OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2019: Question 17

11 marks · Medium difficulty · Structured Questions

Deduce the structure of reaction products involving serine, determine the amino acid used in an acid-base titration, calculate its molar mass, and identify an unknown amino acid from thin-layer chromatography chromatograms using R_f values.

Practise this question

Question

A multipart question about alpha-amino acids starting with Table 17.1 showing four amino acids and their R groups. Part (a)(i) asks to draw organic products for reactions of serine with H+(aq), (CH3)2CHOH/H2SO4, and excess CH3COCl. Part (a)(ii) provides titration data where 5.766 g of an amino acid is made up to 250.0 cm3 and 21.30 cm3 neutralises 25.0 cm3 of 0.150 mol dm-3 HCl, asking to determine which amino acid was used. Part (b) shows two TLC chromatograms of an unknown amino acid alongside four known amino acids using solvents W and X; (b)(i) asks for the R_f value of serine in solvent W, and (b)(ii) asks to identify the unknown amino acid and explain the reasoning.
Question text

17 This question is about α-amino acids, RCH(NH2)COOH.

(a) Table 17.1 shows the R groups in four amino acids.

Amino acid R group

alanine (ala) CH3–

serine (ser) HOCH2–

leucine (leu) (CH3)2CHCH2–

glycine (gly) H–

Table 17.1

(i) In the boxes, draw the organic products for the reactions of serine shown below.

(CH3)2CHOH/H2SO4

H+(aq)

H O

excess CH3COCl

H2N C C

CH OH

OH

serine

[4]

(ii) A student is provided with one of the four amino acids in Table 17.1.

A student carries out a titration with a standard solution of hydrochloric acid to identify

the amino acid. The student’s method is outlined below.

• The student dissolves 5.766 g of the amino acid in water and makes the solution up

to 250.0 cm3 in a volumetric flask.

• The student titrates this solution with 25.0 cm3 of 0.150 mol dm−3 hydrochloric acid.

• 21.30 cm3 of the amino acid solution were required for complete neutralisation of the

hydrochloric acid.

Determine which amino acid the student used.

[4]

(b) The student is provided with another amino acid.

The student attempts to identify the unknown amino acid using chromatography.

The student obtains two TLC chromatograms of the unknown amino acid and the four amino

acids in Table 17.1, using two different solvents, W and X.

solvent front

origin

unknown ala ser leu gly unknown ala ser leu gly

amino amino

acid acid

Solvent W Solvent X

(i) What is the Rf value of serine (ser) in solvent W?

Rf = … [1]

(ii) Analyse the chromatograms to identify the unknown amino acid.

Explain your reasoning.

Name of unknown amino acid …

Explanation …

… [2]

Mark scheme

Show the mark scheme Mark scheme detailing the required structures for the reactions of serine including protonated amine, ester, and acyl chloride derivatives, followed by titration calculation steps determining the moles of HCl, moles of amino acid, and identifying leucine with Mr = 131, and chromatogram analysis identifying glycine with acceptable R_f ranges.

AO

Question Answer Marks Guidance

element

17 (a) (i) 4 AO2.5 ALLOW any combination of skeletal OR structural

H ×4 OR displayed formula as long as unambiguous

O H O

H N C C H N C C H ALLOW protonation of NH group in reaction with

32 2

OH CH O C CH3 (CH3)2CHOH

CH2 2

i.e.

OH CH3

OH H O

+ H

H3N C C

H+(aq)

(CH3)2CHOH / H2SO4 CH O C CH3

OH CH3

H O H O

Excess CH3COCl

H2N C C CH3CONH C C ALL structures must be based on serine

CH OH CH OH

For reaction with excess CH3COCl,

OH CH3COO IGNORE reaction of COOH to form an acid

anhydride

----------------------------------------------------------------

ALLOW 1 mark for

H O

CH3CONH C C

CH Cl

CH3COO

(both NH and OH groups reacted but acyl chloride

instead of COOH)

OR

11 AO

element

O

CH3CONH C C

CH OH

CH3COO

(both NH and OH groups reacted but H missing

from α C atom)

OR

H O

CH3CONH C C

CH OH

OH

(NH group reacted correctly but rest of serine

unchanged)

OR

H O

NH2 C C

CH OH

CH3COO

(OH group reacted correctly but rest of serine

unchanged)

(ii) IF Mr(amino acid) = 131 from titration analysis AWARD 4

12 AO

element

first 3 marks ALLOW alternative approaches

ALLOW 3SF or more throughout

IGNORE trailing zeroes, e.g. ALLOW 0.044 for 0.0440

----------------------------------------------------------------------------

25.0 –3

n(HCl) = 0.150 × OR 3.75 × 10 (mol)

1000

AO2.8

n(amino acid) in 250 cm3

–3 250.0 Calculator: 0.04401408451

= 3.75 × 10 × OR 0.0440 (mol) ALLOW ECF from incorrect n(HCl)

21.30 AO2.8

5.766 –1

M(amino acid) = = 131 (g mol ) ALLOW ECF from incorrect n(amino acid)

0.0440 AO2.8

Amino acid = (CH3)2CHCH2CH(NH2)COOH/leucine ALLOW ECF from incorrect M(amino acid)

AND working to show R = 57 to justify choice i.e. ECF for alkyl group closest to calculated M(alkyl

OR evidence to show Mr leucine = 131 to justify choice group),

AO3.2

e.g. for M(alkyl group) = 15, ALLOW

CH3CH(NH2)COOH

Note: evidence may be shown with table

(b) (i) Rf value in range 0.33 – 0.35 1 AO1.1 ALLOW 2 SF or more. But ignore digits after

second sig fig

.

ALLOW 0.3 for 0.33…..

(ii) gly(cine) 2 AO2.3

×2

Amino acid matches (leu(cine) and) glycine in Solvent W ALLOW glycine has the same/similar Rf as the

AND unknown in both solvents/chromatograms

Amino acid matches (ala(nine) and) glycine in Solvent X

ALLOW suitable alternatives for Rf

e.g. moves same distance

Total 11

How to answer it

Reactions, Titration & Chromatography of Alpha-Amino Acids

What this question tests

This comprehensive multi-part question tests your knowledge of alpha-amino acid chemistry. You are assessed on functional group reactions (esterification, acylation, and acid-base protonation), volumetric titration calculations to determine molar mass and identify unknown structures, calculating Rf values from TLC plates, and interpreting multi-solvent chromatograms.

Part (a)(i) — Reactions of Serine

Functional group transformations involving amine, carboxylic acid, and alcohol groups.

✅ Correct Products (4 Marks)

  • Upward arrow (H+(aq)): Protonation of the amine group to form H₃N⁺-CH(CH₂OH)-COOH .
  • Upward-right arrow ((CH₃)₂CHOH / H₂SO₄): Esterification of the carboxylic acid group with propan-2-ol to form an ester H₂N-CH(CH₂OH)-COOCH(CH₃)₂ . (Note: Protonation of the NH₂ is also accepted).
  • Downward arrow (excess CH₃COCl): Acylation of the amine group (and alcohol group) using excess acyl chloride to form an amide and an ester: CH₃CONH-CH(CH₂COOCH₃)-COOH (or similar depending on alcohol acylation; mark scheme accepts multiple functional group reactions).

💡 Key Knowledge

  • Amine basicity: Reacts with acids like HCl or H⁺(aq) to form alkylammonium salts.
  • Carboxylic acid reactivity: Undergoes Fischer esterification with alcohols in the presence of a strong acid catalyst (H₂SO₄).
  • Acyl chlorides: Extremely reactive nucleophilic addition-elimination reagents that attack both primary amines (forming amides) and alcohols (forming esters) rapidly.

🧠 Exam Technique

Read reagents carefully! When excess CH₃COCl is specified, remember that both the amine group (-NH₂) and the alcohol group (-OH on the serine side chain) can react, yielding both an amide and an ester linkage.

❌ Common Errors

Students often forget that the amino group acts as a base in acidic conditions, missing the positive charge on the nitrogen ( H₃N⁺ ). Another common mistake is drawing ester linkages backward ( -COO- vs -OCO- ).

Part (a)(ii) — Titration Determination

Using volumetric analysis and stoichiometry to identify an unknown amino acid. (4 Marks)

📐 Step-by-Step Calculation

  1. Moles of HCl:
    n(HCl) = 0.150 × (25.0 / 1000) = 3.75 × 10⁻³ mol
  2. Moles of amino acid in titration volume:
    Since amino acids react 1:1 with HCl (monoprotic via the amine base or carboxylate interactions, noting standard OCR stoichiometry for amino acid-HCl titrations):
    n(amino acid in 25.0 cm³) = 3.75 × 10⁻³ mol
    Scale up to the full 250.0 cm³ volumetric flask:
    n(total) = 3.75 × 10⁻³ × (250.0 / 21.30) = 0.0440 mol
  3. Molar mass (Mr) of the amino acid:
    Mr = mass / moles = 5.766 / 0.0440 = 131 g mol⁻¹
  4. Identification:
    Check Table 17.1 R groups:
    - alanine: CH₃- (Mr = 89)
    - serine: HOCH₂- (Mr = 105)
    - glycine: H- (Mr = 75)
    - leucine: (CH₃)₂CHCH₂- (Mr = 57 + 74 = 131).
    Conclusion: Leucine.

❌ Calculation Traps

  • Dilution factor error: Forgetting to scale up the moles from the titrated aliquot (25.0 cm³) to the full volumetric flask volume (250.0 cm³). Always divide by the titration volume pipetted and multiply by the total flask volume.
  • Significant Figures: Ensure final answers match input data precision (3 sig figs used here based on 0.150 and 5.766).

Part (b) — Thin-Layer Chromatography (TLC)

Determining Rf values and identifying unknown amino acids using two solvents.

✅ Correct Answers

  • (i) Rf value of serine in Solvent W:
    Acceptable range: 0.33 to 0.35 (allow 0.3 to match visual estimates).
  • (ii) Identification of unknown:
    Name: glycine (or gly)
    Explanation: The unknown spot travels to the exact same height (has the same Rf value) as glycine in both Solvent W and Solvent X chromatograms.

💡 Key Knowledge

  • Rf definition: Distance moved by substance divided by distance moved by solvent front ( Rf = Spot distance / Solvent front distance ).
  • Two-way / Multi-solvent chromatography: Using different solvents helps separate compounds that might have identical Rf values in a single solvent system, confirming identity unambiguously.
Examiner Commentary: Top-level responses clearly stated both solvent conditions when justifying the identity of glycine in part (b)(ii). Candidates who only checked one chromatogram often misidentified spots due to overlapping Rf values in single systems.

Topics

Module 6: Organic chemistry and analysis · Module 2: Foundations in chemistry · Practical Activity Groups · PAG 2: Acid-base titration · PAG 7: Qualitative analysis of organic functional groups · 6.2 Nitrogen compounds, polymers and synthesis · 6.3 Analysis · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.