OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2018: Question 10

1 mark · Easy difficulty · Multiple Choice

Identify the organic reagent required for the acylation of benzene to form phenylethanone in the presence of a halogen carrier.

Practise this question

Question

A multiple-choice question showing a chemical reaction equation where benzene reacts to form phenylethanone. Below the reaction, four organic reagents are listed as options A to D: A is CH3CH2OH, B is CH3CHO, C is CH3COCl, and D is CH3COOH. A box for the student's answer is provided at the bottom left, with a mark allocation of [1] at the bottom right.
Question text

10 Benzene reacts with an organic reagent in the presence of a halogen carrier to form

phenylethanone.

O

C

CH3

phenylethanone

Which organic reagent is required?

A CH3CH2OH

B CH3CHO

C CH3COCl

D CH3COOH

Your answer

[1]

Mark scheme

Show the mark scheme The mark scheme table shows question number 10 corresponding to the correct answer C, with 1 mark.

10 C 1

How to answer it

Identifying Reagents in Friedel-Crafts Acylation

What this question tests

This question assesses your knowledge of aromatic chemistry, specifically the Friedel-Crafts acylation of benzene. You must be able to recognise the correct organic reactant (an acyl chloride) required to introduce a specific ketone functional group onto an aromatic ring in the presence of a halogen carrier.

Question 10 [1 mark]

Determining the Organic Reagent for Phenylethanone Formation

✅ Correct Answer

C: CH₃COCl

Ethanoyl chloride is an acyl chloride. When reacted with benzene in the presence of a halogen carrier (like AlCl₃), it forms phenylethanone (a phenyl ketone) and hydrogen chloride gas.

💡 Key Knowledge

  • Friedel-Crafts Acylation: Introduces an acyl group (R-C=O) onto an aromatic ring.
  • Acyl Chlorides: General formula RCOCl. For phenylethanone (which contains a two-carbon acyl chain attached to the benzene ring), ethanoyl chloride ( CH₃COCl ) is needed.
  • Role of Halogen Carrier: Generates the reactive electrophile, the acylium ion ( CH₃CO⁺ ).

🧠 Exam Technique

Break down the target molecule structure shown in the diagram. Split it into the benzene ring fragment and the attached side chain ( -C(=O)CH₃ ). Match this two-carbon acyl group to the corresponding acyl chloride.

❌ Common Errors

  • Choosing A ( CH₃CH₂OH , ethanol) — alcohols undergo substitution/elimination, not Friedel-Crafts acylation.
  • Choosing B ( CH₃CHO , ethanal) — aldehydes are not used as acylating agents in this context.
  • Choosing D ( CH₃COOH , ethanoic acid) — carboxylic acids are generally unreactive towards benzene without conversion into more reactive derivatives like acyl chlorides or anhydrides.
Mark Scheme Guidance: 1 mark awarded for selecting option C. No other alternative answers are accepted.

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 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.