6.4.3Organic Chemistry and Analysis
Amino Acids — OCR A Level Chemistry A H432 Revision Notes & Topical Questions
5 min read 4 parts Lesson 14 of 33
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 4Structure of 2-Amino Acids
The general formula RCH(NH₂)COOH shows an amino group and a carboxyl group on the same carbon.
Functional groups
- Each molecule has a basic –NH₂ group and an acidic –COOH group, so it can react as both a base and an acid.
Glycine
- In glycine, R is hydrogen, giving H₂NCH₂COOH, the simplest amino acid and the only common one that is not chiral.
Alanine
- In alanine, R is –CH₃, so CH₃CH(NH₂)COOH has four different groups on carbon 2 and is therefore chiral.
Enantiomers
- Chiral amino acids exist as two optical isomers, and natural proteins use only one of the two forms.
Peptide building blocks
- About twenty different amino acids occur in proteins, each differing only in the side group R attached to carbon 2.
Part 2 of 4Acid–Base Behaviour
Amino acids are amphoteric because they react with both acids and bases.
Acid–Base Behaviour
Zwitterion
- In the solid and in neutral solution the proton moves from –COOH to –NH₂, giving RCH(NH₃⁺)COO⁻.
Physical properties
- The ionic zwitterion form explains why amino acids are crystalline solids with high melting points and are soluble in water.
In acid
- With excess H⁺ the carboxylate is protonated, giving the cation RCH(NH₃⁺)COOH.
In alkali
- With excess OH⁻ the ammonium group loses a proton, giving the anion RCH(NH₂)COO⁻.
Isoelectric idea
- At one particular pH the zwitterion predominates and the amino acid has no net charge, which is useful in separating mixtures.
Part 3 of 4Peptides and Proteins
Amino acids join through condensation, and the reverse process breaks proteins apart.
1
The –COOH of one amino acid and the –NH₂ of another react, releasing a molecule of water.
2
The new –CONH– group is called a peptide link, an amide bond joining the two units.
3
Two amino acids give a dipeptide, and many give a polypeptide, which folds to form a protein.
4
Glycine and alanine can form two different dipeptides depending on which acid group and which amine group react.
5
Boiling a protein with 6 mol dm⁻³ HCl under reflux hydrolyses every peptide link and returns the amino acids.
Part 4 of 4Exam Checklist & Tips
6.4.3 – Key Points for the Exam
- Amphoteric Able to react as both an acid and a base.
- Zwitterion An ion carrying both a positive and a negative charge with no overall charge.
- Peptide link The –CONH– amide group joining two amino acids.
- Condensation A reaction in which two molecules join and a small molecule is lost.
- Hydrolysis Breaking a bond by reaction with water, often in acid or alkali.
Common Mistakes
- Drawing the neutral form H₂NCHRCOOH as the structure in the solid state instead of the zwitterion.
- Saying all amino acids are chiral; glycine has two hydrogens on carbon 2.
- Showing the peptide link as –COOH–NH– rather than –CONH–.
- Forgetting that a dipeptide forms with loss of water, not by simple addition.
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