OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2018: Question 21

12 marks · Hard difficulty · Structured Questions

Deduce organic products of reactions of an aromatic carboxylic acid, complete synthesis of a polyester, calculate ester concentration from gas chromatography peak areas, and determine ester structures using mass spectrometry data.

Practise this question

Question

A three-part structured chemistry question (Question 21) about aromatic carboxylic acids and their derivatives. Part (a) shows compound H (4-hydroxybenzoic acid) reacting with Na2CO3, NaOH, and Br2 in a flowchart with boxes for products (3 marks). Part (b) shows the synthesis of polymer I from compound H via an acyl chloride intermediate, requiring reagents and structures (4 marks). Part (c) provides a gas chromatogram for four esters (J, K, L, M) with retention times and peak areas, a table of m/z values for their molecular ion peaks, a calculation of concentration for ester M based on ester K (2 marks), and boxes to deduce structures for esters J, L, and M (3 marks).
Question text

21 This question is about aromatic carboxylic acids and their derivatives.

(a) The flowchart below shows some reactions of compound H.

In the boxes, draw the organic products of these reactions.

Na2CO3(aq) NaOH(aq)

O

C

OH

HO

compound H

Br2

[3]

(b) Compound H is used in the synthesis of polymer I, as shown in the flowchart below.

Complete the flowchart by drawing the structure of the acyl chloride and two repeat units of

polymer I, and stating the formula of the reagent(s) required for the first stage on the dotted

line.

O

C

OH

HO

compound H

acyl chloride

two repeat units of polymer I

[4]

(c) A cosmetic product containing four esters, J, K, L and M, is analysed by gas chromatography

and mass spectrometry. The results are shown below.

Gas chromatogram

M

5.9

J K

4.3

4.2

L

1.0

01 2 3 4 5

retention time/minutes

The numbers by the peaks are the relative molar proportions of the compounds in the mixture.

Mass spectrometry

ester m/z of molecular ion peak

J 152

K 166

L 180

M 180

(i) The concentration of ester K in the cosmetic product is 9.13 × 10−2 g dm−3.

Using the results, calculate the concentration, in mol dm−3, of ester M in the cosmetic

product.

Give your answer to two significant figures.

concentration of ester M = … mol dm−3 [2]

(ii) A general structure for esters J, L and M is shown below.

O

C R

O

HO

Where ‘R’ is an alkyl group.

Use the mass spectrometry results to deduce possible structures for esters J, L and M.

J L M

[3]

Mark scheme

Show the mark scheme The mark scheme for Question 21 shows expected structures for the products in part (a), the acyl chloride and polymer repeat units in part (b), full working and answers for the concentration calculation in part (c)(i), and possible structures for esters J, L, and M in part (c)(ii) along with specific marking guidance.

Question Answer Marks Guidance

21 (a) 3 ALLOW any combination of skeletal OR

Product from Na2CO3 structural OR displayed formula as long as

unambiguous

O

ALLOW –COO– OR –COONa

C DO NOT ALLOW negative charge on C atom

- +

O (Na ) DO NOT ALLOW –COO–Na (covalent bond)

IGNORE connectivity of phenol OH group

(marks are for correct conversions)

HO

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

Product from NaOH(aq) ALLOW 1 mark if top two structures are

shown in wrong boxes

O

C

+

O (Na )

(Na+)

O

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

ALLOW substitution of any H from benzene

Product from Br2 ring

O

ALLOW multiple substitution, i.e. di-, tri- and

tetrabromo products.

Br C

OH IGNORE connectivity of phenol OH group

(marks are for correct conversions)

e.g. HO

Question Answer 27 Marks Guidance

(b) One mark for each correct structure/reagent as shown below 4 ALLOW any combination of skeletal OR

structural OR displayed formula as long as

unambiguous

O

O ALLOW PCl OR PCl for reagent mark.

C IGNORE references to temperature for

C

OH SOCl2 reagent mark

Cl

IGNORE additional reagents shown with

SOCl2/PCl5/PCl3 e.g. H2O, AlCl3, HCl etc.

HO

HO

IGNORE names (question asks for structures

compound H of organic compounds and formula of

acyl chloride reagent)

DO NOT ALLOW more than two repeat units

ALLOW 1 mark for one correct repeat unit

e.g.

O

O C

‘End bonds’ MUST be shown (do not have to

O O be dotted)

O C O C ALLOW the ‘O’ at either end

i.e.

O O

ester link

C O C O

rest of structure

IGNORE brackets

two repeat units of polymer I

IGNORE n

(c) (i) FIRST CHECK ANSWER ON ANSWER LINE 2 If there is an alternative answer,

IF answer = 7.5 10–4 award 2 marks Apply ECF

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

Alternative method

[K] in mol dm–3

[K] in g dm–3 with peak area of 5.9

9.13 × 10–2

= 5.50 × 10–4 (mol dm–3) 5.9

9.13 × 10–2 OR 9.13 × 10–2 1.37

166 4.3

= 0.125 OR 0.13 (g dm–3)

[L] from peak areas Calculator: 0.125272093

–4 5.9 –4

5.50 × 10 4.3 OR 5.50 × 10 1.37.…. –3

[L] in mol dm

= 7.5 10–4 (mol dm–3) 0.125

= 7.5 10–4

2 SF Required 0.13 –4 –3

OR 166 = 7.8 10 (mol dm )

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

Common errors:

Award 1 mark for:

9.13 × 10–2

0.099(from 166

180)

–4 0.125

6.9 10 (from )

–4 0.13

7.2 10 (from 180 )

7.0 10–4 (from

0. 25272093

180 )

(ii) ester J 3 ALLOW any combination of skeletal OR

O structural OR displayed formula as long as

unambiguous

C CH3

O

HO

L and M can be identified either way round

esters L and M

O IGNORE ‘C3H7’ in L and/or M as ambiguous

(question requires structures)

C CH2CH2CH3

O IGNORE connectivity of phenol OH group

(marks are for structures of alkyl groups)

HO

O

C CH(CH3)2

O

HO

Total 12

How to answer it

Reactions, Synthesis and Analysis of Aromatic Carboxylic Acid Derivatives

What this question tests

This multi-step question assesses your understanding of aromatic functional group chemistry, acylation reactions, condensation polymerisation, and analytical chemistry techniques (gas chromatography combined with mass spectrometry and peak ratio calculations).

Part (a): Reactions of Compound H

Functional group reactivity of hydroxybenzoic acid

✅ Correct Answers (Products)

  • With Na₂CO₃(aq): Forms a sodium carboxylate salt ( -COO⁻ Na⁺ ) while the phenol -OH group remains unreacted.
  • With NaOH(aq): Reacts at both sites to form a disodium salt (carboxylate salt + sodium phenoxide ring substituent).
  • With Br₂(aq): Electrophilic substitution into the benzene ring (typically ortho/para to the activating -OH group, e.g., 2-bromo or multi-substituted bromo derivatives), while the -COOH group stays intact.

💡 Key Knowledge

  • Carboxylic acids are strong enough to react with weak bases like Na₂CO₃ , whereas phenols only react with strong bases like NaOH .
  • The phenolic -OH is strongly activating and directs incoming electrophiles ( Br₂ ) to the 2, 4, and 6 positions.
Mark breakdown: 3 marks total (1 mark for each correct product box).

Part (b): Synthesis of Polymer I

Acylation and Condensation Polymerisation

✅ Correct Answers

  • Reagent for 1st stage: SOCl₂ (or PCl₅ / PCl₃ ).
  • Acyl chloride structure: Replaces the -OH of the carboxylic acid with -Cl , leaving the phenolic -OH untouched.
  • Polymer I structure: Polyester formed via self-condensation containing two repeat units with clear, dotted end-bonds showing continuity ( -O-Ar-CO-O-Ar-CO- ).

❌ Common Errors & Examiner Pitfalls

  • Forgetting to show clear, dotted continuation bonds on the ends of the polymer repeat unit structure.
  • Drawing more than two repeat units when explicitly asked for two.
  • Failing to specify the formula/reagent correctly for acyl chloride formation (e.g., writing water or invalid catalysts).
Mark breakdown: 4 marks total (1 for reagent, 1 for acyl chloride structure, 2 for the correct two-repeat unit polyester chain).

Part (c)(i): Chromatography & Mass Spec Calculation

Determining Concentration from Peak Areas and Molar Proportions

📐 Step-by-Step Calculation

  1. Find moles of K: Use concentration and the relative molar mass ( Mr of K = 166).
    Moles of K = (9.13 × 10⁻²) / 166 = 5.50 × 10⁻⁴ mol dm⁻³
  2. Use peak area ratios: From the chromatogram, peak area for K = 4.3 and peak area for M = 5.9.
    Ratio of M to K = 5.9 / 4.3
  3. Calculate concentration of M:
    Conc of M = (5.50 × 10⁻⁴) × (5.9 / 4.3) = 7.546... × 10⁻⁴
  4. Apply significant figures: Round to two significant figures.
    Final Answer: 7.5 × 10⁻⁴ mol dm⁻³

🧠 Exam Technique & Trap Alert

  • Significant figures: The question explicitly requested two sig figs. Giving 7.55 × 10⁻⁴ loses the final accuracy mark.
  • Always check whether peak areas correlate directly to molar concentrations in mixtures. Here, relative peak heights/areas directly represent relative molar proportions.
Mark breakdown: 2 marks (1 mark for correct working/moles of K converted via ratio, 1 mark for final calculated concentration evaluated to 2 significant figures).

Part (c)(ii): Elucidating Ester Structures

Using Mass Spectrometry Molecular Ion Peaks ( m/z )

✅ Correct Structures

  • Ester J ( m/z = 152): Methyl ester derivative. Alkyl group R = -CH₃ .
  • Ester L ( m/z = 180): Propyl ester derivative (straight chain). Alkyl group R = -CH₂CH₂CH₃ .
  • Ester M ( m/z = 180): Isopropyl ester derivative (branched chain isomer). Alkyl group R = -CH(CH₃)₂ .

💡 Key Knowledge

  • The molecular ion peak ( m/z ) gives the relative molecular mass ( Mr ) of each ester molecule.
  • By subtracting the mass of the common hydroxybenzoate core from the total Mr , you deduce the exact mass and formula of the alkyl chain ( R ).
  • Constitutional isomers (like propyl and isopropyl groups) share the same Mr (180) and molecular formula but differ in structural arrangement.
Mark breakdown: 3 marks total (1 mark for each correctly identified ester structure corresponding to J, L, and M).

Topics

Module 6: Organic chemistry and analysis · Module 4: Core organic chemistry · 6.1 Aromatic compounds, carbonyls and acids · 6.2 Nitrogen compounds, polymers and synthesis · 6.3 Analysis · 4.2 Alcohols, haloalkanes and analysis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.