A-LevelChemistryCarboxylic acids and derivatives9701/42 · Feb/Mar 2024 · Q617 Marks

Amino acids are molecules that contain –NH2 and —COOH functional groups. Glycine, H2NCH2COOH, is the simplest stable amino acid.

(a)The isoelectric point of glycine is 6.2.
(a)(i)Define isoelectric point.[1]
(a)(ii)Draw the structure of glycine at pH4.[1]
(b)Fig. 6.1 shows two syntheses starting with glycine. [Figure 6.1, reaction scheme starting with glycine leading to hippuric acid and then U]
(b)(i)State the essential conditions for reaction 1.[1]
(b)(ii)Identify the reagent used in reaction 2.[1]
(b)(iii)Draw the structure of the organic product U that forms when hippuric acid reacts with an excess of LiAlH4 in reaction 3.[2]
(b)(iv)A molecule of phenylalanine, R, can react with a molecule of glycine to form two dipeptides, S and T. S and T are structural isomers. [Figure showing structures of R (phenylalanine) and glycine] Draw the structures of these dipeptides. The peptide bond formed should be shown fully displayed. S T[2]
(c)A student proposes a synthesis of hippuric acid by the reaction of benzamide, C6H5CONH2, and chloroethanoic acid, ClCH2COOH. The reaction does not work well because benzamide is a very weak base.
(c)(i)Explain why amides are weaker bases than amines.[2]
(c)(ii)The pKa of chloroethanoic acid is 2.86 whereas the pKa of ethanoic acid is 4.76. Explain the difference between these two pKa values.[2]
(d)Compound V is another amino acid. The proton (1H) NMR spectrum of V shows hydrogen atoms in five different environments, a, b, c, d and e, as shown in Fig. 6.2. [Figure 6.2, structure of compound V with labelled proton environments a, b, c, d, e]
(d)(i)Complete Table 6.2 for the proton (1H) NMR spectrum of V taken in CDCl3. Table 6.1 gives some relevant data. Table 6.2 | proton | a | b | c | d | e | |:----------------------|:----|:----|:----|:----|:----| | chemical shift range, δ/ppm | | | | | | | name of splitting pattern | | multiplet | | | | [4]
(d)(ii)Complete Table 6.3 by placing a tick (✔) to indicate any protons whose peaks are still present in the proton (1H) NMR spectrum of V taken in D2O. Table 6.3 | proton | a | b | c | d | e | |:----------------|:-|:-|:-|:-|:-| | present in D2O | | | | | | [1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2024). The mark or mark type is shown in brackets.

[1] pH at which a molecule has no overall charge / is neutral/ is a Zwitterion / charges cancel out
[1] H3N+CH2COO-
[1] ethanol AND heat in a sealed tube OR ethanol AND high pressure
[1] C6H5COCl / benzoyl chloride / benzoyl anhydride
[1] HOCH2CH2NHCH2C6H5 (alcohol, amine, benzene ring)
[1] carboxylic acid to primary alcohol COOH to CH2OH
[1] amide to amine CONH to CH2NH
[1] rest of the molecule is correct (carbons and benzene ring) mark as •✓✓
[1] dipeptide S: H2N-CH(CH2C6H5)-CO-NH-CH2-COOH (peptide bond between CO of phenylalanine and NH of glycine)
[1] dipeptide T: H2N-CH2-CO-NH-CH(CH2C6H5)-COOH (peptide bond between CO of glycine and NH of phenylalanine)
[1] M1 nitrogen lone pair in amides is delocalised with C=O
[1] M2 lone pair less available for donation / to accept H+ OR less electron density on N / NH2 so less able to accept H+
[1] chloroethanoic acid is a stronger acid (than ethanoic acid)
[1] because electron-withdrawing (–I / inductive) effect of Cl substituent AND weakens O—H / carboxylate anion stabilised
[1] proton a δ 9.0-13.0 splitting singlet
[1] proton b δ 6.0-9.0 splitting multiplet
[1] proton c δ 2.3-3.0 splitting triplet
[1] proton d δ 3.2-4.0 splitting triplet
[1] proton e δ 1.0-5.0 splitting singlet
[1] proton a present in D2O NO
[1] proton b present in D2O YES
[1] proton c present in D2O YES
[1] proton d present in D2O NO
[1] proton e present in D2O NO