A-LevelChemistryCarboxylic acids and derivatives9701/42 · May/June 2018 · Q713 Marks

Asparagine and valine are two naturally occurring amino acids. [Figure showing structures of asparagine and valine]

(a)Give the molecular formula of asparagine.[1]
(b)Name all of the functional groups in an asparagine molecule.[2]
(c)Draw the structure of the dipeptide formed by valine and asparagine. The peptide bond should be shown displayed and should be clearly labelled.[2]
(d)(i)Explain what is meant by a buffer solution.[2]
(d)(ii)Write two equations to explain how valine can act as a buffer. Use the formula H2NCHRCO₂H for valine in your equations.[2]
(e)Each valine molecule has one chiral carbon atom. Draw three-dimensional diagrams to show the two optical isomers of valine. The (CH3)2CH group can be represented as R.[2]
(f)Asparagine is hydrolysed when heated with aqueous sulfuric acid. Write an equation for this reaction.[2]

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 (May/June 2018). The mark or mark type is shown in brackets.

[1] C4H8N2O3
[2] amide, amine, carboxylic acid
[1] peptide / amide bond / unit labelled or circled
[1] (val-asp or asp-val) rest of the dipeptide structure is correct
[1] resists change in pH/pH kept within a small range
[1] when small amount of acid or alkali / base is added
[1] H2NCHRCO2H + H⁺ → H3N⁺CHRCO2H
[1] H2NCHRCO2H + OH⁻ → H2NCHRCO2⁻ + H2O
[1] correct 3D structure for one optical isomer of valine (e.g., central carbon with -COOH, -NH2, -H, and -R groups, with -H on dash and -NH2 on wedge)
[1] correct 3D structure for the other optical isomer of valine (e.g., central carbon with -COOH, -NH2, -H, and -R groups, with -H on wedge and -NH2 on dash)
[2] H2NCH(CH2CONH2)CO2H + H2O → H2NCH(CH2CO2H)CO2H + NH3 scores OR H2NCH(CH2CONH2)CO2H + H2O + 2H⁺ → H3N⁺CH(CH2CO2H)CO2H + NH4⁺

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