OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2019: Question 19
23 marks · Hard difficulty · Structured Questions
Describe the bonding and structure of benzene, experimental evidence for the delocalised model, polymerisation of benzene derivatives, electrophilic substitution mechanisms, and organic synthesis reaction pathways involving benzene derivatives.
Practise this questionQuestion
Question text
19 This question is about benzene.
(a) Over time, the Kekulé and delocalised models have been used to describe the bonding and
structure of a benzene molecule.
(i) Describe, in terms of orbital overlap, the similarities and differences between the bonding
in the Kekulé model and the delocalised model of benzene.
… [3]
(ii) Experimental evidence led to the general acceptance of the delocalised model over the
Kekulé model.
Describe two pieces of evidence to support the delocalised model of benzene.
… 25 [2]
(b) Benzene can be used as the starting material for the synthesis of compounds D and E, shown
below.
In the diagrams C6H5 is a phenyl group.
CH3
H H
C C H2N C COOH
C6H5 H
C6H5
compound D compound E
Compounds D and E can be converted into polymers.
(i) Draw two repeat units of these polymers.
Two repeat units of polymer formed from D
Two repeat units of polymer formed from E
[3]
(ii) State the type of polymer formed from compounds D and E.
From compound D …
From compound E …
[1]
(iii) In the synthesis of compounds D and E, benzene is first reacted with ethanoyl chloride,
CH3COCl, to form phenylethanone, shown below.
H C O
C
phenylethanone
The reaction takes place in the presence of aluminium chloride, AlCl3, which acts as a
catalyst.
In the mechanism for this reaction,
• ethanoyl chloride first reacts with aluminium chloride to form the CH –C+=O cation
• the CH –C+=O cation then behaves as an electrophile.
Complete the mechanism for the reaction.
Include equations to show the role of the AlCl3 catalyst, relevant curly arrows and the
structure of the intermediate.
Formation of electrophile …
+
H3C C O
Intermediate
H C O
C
+ H+
Regeneration of catalyst …
27 [5]
(iv) Complete the flowchart for the synthesis of compounds D and E from phenylethanone.
H C O
C
NaBH4
phenylethanone
NaCN(aq) / H+(aq)
H
H C
C H
CH3
compound D
Br C CN
… CH
H2N C COOH
compound E [7]
Mark scheme
Show the mark scheme
AO Guidance
Question Answer Marks
element
19 (a) (i) 3 AO1.1 ANNOTATE ANSWER WITH TICKS AND
× 3 CROSSES ETC
ALLOW diagram showing orbital overlap e.g.
Similarities
OR
Orbital overlap
(sideways) overlap of p orbitals
p orbital label is required for first mark
IGNORE C=C in diagram showing π bond
π bond IGNORE reference to s orbital overlap/σ bonds
-----------------------------------------------------------
π bond/system/ring above and below (bonding (C) ALLOW from labelled diagram showing π bond e.g.
atoms/ring/plane)
π bond/π electrons label is required for second mark
AO Guidance
element
Difference ------------------------------------------------------------
ALLOW diagram showing π bond in both Kekule
Kekule has: alternating π bonds OR 3 π bonds / AND delocalised models e.g
localised (π electrons) / overlap in one direction /
2 electrons in π bond
AND
Delocalised has: π ring (system) / all p orbitals
overlap OR (π electrons) spread around ring / overlap
in both directions / 6 electrons in π bond /
π bond labels not required for third mark
(ii) Any 2 pieces of evidence from ( ) 2 AO1.1
×2
Bond length
(C–C) bond length is between single (C–C) and ALLOW (C–C) bond enthalpy is between single (C–
double bond (C=C) C) and double bond (C=C)
OR all (C–C) bond lengths are the same OR all (C–C) bond enthalpies are the same
ΔH hydrogenation
∆H hydrogenation less (exothermic) than IGNORE enthalpy of hydration
expected
Resistance to reaction Benzene is unreactive is not sufficient
Benzene is less reactive than alkenes (no comparison to alkene)
OR bromination of benzene requires a
catalyst/halogen carrier For halogen carrier,
OR benzene does not react with/decolourise ALLOW name or formula of suitable catalyst
bromine (at room temperature) e.g. Fe, AlCl3, FeBr3
OR benzene reacts by substitution
OR benzene does not (readily) react by addition
AO Guidance
element
(b) (i) 3 -----------------------------------------------------------
Polymer from D For BOTH structures,
H H H H ALLOW any combination of skeletal
OR structural OR displayed formula as long as
unambiguous
C C C C
‘End bonds’ MUST be shown
H H BUT ALLOW ECF IF end bonds omitted in both
C6H5 C6H5 AO2.5 structures
DO NOT ALLOW more than 2 repeat units BUT
Polymer from E ALLOW ECF in subsequent structure
CH3 O CH3 O
IGNORE connectivity of C6H5
------------------------------------------------------------
N C C N C C CARE: ALLOW any consistent repeat unit:
C6H5 and H groups can alternate or be on opposite
H C6H5 H C6H5 sides of chain
e.g.
Amide link AO1.2 H H H H
2 repeat units of correct polymer AO2.5 C C C C
H H
C6H5 C6H5
end –NH– may be at either side
e.g.
CH3 O CH3 O
C C N C C N
C6H5 H C6H5 H
IGNORE brackets
IGNORE n
22 AO Guidance
element
(ii) D Addition / polyalkene 1 AO1.1 DO NOT ALLOW ‘additional’
AND
E: Condensation / polyamide
(iii) 5 ANNOTATE ANSWER WITH TICKS AND
CROSSES
ALLOW ‘+’ charge anywhere on CH C+O
Formation of electrophile 3
i.e. CH CO+
CH COCl + AlCl → CH –C+=O + AlCl –
33 3 4 AO2.5
NOTE: curly arrows can be straight, snake-like, etc.
Mechanism but NOT double headed or half headed arrows
Curly arrow from π-bond to CH C+=O
3 AO2.5
1st curly arrow must
• go to the C of C=O
AND
• start from, OR close to circle of benzene
ring
-------------------------------------------------------------
IGNORE curly arrow shown on C=O
23 AO Guidance
element
Correct intermediate AO3.1
Curly arrow from C–H bond to reform π-ring AO2.5
H COCH COCH3
3 DO NOT ALLOW the following intermediate:
+
π-ring should cover approximately 4 of the 6 sides of
the benzene ring structure
AND
the correct orientation, i.e. gap towards C with
COCH3
ALLOW + sign anywhere inside the ‘hexagon’ of
intermediate
Regeneration of catalyst
curly arrow must start from, OR be traced back to,
+ – AO1.2
H + AlCl4 → AlCl3 + HCl any part of C-H bond and go inside the ‘hexagon’
AO Guidance
element
(iv) one mark for each correct structure/reagent 7 AO2.5 ALLOW any vertical bond to the OH OR NH2 groups
×7 e.g. ALLOW
OR AND OR
OH HO NH2 H2N
DO NOT ALLOW OH–, OR NH2– but ALLOW ECF
for subsequent use in this part
For elimination,
IGNORE ‘concentrated’, ‘dilute’ with acids
BUT DO NOT ALLOW H O/steam/(aq)
ALLOW HBr for NaBr/H2SO4
For hydrolysis.
IGNORE missing (aq)
ALLOW HNO3 for hydrolysis but
DO NOT ALLOW ‘HNO3 and H2SO4’
ALLOW final 2 stages in opposite order
i.e. NH3 before acid hydrolysis
NH3 AND ethanol
OR excess NH3
CH3
H2N C CN
H+/H SO /HCl
Total 23
How to answer it
OCR A-Level Chemistry: Benzene Bonding & Synthesis Study Guide
What this question tests
This comprehensive question assesses your understanding of aromatic chemistry, comparing the Kekulé model of benzene with the delocalised model, experimental evidence supporting delocalisation, electrophilic substitution mechanisms (Friedel-Crafts acylation), polymerisation types (addition vs condensation), and multi-step organic synthetic routes involving functional group conversions.
Kekulé vs Delocalised Model Bonding
✅ Correct Answer Structure
- Similarities: Sideways overlap of p-orbitals forming a pi-bond/ring of electron density above and below the bonding carbon atoms/ring plane.
- Differences: Kekulé has alternating single/double bonds (localised electrons in specific p-bonds, overlapping in one direction), whereas the delocalised model has a continuous ring where all p-orbitals overlap in both directions, spreading 6 pi-electrons.
💡 Key Knowledge
You must explicitly refer to p-orbitals and orbital overlap. Mentioning the region of electron density above and below the ring is essential for full marks.
❌ Common Errors
Failing to label p-orbitals in diagrams or stating that Kekulé has delocalised electrons. Students also frequently confuse sigma and pi-bond overlapping directions.
Experimental Evidence for Delocalisation
✅ Correct Answers (Choose any two)
- Bond Lengths: C-C bond lengths in benzene are all equal (intermediate between single C-C and double C=C bonds), unlike Kekulé's alternating lengths.
- Enthalpy of Hydrogenation: Experimental enthalpy of hydrogenation is less exothermic than expected (by about 152 kJ mol⁻¹) compared to a hypothetical Kekulé structure, showing benzene is more thermodynamically stable.
- Resistance to Reaction: Benzene does not readily undergo addition reactions (does not decolourise bromine water at room temperature), requiring a halogen carrier and substitution instead.
🧠 Exam Technique
When citing enthalpy evidence, always specify that it is less exothermic than expected. Saying "lower enthalpy" is ambiguous and often penalised.
Polymers from Compounds D and E
✅ Correct Answers
(i) Repeat Units:
- Polymer D: Polyalkene repeat unit formed by opening the C=C double bond. Must show correct backbone with attached phenyl (C₆H₅) groups and clear extension bonds (brackets optional, but end bonds mandatory).
- Polymer E: Polyamide repeat unit showing the amide link ( -NH-CO- ) with correct repeating sections and end bonds. Must show two repeat units.
(ii) Polymer Types:
- Compound D forms an addition (or polyalkene) polymer.
- Compound E forms a condensation (or polyamide) polymer.
❌ Common Errors
Writing "additional" instead of "addition" loses the mark in part (ii). Omitting end-bonds (dashes) on repeat units or failing to show two repeat units in part (i) will lose marks.
Friedel-Crafts Acylation Mechanism
✅ Correct Answer Steps
- Formation of Electrophile: CH₃COCl + AlCl₃ → CH₃C⁺=O + AlCl₄⁻
- Mechanism Curly Arrows: First arrow starts from inside the benzene ring (or pi-ring) and goes to the positive carbon ( C⁺=O ). Intermediate must show a incomplete horseshoe ring with a plus sign inside and a tetrahedral carbon holding H and COCH₃ .
- Regeneration of Catalyst: H⁺ + AlCl₄⁻ → AlCl₃ + HCl
🧠 Exam Technique
Make sure the curly arrow for the electrophilic attack originates clearly from the delocalised pi-system of the benzene ring, not just floating in space. The horseshoe in the intermediate must enclose approximately 4 or 5 carbons with the open end pointing away from the attachment carbon.
❌ Common Errors
Drawing the first curly arrow starting from the hydrogen atom rather than the ring, or having the intermediate horseshoe accidentally covering the entire 360 degrees of the ring.
Multi-Step Synthesis Flowchart
✅ Correct Reagents and Intermediates
- Phenylethanone to Compound D top branch: Reduction using NaBH₄ followed by dehydration (elimination) using acid catalyst like conc. H₃PO₄ / H₂SO₄ to form the alkene (Compound D).
- Phenylethanone to intermediate: Nucleophilic addition with NaCN(aq) / H⁺(aq) producing a hydroxynitrile ( CH₃-C(OH)(CN)-C₆H₅ ).
- Hydroxynitrile conversion: Elimination of H₂O (using HBr / H₂SO₄ ) creates the unsaturated nitrile, followed by hydrolysis ( H⁺/H₂O ) to form the carboxylic acid precursor, then amination or direct conversion to compound E.
💡 Key Knowledge
Be precise with reagents and conditions. For carbonyl reductions, NaBH₄ in aqueous/alcoholic conditions is standard. Cyanide addition requires NaCN / H⁺ (or KCN / HCN ).
Topics
Module 6: Organic chemistry and analysis · 6.1 Aromatic compounds, carbonyls and acids · 6.2 Nitrogen compounds, polymers and synthesis
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.