5.5.7Unit 5. Transition Metals and Organic Nitrogen Chemistry
Characteristic Behaviour of Amino Acids — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 37 of 43 Unit quiz
Curriculum Equation Key pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: pH + pOH = 14
Part 1 of 5Structure and Zwitterions
Both an acidic and a basic group are present on one molecule.
General structure
- A 2-amino acid has the formula H₂N–CHR–COOH, with the amine and carboxylic acid groups on the same carbon.
Zwitterion
- In the solid and in solution near neutral pH, the molecule transfers H⁺ from –COOH to –NH₂, giving ⁺H₃N–CHR–COO⁻.
Physical properties
- Ionic interactions between zwitterions give high melting points, and most amino acids dissolve in water.
Chirality
- Every 2-amino acid except glycine has four different groups on the central carbon and so shows optical isomerism.
Part 2 of 5Acid–Base Behaviour
Acid–Base Behaviour
With acid
- In strongly acidic solution the –COO⁻ group takes up H⁺, giving the cation ⁺H₃N–CHR–COOH.
With alkali
- In strongly alkaline solution the –NH₃⁺ group loses H⁺, giving the anion H₂N–CHR–COO⁻.
Amphoteric
- The ability to react as both an acid and a base is called amphoteric behaviour.
Isoelectric point
- The pH at which the overall charge is zero and the zwitterion predominates;
- the amino acid does not move in an electric field.
| pH of solution | Dominant form of glycine | Overall charge |
|---|---|---|
| Low (strong acid) | ⁺H₃NCH₂COOH | +1 |
| Near isoelectric point | ⁺H₃NCH₂COO⁻ | 0 |
| High (strong alkali) | H₂NCH₂COO⁻ | –1 |
Part 3 of 5Reactions of Amino Acids
1
With an alcohol and a few drops of concentrated acid catalyst under reflux, the –COOH group forms an ester, for example the methyl ester of glycine.
2
Two amino acids can join when the –NH₂ of one reacts with the –COOH of another, releasing water to form a dipeptide.
3
Repeating this step builds polypeptides, and the sequence of side groups R decides the final protein structure.
4
Heating a peptide with aqueous acid for many hours hydrolyses every amide link and returns the separate amino acids.
5
Ninhydrin spray on a chromatogram turns the amino acid spots purple so they can be identified by Rf value.
Part 4 of 5Separation and Identification
Electrophoresis
- At a fixed pH, amino acids move at different rates towards the electrodes depending on their charge and size.
Paper chromatography
- Amino acids have different solubilities in the solvent, so they travel different distances up the paper.
Rf value
- Rf equals the distance moved by the spot divided by the distance moved by the solvent front.
Developing agent
- Ninhydrin is used because the amino acids are colourless on the paper.
Part 5 of 5Exam Checklist & Tips
5.5.7 – Key Points for the Exam
- 2-Amino acid A molecule with an amine and a carboxylic acid group on the same carbon atom.
- Zwitterion An ion carrying both a positive and a negative charge with zero overall charge.
- Amphoteric Able to react both as an acid and as a base.
- Isoelectric point The pH at which an amino acid has no net charge.
- Peptide link The amide link formed between two amino acids.
Common Mistakes
- Drawing amino acids as uncharged H₂N–CHR–COOH in solid or neutral solution rather than as zwitterions.
- Stating that glycine is chiral; its R group is hydrogen so it has no optical isomer.
- Saying the isoelectric point is pH 7 for all amino acids; it depends on the side chain.
- Confusing the cation (low pH) with the anion (high pH) forms.
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