OCR A-Level Chemistry Synthesis and analytical techniques (02), November 2020: Question 19

14 marks · Medium difficulty · Structured Questions

Analyze carbonyl compounds F and G regarding homologous series, chemical tests for functional groups, nucleophilic addition mechanism, and heterolytic fission.

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Question

Question text

19 The carbonyl compounds, F and G, shown below, contribute to the flavour of coffee.

CH3 H

CH C

H C C O

3 H

F G

(a) Compound F is a member of a homologous series.

(i) Explain the term homologous series.

… [2]

(ii) Predict the molecular formula for the member of this homologous series containing

24 carbon atoms.

… [1]

(b) Describe suitable chemical tests, with observations, that would confirm the presence of the

functional groups in F and G.

… [4]

(i) Outline the mechanism for the reaction of F with NaCN(aq) and H+(aq) and state the

name of the mechanism. The structure of F has been provided.

Include relevant dipoles, lone pairs and the structure of the organic product.

CH3 H

CH C

H C C O

3 H

Name of mechanism: … [5]

(ii) Explain why the mechanism in (c)(i) involves heterolytic fission.

… [2]

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

element

19 (a) (i) (series of organic compounds with the) same functional 2 1.1 IGNORE reference to physical properties

group OR same/similar chemical properties/reactions ×2 IGNORE same general formula

DO NOT ALLOW same empirical OR

molecular formula

each successive/subsequent member differs by CH2 Differs by CH2 is not sufficient (no successive)

(ii) C24H48O 1 2.1

(b) F/aldehyde 4 IGNORE use of 2,4-DNP with F

AND

Tollens’ (reagent)

AND ALLOW ammoniacal silver nitrate OR Ag+/NH

Silver (mirror/precipitate/ppt/solid) 2.3 ALLOW black ppt OR grey ppt

G/alkene/C=C

AND

Bromine/Br2 ALLOW bromine water/ Br2(aq)

AND

goes colourless/decolourised 3.3

G/ketone ALLOW errors in spelling for 2,4-DNP

AND ALLOW 2,4(-)DNP OR 2,4(-)DNPH

2,4-dinitrophenylhydrazine ALLOW Brady’s reagent or Brady’s Test

AND ALLOW solid OR crystals OR ppt as

orange/yellow/red precipitate 3.3 alternatives for precipitate

G/ketone ALLOW ammoniacal silver nitrate OR Ag+/NH

AND ALLOW black ppt OR grey ppt

Tollens’ (reagent)

AND ALLOW alterative approach using acidified

no silver mirror/no change/no reaction 3.3 potassium dichromate for tests with F and/or G,

with correct observations, alongside use of 2,4-

DNP

Question Answer 15 Marks Guidance

element

CROSSES

Curly arrow must come from lone pair on C

of –CN OR CN–

OR from minus sign on C of –CN ion (then lone

pair on CN– does not need to be shown)

Curly arrow from C=O bond must start from,

Curly arrow from –CN to C atom of C=O

1.2 OR be traced back to, any part of C=O bond

and go to O

Dipole shown on C=O bond, Cδ+ and O δ−,

AND curly arrow from C=O bond to O atom 1.2

Curly arrow from lone pair OR – charge on O– of -------------------------------------------------------------

correct intermediate to H+ 2.5 ALLOW curly arrow to H atom of H2O, i.e.

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

Product 1 mark

IGNORE attempt to draw curly arrow showing

2.5 breaking of H–O in H2O

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

Name of mechanism 1 mark IGNORE lack of dipole on H2O

Nucleophilic addition 1.1

16 element

(ii) Heterolytic 2 1.2 ALLOW 2 electrons go to one (bonded)

One (bonded) atom/O receives both/2 electrons atom/O

DO NOT ALLOW both pairs of electrons go to

Fission O

Breaking of a covalent bond

IGNORE formation of ions/radicals

For O atom,

ALLOW species

DO NOT ALLOW element or molecule

ALLOW π bond in C=O breaks

IGNORE breaking of C=O bond (no reference

to only one bond breaking)

‘Bond breaking’ is not sufficient

(no reference to covalent)

How to answer it

Carbonyl Compounds, Homologous Series & Mechanisms

What this question tests

This question assesses core organic chemistry knowledge including definitions of a homologous series, predicting molecular formulas, functional group identification tests (Tollens', bromine water, 2,4-DNP), curly arrow mechanisms for nucleophilic addition of HCN to carbonyls, and definitions of heterolytic fission.

Part (a)(i)

Homologous Series Definition

✅ Correct Answer

Series of organic compounds with the same functional group AND similar chemical properties.
Each successive/subsequent member differs by CH₂ .

❌ Common Errors

Stating "differs by CH₂ " without the word successive loses a mark.
Mentioning "same general formula" or "same empirical/molecular formula" is ignored or disallowed.
Reference to physical properties is ignored by examiners.

Total: 2 marks

Part (a)(ii)

Predicting Molecular Formula

✅ Correct Answer

C₂₄H₄₈O

💡 Key Knowledge

Compound F is an aldehyde with formula C₅H₁₀O . Adding 19 more CH₂ units to reach 24 carbon atoms yields C₅₊₁₉H₁₀₊₃₈O = C₂₄H₄₈O .

Total: 1 mark

Part (b)

Functional Group Chemical Tests

✅ Correct Answers

For aldehyde (F): Tollens' reagent ➔ Silver mirror (or black/grey precipitate).
For alkene (G): Bromine / Br₂(aq) ➔ Goes colourless / decolourized.
For ketone (G): 2,4-dinitrophenylhydrazine (Brady's reagent) ➔ Orange/yellow/red precipitate.
Alternative for ketone (G): Tollens' reagent ➔ No silver mirror / no change / no reaction.

🧠 Exam Technique

To score all 4 marks, you must state the reagent AND the specific observation for the targeted functional groups present in F and G. Note that compound G contains both an alkene and a ketone group, while F is an aldehyde.

Total: 4 marks

Part (c)(i)

Nucleophilic Addition Mechanism

✅ Correct Answer & Mechanism Steps

Name of mechanism: Nucleophilic addition.
Step 1: Curly arrow from lone pair on :CN⁻ to the C atom of C=O .
Dipole & Arrow 2: Delta plus ( C δ+ ) and delta minus ( O δ- ) clearly shown on C=O , with a curly arrow starting from the C=O bond going to the O atom.
Step 3: Curly arrow from lone pair (or minus charge on O⁻ ) of the intermediate to an H⁺ ion.
Product: Correct cyanohydrin structure drawn with -OH and -CN groups attached.

❌ Common Errors

Starting curly arrows from the wrong place (e.g., arrow starting from a minus sign rather than the lone pair, or double bonds drawn incorrectly).
Omitting partial charges ( δ⁺ / δ⁻ ) on the carbonyl group.
Failing to show the lone pair on the cyanide ion attacking the carbon.

Total: 5 marks (3 for mechanism steps, 1 for product, 1 for mechanism name)

Part (c)(ii)

Heterolytic Fission Explanation

✅ Correct Answer

Heterolytic: One (bonded) atom / Oxygen receives both / 2 electrons.
Fission: Breaking of a covalent bond.

❌ Common Errors

Writing "bond breaking" without specifying that it is a covalent bond.
Stating "both pairs of electrons go to O" instead of "both/two electrons go to one atom/O".

Total: 2 marks

Topics

Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · 6.1 Aromatic compounds, carbonyls and acids · 4.1 Basic concepts and hydrocarbons · 4.2 Alcohols, haloalkanes and analysis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.