Amines and amides

An amine is an organic compound based on ammonia, NH₃, with one or more of its hydrogen atoms replaced by alkyl or aryl groups. Ethylamine, CH₃CH₂NH₂, is a simple example. The lone pair on the nitrogen makes amines weak bases and nucleophiles. An amide also contains nitrogen, but next to a C=O group (as in -CONH₂), and that one change means amides are not basic.

What is an amine?

AQA's A-level specification describes amines as compounds based on ammonia where hydrogen atoms have been replaced by alkyl or aryl groups. How many are replaced decides the type:

If the group on the nitrogen is an alkyl chain, the amine is aliphatic. If the nitrogen is joined straight to a benzene ring, it is aromatic: phenylamine, C₆H₅NH₂, is the main example.

Amines as bases

The nitrogen's lone pair can accept a proton, H⁺, so amines are bases. OCR asks for their reaction with dilute acids to form salts. For example:

CH₃CH₂NH₂ + HCl → CH₃CH₂NH₃⁺Cl⁻ (ethylammonium chloride)

AQA asks you to compare the base strength of ammonia, primary aliphatic amines and primary aromatic amines, and to explain it by how available the nitrogen's lone pair is. A useful measure is the pKa of each base's conjugate acid (the ion it forms): the higher it is, the stronger the base. The values are from OpenStax:

Those pKa values are on a log scale, so the gaps are big: a difference of 10.75 − 4.63 = 6.12 units means ethylamine's conjugate acid is about 10⁶ times weaker an acid than phenylamine's. See pKa for how the scale works.

Phenylamine

Phenylamine (aniline), C₆H₅NH₂, has an NH₂ group joined directly to a benzene ring. OpenStax explains its weak basicity: the nitrogen lone pair is delocalised by interaction with the ring's π electrons, and much of that stabilisation would be lost if it accepted a proton.

AQA notes that aromatic amines are prepared by reducing nitro compounds and are used to make dyes. OCR gives the conditions: reduction of nitroarenes using tin and concentrated hydrochloric acid. So phenylamine is made in two steps from benzene: nitration to nitrobenzene (see electrophilic substitution), then reduction:

C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O

How amines are made

• From halogenoalkanes and ammonia. OCR specifies excess ethanolic ammonia. Ammonia's lone pair attacks the carbon bonded to the halogen in a nucleophilic substitution. The excess makes a primary amine the main product.
• By reducing nitriles. Both boards include it; OCR gives hydrogen with a nickel catalyst as an example. CH₃CH₂CN + 4[H] → CH₃CH₂CH₂NH₂. This route adds a carbon to the chain and gives only the primary amine.
• Aromatic amines from nitro compounds, as above.

The halogenoalkane route has a catch. The amine product has a lone pair too, so it can react with more halogenoalkane. AQA describes the result: substitution goes on to form secondary and tertiary amines and finally quaternary ammonium salts. AQA also notes that quaternary ammonium salts are used as cationic surfactants. See nucleophilic substitution for the mechanism.

Amides

An amide has a nitrogen joined to a carbonyl carbon: the -CONH- group. OCR asks for the structures of primary and secondary amides. A primary amide has -CONH₂, as in propanamide, CH₃CH₂CONH₂. A secondary (N-substituted) amide has one alkyl group on the nitrogen, -CONHR.

AQA covers making them by nucleophilic addition-elimination: ammonia or a primary amine reacts with an acyl chloride or an acid anhydride. For example, propanoyl chloride and methylamine give N-methylpropanamide:

CH₃CH₂COCl + CH₃NH₂ → CH₃CH₂CONHCH₃ + HCl

With excess amine, the HCl reacts with a second molecule of amine to form a salt. The mechanism is in nucleophilic addition-elimination. The same amide link joins the units in polyamides such as nylon: see condensation polymerisation.

Amine vs amide

Why the difference? OpenStax explains that an amide's nitrogen shares its lone pair with the neighbouring carbonyl group through orbital overlap. Protonating the nitrogen would lose that stabilisation, so amides aren't protonated by aqueous acids and are poor nucleophiles. To tell them apart in a formula, look at the carbon next to the nitrogen: if it is C=O, it is an amide.

Worked example: classifying and naming

Classify each compound: (a) CH₃CH₂CH₂NH₂, (b) CH₃NHCH₂CH₃, (c) CH₃CONHCH₃, (d) C₆H₅NH₂.

1. (a) One alkyl group on N, no C=O: a primary aliphatic amine, propylamine (propan-1-amine).
2. (b) Two alkyl groups on N: a secondary amine, N-methylethanamine.
3. (c) N next to C=O, with one methyl on N: a secondary (N-substituted) amide, N-methylethanamide. It is not basic.
4. (d) NH₂ on a benzene ring: a primary aromatic amine, phenylamine, the weakest base of the amines here.

Common mistakes

• Calling any nitrogen compound an amine. If the nitrogen is bonded to a C=O, it is an amide.
• Saying phenylamine is a stronger base because the ring is big. The ring pulls the lone pair into delocalisation, making it weaker.
• Explaining base strength without the lone pair. AQA wants the availability of the lone pair on N in every answer.
• Forgetting further substitution. A halogenoalkane and ammonia give a mixture unless ammonia is in excess.
• Mixing up primary amines and primary amides. CH₃CH₂NH₂ is an amine; CH₃CONH₂ is an amide.

Amines are A-level only in AQA's specification, so practise them on today's A-level Chemistry: Year 2 short timed paper. For planning multi-step routes that use these reactions, see organic synthesis.

Read next

Chemistry · Organic chemistryNucleophilic addition-elimination: mechanism and reactionsThe nucleophilic addition-elimination mechanism for acyl chlorides with water, alcohols, ammonia and amines, anhydrides, and why aspirin is made with one.