Edexcel A-Level Chemistry Paper 2, June 2017: Question 8
18 marks · Hard difficulty · Extended Writing
Analyse the reactions, mechanism, distinguishing tests, proton NMR spectra, and reduction of phenylethanone and related aromatic compounds.
Practise this questionQuestion
Question text
8 Phenylethanone is an ingredient in many types of chewing gum.
O CH3
C
One method for the production of phenylethanone involves the reaction of
benzene with ethanoyl chloride, CH3COCl.
(a) (i) Write the equation for the formation of the electrophile, CH CO+,
from ethanoyl chloride using the catalyst aluminium chloride.
(1)
(ii) Complete the diagram, including curly arrows, to show the mechanism for the
reaction between this electrophile and benzene to produce phenylethanone.
Include the regeneration of the catalyst.
(4)
CH3
C O
+
(b) Phenylethanone can be distinguished from its structural isomer, phenylethanal,
in a number of different ways.
O
C
CH2 H
(i) Which would react with phenylethanone but*P48059A01424*notwith phenylethanal?
(1)
A acidified sodium dichromate(VI)
B alkaline iodine solution
C Fehling’s solution
D Tollens’ reagent
(ii) Give the steps to show how 2,4-dinitrophenylhydrazine could be used to
distinguish between phenylethanone and phenylethanal.
(4)
*(iii) Compare and contrast the high resolution proton NMR spectra of
phenylethanone and phenylethanal. 15
You should use the Data Booklet.*P48059A01524*
O
O CH C
C CH2 H
Phenylethanone Phenylethanal
(6)
(c) The compound 1-phenylethanol can be formed from phenylethanone.*P48059A01624*
HO CH3
CH
Give the reagent and conditions that would be used to form 1-phenylethanol.
(2)
(Total for Question 8 = 18 marks)
Mark scheme
Show the mark scheme
Question
Answer Additional Guidance Mark
Number
8(a)(i) CH COCl + AlCl → CH CO+ + AlCl − Accept use of FeCl3/Fe +Cl2 (1)
33 3 4
Allow displayed formulae
Do not award C2H3OCl
Ignore state symbols even if incorrect
Question
Answer Additional Guidance Mark
Number
8(a)(ii) (4)
electron pair movement from ring to
Do not award curly arrow that ends at the CH3
electrophile (1)
Allow arrow starting anywhere within the hexagon
formula of intermediate ion (1) ‘Horseshoe’ to cover at least three carbon atoms
and face the tetrahedral carbon and with some part
of the plus sign inside ‘horseshoe’
Allow Kekulé diagrams
Do not award dotted bonds unless part of a
3D structure
curly arrow from C−H bond to reform
delocalised ring (1)
correct product and equation to show Could be shown in reaction mechanism
regeneration of catalyst and HCl (1) Ignore curly arrows
Question
Answer Mark
Number
8(b)(i) The only correct answer is B (alkaline iodine solution) (1)
A is not correct because this oxidising agent would react with phenylethanal and not with
phenylethanone which is the wrong way round
C is not correct because test is for aldehydes and so would react with phenylethanal and not with
phenylethanone which is the wrong way round
D is not correct because test is for aldehydes and so would react with phenylethanal and not with
phenylethanone which is the wrong way round
Question
Answer Additional Guidance Mark
Number
An answer that makes reference to the following points: (4)
8(b)(ii)
formation of yellow/orange/red (crystalline) precipitate (1) Colour and state are both required
Allow solid for ppt
Ignore any conditions given with the
use of 2,4-DNPH
(Filter then) recrystallisation of products (1)
determination of melting temperature (1) Penalise M3 if any reference to
boiling temperature
comparison (and hence identification) from use of Award only in the context of melting
database/known values (1) temperature of the hydrazones or as
a TE of boiling temperature
Max 3 out of 4 if test is only carried
out with one of the carbonyls
Question
Answer Additional Guidance Mark
Number
This question assesses the student’s ability to show a Guidance on how the mark scheme (6)
*8(b)(iii) coherent and logically structured answer with linkages should be applied:
and fully sustained reasoning. The mark for indicative content should
be added to the mark for lines of
Marks are awarded for indicative content and for how the
reasoning. For example, a response with
answer is structured and shows lines of reasoning.
four indicative marking points that is
The following table shows how the marks should be partially structured with some linkages
awarded for indicative content. and lines of reasoning scores 4 marks (3
Number of indicative Number of marks awarded marks for indicative content and 1 mark
marking points seen in for indicative marking points for partial structure and some linkages
answer and lines of reasoning).
64 If there were no linkages between the
5-4 3 points, then the same indicative
3-2 2 marking points would yield and overall
11 score of 3 marks (3 marks for indicative
00 content and zero marks for linkages).
The following table shows how the marks should be In general it would be expected
awarded for structure and lines of reasoning that 5 or 6 indicative points would
Number of marks awarded get 2 reasoning marks, and 3 or 4
for structure of answer and
indicative points would get 1 mark
sustained lines of
reasoning for reasoning, and 0, 1 or 2
Answer shows a coherent 2 indicative points would score zero
logical structure with linkages marks for reasoning. If there is any
and fully sustained lines of incorrect chemistry, deduct mark(s)
reasoning demonstrated from the reasoning. If no reasoning
throughout mark(s) awarded do not deduct
Answer is partially structured 1 mark(s).
with some linkages and lines of
reasoning If there is no mention of
Answer has no linkages 0 protons/hydrogens in the response then
between points and is
deduct one structure and reasoning
unstructured
mark
Ignore references to C13 nmr
Accept annotations on a structure
towards crediting the following IPs
Allow either a single chemical shift value
Indicative content or a range within the stated values
Penalise incorrect chemical shifts
Similarities
IP1: aromatic hydrogens will give similar/same peaks Both have peaks in the range 6.5-8.4
(ppm)
Ignore any splitting description
IP2: both have a peak in the range 1.7-3.0 (ppm) Ignore any splitting pattern given for
(due to the hydrogen of the H−C−C=O type) this peak to award this mark
Differences
IP3 (Hydrogen environments): Phenylethanone has Allow any difference of one in the
one less peak/hydrogen environment than number of peaks stated
phenylethanal
IP4 (Splitting patterns): a singlet for phenylethanone All these splitting patterns required for
but a doublet and a triplet in phenylethanal this IP
IP5 (Peak area ratios): relative peak (area) ratio in Ignore the splitting pattern for this IP
phenylethanone is 3 but in phenylethanal the peak and ignore any peak areas given for the
(area) ratio is 2 to 1 aryl hydrogens
IP6 (Chemical shifts): (Only) phenylethanal has an Ignore the splitting pattern for this IP
aldehyde (hydrogen) peak in the range 9 – 10.1 (ppm)
Question
Answer Additional Guidance Mark
Number
Reagent: lithium tetrahydridoaluminate((III)) /LiAlH4 (1) Allow lithium aluminium hydride (2)
8(c)
Accept sodium tetrahydridoborate
/sodium borohydride/ NaBH4
In water/alcohol for 2 marks
Ignore reference to addition of acid
after use of LiAlH4 in dry ether
Do not award with additional
reagents
Conditions: (dry) ether/ethoxyethane (1) Ignore heat
The mark for conditions is
dependent on correct reagent or
near miss such as incorrect formula
LiAlH3/LiAlH/LiAl
(Total Question 8 = 18 marks)
How to answer it
Phenylethanone Reactions and Spectra Study Guide
This question covers key aromatic chemistry topics: Friedel-Crafts acylation mechanisms, functional group identification using chemical tests (such as iodine/alkaline iodine and derivatives), distinguishing structural isomers using high-resolution proton NMR spectroscopy (chemical shifts, integration, and splitting patterns), and carbonyl reduction reagents and conditions.
Electrophile Generation Equation
✅ Correct Answer
CH₃COCl + AlCl₃ → CH₃CO⁺ + AlCl₄⁻
💡 Key Knowledge
- Friedel-Crafts acylation requires a Lewis acid catalyst like aluminium chloride ( AlCl₃ ) to generate the powerful CH₃CO⁺ electrophile.
❌ Common Errors
- Writing incorrect formulae for the acylium ion or forgetting the negative charge on the tetrachloroaluminate ion ( AlCl₄⁻ ).
Electrophilic Substitution Mechanism
✅ Correct Answer
Mechanism requirements:
- Curly arrow starting from inside the benzene ring delocalised system pointing to the CH₃CO⁺ carbon.
- Intermediate 'horseshoe' structure enclosing at least 3 carbon atoms with a positive charge located inside.
- Curly arrow from the C-H bond breaking and returning electrons to restore the delocalised ring.
- Equation/showing regeneration of AlCl₃ catalyst and formation of HCl .
🧠 Exam Technique
- Ensure the curly arrow from the ring clearly originates from the electrons inside the benzene ring, not from a random C-C bond or hydrogen atom.
- The horseshoe intermediate must be drawn precisely: the open end of the horseshoe must cover at least carbons 2 through 6, and the + sign must sit inside it.
Distinguishing Chemical Test
✅ Correct Answer
B (alkaline iodine solution)
💡 Key Knowledge
- Phenylethanone contains a methyl ketone group ( -COCH₃ ), which gives a positive triiodomethane (iodoform) test with alkaline iodine ( I₂ / NaOH ), forming a yellow crystalline precipitate.
- Phenylethanal is an aldehyde and does not have the methyl ketone grouping, meaning it will not react with alkaline iodine in this manner.
❌ Common Errors
- Choosing oxidising agents like acidified dichromate, Tollens', or Fehling's, which react with aldehydes (phenylethanal) rather than specifically targeting the methyl ketone in phenylethanon.
Derivatives and Melting Point Determination
✅ Correct Answer
- Add 2,4-DNP (2,4-dinitrophenylhydrazine) reagent to form a yellow/orange/red crystalline precipitate.
- Filter and recrystallise the precipitate product.
- Determine the melting temperature of the purified derivative.
- Compare the measured melting temperature with literature values or a database to identify the compound.
🧠 Exam Technique
- Always state "melting temperature" rather than boiling temperature when dealing with solid crystalline derivatives.
- Recrystallisation is a vital purification step prior to melting point determination to ensure sharp, accurate melting points.
Proton NMR Comparison
✅ Correct Answer
Similarities:
- Aromatic ring protons ( Ar-H ) appear in the same chemical shift region ( 6.5 - 8.4 ppm ) for both isomers.
- Both have a peak corresponding to protons adjacent to the carbonyl ( H-C-C=O ) in the 1.7 - 3.0 ppm range.
Differences:
- Number of environments: Phenylethanone has fewer hydrogen environments than phenylethanal.
- Splitting patterns: Phenylethanone exhibits a singlet for its methyl group ( -CH₃ ), whereas phenylethanal exhibits a doublet and triplet system for its -CH₂- and -CHO protons.
- Integration ratios: Peak area ratios differ between the non-aromatic protons.
- Aldehyde proton: Only phenylethanal has a characteristic aldehyde proton peak in the 9 - 10.1 ppm range.
🧠 Exam Technique
This is a levelled extended-response question. To score top marks for reasoning, clearly group your points into structural similarities and specific differences, explicitly mentioning chemical shifts, splitting patterns, and integration ratios from the Data Booklet.
Reduction of Phenylethanone
✅ Correct Answer
- Reagent: Lithium tetrahydridoaluminate(III) ( LiAlH₄ ) or sodium borohydride ( NaBH₄ ).
- Conditions: Dry ether / ethoxyethane (for LiAlH₄ ) or water/alcohol solvent (for NaBH₄ ).
❌ Common Errors
- Using incorrect spelling or formulae for reducing agents (e.g. writing LiAlH₃ ).
- Failing to specify dry conditions when using LiAlH₄ , which reacts violently with water.
Topics
Organic Chemistry · Topic 18: Organic Chemistry III · Topic 19: Modern Analytical Techniques II · Topic 17: Organic Chemistry II · Topic 6: Organic Chemistry I
Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 2, June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.