Amides are another example of a substituted carboxylic acid, with the OH group replaced by -NH2, -NH-R or -N-R2 (where R is shorthand for an alkyl or aryl group).
Primary amides are simple to name …
In secondary and tertiary amides, ‘N’ denotes the name of the alkyl group bonded to N in place of an H atom …
Preparation of an amide
Amides are prepared from a reaction between an acyl chloride and an amine or concentrated ammonia solution at room temperature (carboxylic acids are not reactive enough).
E.g. ethanoyl chloride + ammonia
CH3COCl + 2NH3 ⇾ CH3CONH2 + NH4+Cl–
Excess amine / ammonia reacts with the by-product, HCl, to form a salt – ammonium chloride in the example above.
The mechanisms are shown below …
- with ammonia
- with methylamine
CH3COCl + 2CH3NH2 ⇾ CH3CONHCH3 + CH3NH3+Cl–
The salt formed in this example is methyl ammonium chloride.
Reactions of amides
Amides are not bases (they are neutral in solution) unlike amines. The lone pair on the nitrogen atom is not available for making a dative covalent bond to H+ / a proton due to the proximity of the electron-withdrawing carbonyl group.
Amides are considerably less reactive than acyl chlorides and acid anhydrides because the 𝛿+ C of the carbonyl group is stabilised by electron donation from the N atom (see above).
We can reduce an amide to an amine with the reducing agent lithium tetrahydridoaluminate, LiAlH4, in dry ether.
And we can hydrolyse the amide link by refluxing with either an aqueous acid such as HCl(aq), or an aqueous alkali such as NaOH(aq). The products of hydrolysis are a carboxylic acid and an amine, although it is important to note that in either case, one of the products will be neutralised by the excess acid or alkali.
Acid / alkaline hydrolysis of amides and esters is an exceptionally common exam question so make sure you understand which products further react / are neutralised by the H+ or OH–.
Polyamides
Synthetic polyamides are more commonly known as nylons. They are made from the condensation reaction between a diamine and a dicarboxylic acid (technically they are copolymers).
In condensation polymerisation we have an addition reaction followed by elimination of a small molecule such as H2O or HCl.
There is a very good synopsis of the discovery of the nylon by Wallace Carothers on the Science Museum website.
Nylons are named according to the number of carbon atoms in each diamine and dicarboxylic acid monomer e.g. nylon-6,6 is made from 1,6-diaminohexane and hexanedioic acid.
Nylon-6 is made from one monomer, caprolactam, and this is not a condensation reaction as no small molecule is given off when the monomers polymerise. Caprolactam is heated in a nitrogen atmosphere causing the amide link in the ring to break open, and then the resulting fragment molecules react to form a polymer.
Nylon is manufactured as a fibre by cold-drawing . The polymer chains are pulled into alignment maximising the opportunity for hydrogen bonds to form between them and so nylon fibres have a high tensile strength with some elasticity.
Kevlar
Kevlar is another example of a polyamide formed in a condensation reaction between a diamine, 1,4-phenylenediamine, and a diacyl chloride, terephthaloyl chloride.
The resulting polymer chains are linear and rigid (the benzene rings prevent the chains from rotating). They stack on top of each other in a regular arrangement that allows for strong hydrogen bonds to form between the chains. The polymer can be spun into ropes or woven into a fabric and is known for its very high tensile strength to weight ratio and its fire resistant properties.
Polypeptides
Polypeptides are essentially natural polyamides made from the polymerisation of amino acids.
Practice questions
- Paracetamol is made from the reaction between 4-aminophenol and ethanoyl chloride.
(a) Complete the mechanism below to show how this reaction proceeds.
(b) Other aromatic compounds are formed in this reaction. Draw the structure of one of these organic products.
- Aspartame is an artificial sweetener.
(a) Name the functional groups present in aspartame.
(b) Aspartame is hydrolysed when refluxed with aqueous hydrochloric acid. Draw the products of the reaction.
3. Samples of ethylamine and ethanamide were each dissolved in a little water in separate test tubes. A few drops of universal indicator were added to each test tube.
(a) Describe what you would observe in each test tube.
(b) Explain your observations and write an equation for any reaction that happens.
4. Write an equation for the reaction between ethanoyl chloride and propylamine. Name the organic product.
5. Name and draw the structure of the monomer(s) that react to form the polyamide shown below:
Answers
- (a) There are two answers shown below, both would get the full marks for the curly arrows showing the electron movements, lone pairs of electrons, relevant charges on the intermediate and the correct by-product.
Full mechanism:
Simplified mechanism:
(b)
2. (a) carboxylic acid, primary amine, secondary amide, ester, benzene
(b)
3. (a) The test tube containing a solution of ethylamine would turn from colourless to blue when the UI was added. The test tube containing a solution of ethanamide would turn from colourless to green when the UI was added.
(b) ethylamine is a base and reacts with water to produce an alkaline solution of ethyl ammonium ions and hydroxide ions:
CH3CH2NH2 + H2O ⇾ CH3CH2NH3+ + OH–
Ethanamide is neutral in solution because the lone pair of electrons on the N is not available for making a dative covalent bond to H+ / a proton in water due to the proximity of the electron-withdrawing carbonyl group.
4. CH3COCl + CH3CH2CH2NH2 ⇾ CH3CONHCH2CH2CH3+ HCl.
The amide is N-propylethanamide.
5.
