delocalised.
3
Electron density is drawn towards the more electronegative oxygen atom.
4
The nitrogen is therefore much less able to bond to H⁺, and amides give neutral solutions.
5
This contrasts with amines, whose localised lone pair makes them weak bases.
6
Amides are hydrolysed more slowly than esters, so harsher conditions and longer heating are needed to break them down.
Section 4 of 4Exam Checklist & Tips
6.4.4 – Key Points for the Exam
- Amide A compound containing the –CONH– group.
- N-substituted amide An amide with an alkyl group attached to the nitrogen atom.
- Hydrolysis Breaking down by reaction with water in the presence of acid or alkali.
- Delocalisation Spreading of electrons over more than two atoms.
Common Mistakes
- Treating an amide as a base in the same way as an amine.
- Naming CH₃CONH₂ as ethylamide rather than ethanamide.
- Giving ammonia as the product of acid hydrolysis instead of an ammonium salt.
- Omitting the N- locant in N-methylethanamide.