How do amines and amino acids behave, and how do proteins form?
Amines as bases and nucleophiles, their preparation from haloalkanes and nitriles, the structure and zwitterion behaviour of amino acids, the formation of peptide bonds, and the isoelectric point.
A CCEA A-Level Chemistry answer on amines and amino acids, covering amine basicity and nucleophilic behaviour, preparation from haloalkanes and nitriles, the zwitterion structure of amino acids, the isoelectric point and the formation of peptide bonds.
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What this dot point is asking
CCEA wants you to treat amines as both bases and nucleophiles, prepare them from haloalkanes and nitriles, describe the zwitterion behaviour of amino acids, define and use the isoelectric point, and show how amino acids join through peptide bonds.
Amines as bases and nucleophiles
Preparation of amines
From a haloalkane with excess ammonia (nucleophilic substitution):
Excess ammonia limits further substitution to secondary and tertiary amines. From a nitrile by reduction (using or with a nickel catalyst):
The nitrile route adds one carbon to the chain, which is useful in synthesis.
Amino acids and zwitterions
Peptide bonds
A peptide (amide) bond forms when the of one amino acid condenses with the of another, releasing water:
Hydrolysis with hot aqueous acid or alkali reverses this, regenerating the amino acids.
Examples in context
In electrophoresis, a CCEA staple, a mixture of amino acids is spotted onto a gel buffered to a chosen pH. Glycine at carries a net positive charge and migrates toward the cathode, while at it carries a net negative charge and moves toward the anode; at its isoelectric point of it stays put. The same chemistry underlies protein digestion: hydrochloric acid in the stomach and enzymes hydrolyse the peptide bonds of dietary protein back to amino acids, exactly the acid hydrolysis written above.
Try this
Q1. Write an equation for the preparation of ethylamine from bromoethane using excess ammonia. [2 marks]
- Cue. .
Q2. State why an alkyl group increases the basicity of an amine. [1 mark]
- Cue. It donates electron density () onto nitrogen, making the lone pair more available to accept a proton.
Q3. Define the isoelectric point. [1 mark]
- Cue. The pH at which the amino acid exists predominantly as a zwitterion with no net charge.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20183 marksExplain why ethylamine is a stronger base than ammonia, but phenylamine is a weaker base than ammonia.Show worked answer →
Basicity depends on how available the lone pair on nitrogen is to accept a proton.
In ethylamine the ethyl group is electron-donating (a positive inductive effect, ). It pushes electron density onto the nitrogen, increasing the electron density of the lone pair and making it more available to accept a proton. So ethylamine is a stronger base than ammonia.
In phenylamine the lone pair on nitrogen is delocalised into the benzene ring. This withdraws electron density from the nitrogen, so the lone pair is less available to accept a proton. So phenylamine is a weaker base than ammonia.
Markers reward (1) the inductive electron-donating effect of the alkyl group, (2) delocalisation of the nitrogen lone pair into the ring, (3) linking lone-pair availability to base strength.
CCEA 20224 marksDescribe the zwitterion structure of the amino acid glycine and explain, with reference to the isoelectric point, why glycine has a high melting point.Show worked answer →
A zwitterion is a species that carries both a positive and a negative charge but is overall neutral. In glycine the carboxylic acid group donates a proton to the amine group, giving .
The isoelectric point is the pH at which the amino acid exists predominantly as the zwitterion and has no net charge. At this pH it does not migrate in an electric field.
Because the zwitterion carries full ionic charges, glycine crystals are held together by strong ionic attractions between the and groups of neighbouring molecules. A large amount of energy is needed to overcome these, so the melting point is high. Markers reward (1) the zwitterion formula with both charges, (2) the definition of the isoelectric point, (3) ionic attractions, (4) linking these to the high melting point.
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Sources & how we know this
- CCEA GCE Chemistry specification — CCEA (2016)