AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Amino acids have both acidic and basic properties, including the formation…
Full specification wording
Amino acids (A-level only): Amino acids have both acidic and basic properties, including the formation of zwitterions.
An α-amino acid carries a basic -NH₂ group and an acidic -COOH group on the same carbon, so each can react with the other.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 2
Study notes
From the checked teaching notes for this specification point.
The idea
An α-amino acid carries a basic -NH₂ group and an acidic -COOH group on the same carbon, so each can react with the other. The proton moves from the carboxyl group to the amine group within the same molecule, giving a zwitterion: ⁺H₃N-CHR-COO⁻, with a positive centre and a negative centre but no overall charge. That internal ionic character is why amino acids are crystalline solids with high melting points and dissolve well in water, unlike the small covalent molecules they otherwise resemble. Add acid and the COO⁻ is protonated, so the molecule becomes an overall cation. Add alkali and the NH₃⁺ loses its proton, so the molecule becomes an overall anion. The amino acid is therefore amphoteric.
What earns the marks
- The zwitterion drawn with NH₃⁺ and COO⁻ and no overall charge.
- The cation in acid, with COOH and NH₃⁺.
- The anion in alkali, with COO⁻ and NH₂.
- Explaining a high melting point by the attraction between the oppositely charged ions.
Common mistakes and how to put them right
Drawing a zwitterion that carries an overall positive or negative charge.
Protonating the amine group but leaving COO⁻ unchanged when acid is added.
Saying an amino acid is neutral so it will not react with hydrochloric acid.
Assuming the zwitterion exists only in solution, when it is also the form in the solid.
Exam tips
- Decide what the added reagent does before you draw: acid supplies H⁺, so something gains a proton; alkali removes H⁺, so something loses one.
- Check the charges cancel in a zwitterion, and that they do not in the ions formed in acid or alkali.
Try the lesson
The start of the Amino acids (A-level only): Amino acids have both acidic and basic properties, including the formation of zwitterions. lesson, made from checked teaching notes for this specification point.
Have a go first
Which two functional groups are present in every amino acid?
Listen
An α-amino acid carries a basic -NH₂ group and an acidic -COOH group on the same carbon, so each can react with the other. The proton moves from the carboxyl group to the amine group within the same molecule, giving a zwitterion: ⁺H₃N-CHR-COO⁻, with a positive centre and a negative centre but no overall charge. That internal ionic character is why amino acids are crystalline solids with high melting points and dissolve well in water, unlike the small covalent molecules they otherwise resemble. Add acid and the COO⁻ is protonated, so the molecule becomes an overall cation. Add alkali and the NH₃⁺ loses its proton, so the molecule becomes an overall anion. The amino acid is therefore amphoteric. A strong answer earns marks for these points. the zwitterion drawn with NH₃⁺ and COO⁻ and no overall charge. the cation in acid, with COOH and NH₃⁺. the anion in alkali, with COO⁻ and NH₂. explaining a high melting point by the attraction between the oppositely charged ions. Exam tip: Decide what the added reagent does before you draw: acid supplies H⁺, so something gains a proton; alkali removes H⁺, so something loses one.
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