A-Level Chemistry May/June 2024 Q8(b)(ii): Draw the structures of the three organic products formed from the reaction of neotame w…

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  The reaction of neotame with an excess of hot aqueous sodium hydroxide results in the hydrolysis of all amide and ester functional groups. The excess strong base also deprotonates all carboxylic acid groups to form sodium carboxylate salts. The amine groups remain as neutral -NH- or -NH₂ groups.

  The three organic products formed are:

  Product 1: The sodium salt of N-(3,3-dimethylbutyl)glycine

        CH₃
        |
  CH₃ - C - CH₂ - CH₂ - NH - CH₂ - COO⁻ Na⁺
        |
        CH₃
  

  Product 2: The disodium salt of aspartic acid

  Na⁺ ⁻OOC - CH₂ - CH - COO⁻ Na⁺
                   |
                   NH₂
  

  Product 3: The sodium salt of phenylalanine

            CH₂ - CH - COO⁻ Na⁺
           /      |
        (C₆H₅)    NH₂
  

  (Note: (C₆H₅) represents a benzene ring)

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  How the marks are awarded

  • M1 — Correct structure for the sodium salt of N-(3,3-dimethylbutyl)glycine. The structure must show the (CH₃)₃C-CH₂-CH₂- group attached to the nitrogen of a glycine backbone, with the carboxylic acid correctly deprotonated to -COO⁻Na⁺.
  • M2 — Correct structure for the disodium salt of aspartic acid. Both the alpha-carboxyl group and the side-chain carboxyl group must be shown as deprotonated sodium salts (-COO⁻Na⁺).
  • M3 — Correct structure for the sodium salt of phenylalanine. The structure must show the phenylmethyl (benzyl) side chain, the alpha-amine group, and the alpha-carboxylic acid group deprotonated to a sodium salt (-COO⁻Na⁺).

  Common mistakes

  • Leaving carboxylic acid groups as -COOH. In excess strong base, all acidic protons are removed to form carboxylate salts (-COO⁻Na⁺).
  • Incorrectly protonating amine groups to -NH₃⁺. In strongly alkaline conditions, amine groups remain as neutral -NH₂ or -NH-.
  • Drawing the incorrect side chain for one of the amino acid products, for example confusing phenylalanine with tyrosine or aspartic acid with glutamic acid.
  • Failing to show both carboxylic acid groups in the aspartic acid derivative as deprotonated salts; often the side-chain carboxyl group is missed.

  Examiner tip: Systematically identify all hydrolysable functional groups (e.g., esters, amides) and all acidic/basic sites in a complex molecule to predict the final structures of products under specific pH conditions.

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