Amino acidsAQA A-Level Chemistry: Revision notes
Section 1
Structure of amino acids
An amino acid contains an amino group, –NH₂, and a carboxylic acid group, –COOH. The 2-amino acids in proteins have both groups on the same carbon:
H₂N–CHR–COOH
R is the side chain. In glycine R is H and in alanine R is CH₃. Amino acids are crystalline solids, soluble in water, with high melting points.
Section 2
Acidic and basic properties
The –COOH group is acidic (donates H⁺) and the –NH₂ group is basic (accepts H⁺, using the lone pair on nitrogen). An amino acid therefore reacts with both acids and alkalis. A substance able to act as both acid and base is amphoteric.
Section 3
Zwitterions
In the solid and in neutral solution a proton moves from –COOH to –NH₂ in the same molecule:
H₂N–CHR–COOH ⇌ ⁺H₃N–CHR–COO⁻
This is a zwitterion: it has a positive and a negative charge but no overall charge. Strong ionic attractions between zwitterions explain the high melting points and solubility in water.
A zwitterion is not a mixture of two ions. It is a single molecule with a positive and a negative group.
Section 4
Species in acid and in alkali
In dilute acid the molecule gains H⁺ on the carboxylate group, giving the positive ion:
⁺H₃N–CHR–COOH
In aqueous alkali OH⁻ removes H⁺ from the –NH₃⁺ group, giving the negative ion:
H₂N–CHR–COO⁻
With NaOH in a 1 : 1 reaction the product is the sodium salt: H₂N–CHR–COOH + NaOH → H₂N–CHR–COO⁻Na⁺ + H₂O.
Draw the –NH₃⁺ as ⁺H₃N–. Check the overall charge: acid gives +, alkali gives –, water gives 0.
Section 5
Worked example: titrating an amino acid
0.300 g of H₂N–CHR–COOH needs 22.50 cm³ of 0.150 mol dm⁻³ NaOH.
n(NaOH) = 0.02250 × 0.150 = 3.375 × 10⁻³ mol. The ratio is 1 : 1.
Mr = 0.300 ÷ 3.375 × 10⁻³ = 88.9, so 89.
H₂N–CH–COOH has Mr = 74, so R = 15 = CH₃. The amino acid is alanine.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Amino acids
- Glycine, H₂N–CH₂–COOH, is the simplest amino acid. In the solid state and in neutral aqueous solution it exists mainly as a zwitterion.Explain how glycine forms a zwitterion.2 marks
- A student dissolves alanine, CH₃CH(NH₂)COOH, in solutions of different pH: dilute hydrochloric acid, water and aqueous sodium hydroxide.Write an equation for the reaction of alanine with aqueous sodium hydroxide, showing the structure of the organic product.2 marks
- Glycine, H₂NCH₂COOH, is a white crystalline solid that melts above 200 °C. Propanoic acid, CH₃CH₂COOH, has almost the same relative molecular mass but is a liquid at room temperature.Explain why glycine, Mr = 75.0, is a solid at room temperature but propanoic acid, Mr = 74.0, is a liquid.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).