Edexcel A-Level Chemistry Paper 3, June 2017: Question 3

7 marks · Medium difficulty · Short Open Response

Give the systematic IUPAC name for lysine, draw the structures of its reaction products with various reagents, draw the structure of an alanine-lysine dipeptide, and explain its chromatography retention relative to alanine.

Practise this question

Question

The question presents chemical structures for two amino acids, alanine and lysine. Part (a) asks for the systematic IUPAC name for lysine. Part (b) requires drawing the organic products formed when lysine reacts with aqueous sodium hydroxide, excess dilute hydrochloric acid, and methanol with concentrated sulfuric acid. Part (c) asks for the structure of a dipeptide formed from alanine and lysine. Part (d) asks to explain why lysine leaves a polar stationary phase chromatography column after alanine following acidic hydrolysis of the dipeptide.
Question text

3 Alanine and lysine are amino acids.

H H

H2N C COOH H2N C COOH

CH3 CH2(CH2)3NH2

alanine lysine

(a) Give the systematic (IUPAC) name for lysine.

(1)

(b) Draw the structure of the organic product formed when lysine reacts with the

following reagents:

(3)

aqueous sodium hydroxide, NaOH(aq)

excess dilute hydrochloric acid, HCl(aq)

methanol, with warming, in the presence of a few drops of concentrated sulfuric acid.

(c) Draw the structure of a dipeptide formed when one molecule of alanine reacts with one

molecule of lysine.

(1)

*P48954A01036*

(d) The dipeptide formed in part (c) is hydrolysed under acidic conditions and the resulting

mixture is analysed by column chromatography. The column uses a polar stationary phase.

Explain why lysine leaves the chromatography column after alanine.

(2)

(Total for Question 3 = 7 marks)

Mark scheme

Show the mark scheme The mark scheme provides the acceptable answers for all parts of Question 3. For part (a), it gives 2,6-diaminohexanoic acid. For part (b), it displays the deprotonated, protonated, and ester structures of lysine. For part (c), it shows the possible dipeptide structures. For part (d), it awards marks for noting that lysine has two positive charges in acidic conditions and thus greater attraction for the polar stationary phase.

Question Mark

Number Acceptable Answers Additional Guidance

3(a) 2,6-diaminohexanoic acid Allow 2,6-diaminehexanoic acid (1)

Ignore any additional commas or hyphens or spaces

Do not award 2,6-diamminohexanoic acid

Question Mark

Number Acceptable Answers Additional Guidance

3(b) Deprotonated structure Allow displayed /structural /condensed (3)

(1) formulae

H

Allow NH2–

Allow –CO −

H NCH CH CH CH C COO (Na ) 2

22 2 2 2

Allow –COONa but penalise if O–Na covalent

bond is shown

NH2

Protonated structure

(1)

Both NH2 groups must be protonated

H + +

Allow NH3 – / H3N–

Allow –CO2H

H3NCH2CH2CH2CH2 C COOH (2Cl )

+NH3

Ester structure

(1) Allow CO2CH3

Allow NH +– or NH – for each amine group

H

Penalise wrong side chain only once

H NCH CH CH CH C COOCH If alanine used throughout then only MP3 can

22 2 2 2 3

be awarded

NH2

Question Mark

Number Acceptable Answers Additional Guidance

3(c) Any one of: Must be the dipeptide and not the repeat unit (1)

Allow –CO2H

H O H CH3 Allow –H2N

Allow –CONH– / -COHN- unless C-H-N

H NCH CH CH CH C C N C COOH Allow zwitterions or cyclic dipeptides

22 2 2 2

Allow skeletal / part-skeletal formulae

NH2 H

OR

H O H CH2CH2CH2CH2NH2

H3C C C N C COOH

NH2 H

OR

H O H H

H3C C C N CH2CH2CH2CH2 C COOH

NH2 NH2

Question Mark

Number Acceptable Answers Additional Guidance

3(d) An explanation that makes reference to the following Ignore comments on retention time, solubility, (2)

points: polarity, dipoles or intermolecular forces

(In acidic conditions) lysine (ion) has two positive

charges (whereas alanine has only one) (1) Allow ‘greater positive charge’

Allow lysine has 2 NH2 groups that can be

protonated

(So lysine ion has) has greater attraction for the

stationary phase (1) Allow

‘greater affinity for stationary phase’

‘adheres better to stationary phase’

‘better adsorption onto stationary phase’

Allow

‘polar phase’ for ‘stationary phase’

Allow reverse argument for alanine

Mark points M1 and M2 independently

(Total for Question 3 = 7 marks)

How to answer it

Overall Difficulty Judgement: Medium

Study Guide: Amino Acids, Reactions, and Chromatography

What this question tests

This question assesses your knowledge of amino acid chemistry, including IUPAC nomenclature, chemical reactions of functional groups (carboxylic acids, amines, and esters), peptide bond formation (amide synthesis), and the principles of column chromatography based on ionic charge and stationary phase interaction.

Part (a): IUPAC Nomenclature of Lysine

(1 Mark)

✅ Correct Answer

2,6-diaminohexanoic acid

(Spelling tip: Ensure you write "diamino" and not "diammino").

💡 Key Knowledge

  • Identify the longest carbon chain containing the principal functional group (carboxylic acid = 6 carbons, hexanoic acid).
  • Number the chain starting from the carboxylic acid carbon as C1.
  • Locate amine groups at carbons 2 and 6.
Mark Awarded: 1 mark for the exact correct IUPAC name.

Part (b): Reactions of Lysine with Specific Reagents

(3 Marks - 1 mark per reagent structure)

✅ Correct Answers (Structures)

  • With aqueous NaOH: The carboxylic acid is deprotonated to form -COO⁻ (Na⁺) while the two amine groups -NH₂ remain unchanged.
  • With excess dilute HCl: Both amine groups are protonated to form -NH₃⁺ (2Cl⁻) while the carboxylic acid -COOH remains unprotonated.
  • With methanol + conc. H₂SO₄: Forms an ester where the carboxylic acid group becomes -COOCH₃ and the amine groups remain -NH₂ .

❌ Common Errors

  • Failing to protonate both amine groups in hydrochloric acid.
  • Showing a covalent bond between oxygen and sodium ( O-Na ) instead of ionic species.
  • Modifying the wrong parts of the molecule (e.g., reacting the amine with methanol).
Mark Awarded: 1 mark for each correctly identified product structure.

Part (c): Formation of a Dipeptide

(1 Mark)

✅ Correct Answer

A dipeptide structure formed by condensing one molecule of alanine with one molecule of lysine via a peptide bond ( -CONH- ).

🧠 Exam Technique

Double-check that you have drawn a dipeptide (joining two amino acid residues together) and not a repeating polymer chain. Ensure the amide linkage is clearly represented as -CO-NH- or -NH-CO- .

Mark Awarded: 1 mark for a valid dipeptide structure showing correct peptide linkage.

Part (d): Column Chromatography Separation

(2 Marks)

✅ Correct Answers

  • Point 1: Under acidic hydrolysis/conditions, the lysine ion has two positive charges (due to two protonated amine groups), whereas alanine has only one.
  • Point 2: Therefore, the lysine ion has a greater attraction / affinity for the polar stationary phase and travels more slowly, leaving the column after alanine.

💡 Key Knowledge

Stationary phases in column chromatography are often polar. Components in a mixture with higher polarity or ionic charge form stronger interactions with the stationary phase, leading to longer retention times.

Mark Awarded: 1 mark for identifying lysine's higher positive charge in acid, and 1 mark for linking this to a stronger attraction/affinity for the polar stationary phase.

Topics

Organic Chemistry · Topic 18: Organic Chemistry III · Topic 19: Modern Analytical Techniques II

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.