OCR A-Level Chemistry Unified chemistry (03), June 2018: Question 6

12 marks · Hard difficulty · Extended Response

Name the product of the base-catalysed addition reaction of ethanal, write the mechanism/equations for its formation, draw the product for propanone reacting similarly, and explain the role of electrophiles with aliphatic and aromatic examples including mechanisms.

Practise this question

Question

Exam question with two main parts. Part (a) presents Reaction 6.1 showing ethanal reacting with ethanal to form compound A, with sub-parts asking for the systematic name (ai), reaction type (aii), completion of step 1 acid-base equilibrium with conjugate acid-base pairs and step 2 equation (aiii), and the product of propanone with OH- (aiv). Part (b) is a 6-mark extended response question asking to explain the role of electrophiles in organic chemistry including one reaction of an aliphatic compound and one reaction of an aromatic compound with relevant mechanisms.
Question text

6 This question is about organic reactions.

(a) Compound A is formed when ethanal is mixed with OH–(aq) ions, which act as a catalyst.

The balanced equation is shown in reaction 6.1 below.

H O H O H OH H O

H C C + H C C H C C C C Reaction 6.1

H H H H H H H H

Compound A

(i) Give the systematic name for compound A.

… [1]

(ii) What type of reaction has taken place?

… [1]

(iii) Reaction 6.1 takes place in two steps. OH– ions act as a catalyst.

In step 1, ethanal reacts with OH– ions to set up an acid–base equilibrium.

In step 2, compound A is formed.

• Complete the equilibrium for step 1 and label the conjugate acid–base pairs as:

A1, B1 and A2, B2.

CH CHO + OH– … + …

• Suggest the equation for step 2.

[3]

(iv) A similar reaction takes place when propanone, (CH ) CO, is mixed with OH–(aq) ions.

Draw the structure of the organic product of this reaction.

[1]

(b)* Many organic reactions use electrophiles as reagents.

Explain the role of electrophiles in organic chemistry.

Your answer should include one reaction of an aliphatic compound and one reaction of an

aromatic compound, including relevant mechanisms. [6]

Additional answer space if required.

Mark scheme

Show the mark scheme Mark scheme providing answers and guidance for question 6, including expected systematic names, reaction types, acid-base equilibria for the carbonyl addition mechanism, structures for the propanone reaction product, and a three-level marking grid for the 6-mark extended response on electrophiles with example mechanisms for alkenes and benzene.

Question Answer Marks Guidance

6 (a) (i) 3-hydroxybutanal 1 ALLOW 3-hydroxybutan-1-al

IGNORE lack of hyphens or addition of commas

ALLOW 4-oxobutan-2-ol OR 1-oxobutan-3-ol

DO NOT ALLOW

3-hydroxybutal

3-hydroxylbutanal

(ii) Addition 1 IGNORE nucleophilic OR electrophilic OR radical

DO NOT ALLOW addition–elimination, condensation,

polymerisation

(iii) ALLOW any formula provided that number and type of 3 Throughout, IGNORE ‘connectivity in any formula or

atoms and charge are correct, structures shown.

e.g. For CH3CHO, ALLOW CH3COH, C2H4O, etc. Examples in Answer column and in 6a(iv) guidance below

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

Step 1:

Correct equation

One correct acid–base pair

i.e. A1 and B1 OR A2 and B2

Step 1: ALLOW H+ transfer from OH–,

CH CHO + OH– –CH CHO + H O i.e.

32 2

CH CHO + OH– CH CH O+ + O2–

OR CH CHO + OH– CH CO– + H O 3 3 2

33 2

A1 B2 B1 A2 B2 A1 A2 B1

OR OR

A2 B1 B2 A1 B1 A2 A1 B2

Step 2: Step 2:

CH CHO + CH CH O+ + O2–

CH CHO + –CH CHO + H O 3 3 2

32 2 –

CH CHOHCH CHO + OH– CH3CHOHCH2CHO + OH

Question Answer 18 Marks Guidance

For –CH CHO: ALLOW CH CHO–; CH CO–; C H O– For CH CH O+: ALLOW CH CHOH+, C H O+

22 3 2 3 3 2 3 2 5

For CH3CHOHCH2CHO, ALLOW C4H8O2

(iv) 1 ALLOW correct structural OR displayed

OH H O OR skeletal formulae

OR a combination of above as long as unambiguous

H3C C C C

For connectivity,

CH H CH3

ALLOW | | CH3– C3H– OH—

OH CH3

(Connectivity not being assessed)

(b) Refer to marking instructions on page 5 of mark scheme for 6 Indicative scientific points may include:

guidance on marking this question.

Explanation of role of electrophiles in organic chemistry

Level 3 (5–6 marks)

Describes, in detail, electrophilic reactions and mechanisms of Reaction of aliphatic compound and mechanism

one aliphatic AND one aromatic compound. Suitable reaction, e.g. ethene and Br2

May be shown within mechanism

There is a well-developed line of reasoning which is clear and

logically structured. The information presented is relevant and Mechanism, e.g.

H H H H H H

substantiated.

C C H C C H H C C H

Level 2 (3–4 marks) H H

Brδ+ Br Br Br

Describes, in detail, an electrophilic reaction and mechanism

of one aliphatic OR one aromatic compound. Br

Brδ−

OR Reaction of aromatic compound and mechanism

Describes electrophilic reactions and mechanisms of one

Suitable reaction, e.g. benzene + Cl2 ; HNO3

aliphatic AND one aromatic compound, with few

May be shown within mechanism

omissions/errors.

Mechanism, e.g.

There is a line of reasoning presented with some structure.

The information presented is relevant and supported by some

evidence.

Level 1 (1–2 marks) NO + NO2

2 H NO2

Selects suitable reagents for electrophilic reactions of one

aliphatic AND one aromatic compound.

+ H+

OR

Attempts to describe an electrophilic reaction and mechanism

of one aliphatic OR one aromatic compound, with

omissions/errors. Examples of a detailed description (NOT INCLUSIVE)

There is an attempt at a logical structure with a line of Electrophile as electron pair acceptor

reasoning. The information is in the most part relevant. Types and names of mechanisms

Equations for generation of electrophile and regeneration

0 marks No response or no response worthy of credit. of catalyst

Accurately positioned and directed curly arrows and

charges/ dipoles included

Explanation of major and minor product from electrophilic

addition

Total 12

How to answer it

Organic Reactions & Electrophiles Study Guide

What this question tests

This question assesses your understanding of carbonyl chemistry, base-catalysed aldol addition mechanisms, conjugate acid-base pairs, structural representation of branched organic molecules, and the defining role of electrophiles across both aliphatic and aromatic systems using curly arrow mechanisms.

Question 6(a)(i) - Systematic Naming

Naming Compound A

✅ Correct Answer

3-hydroxybutanal

Mark: 1 / 1

💡 Key Knowledge

  • The principal functional group is the aldehyde (suffix -al ).
  • Numbering starts from the carbonyl carbon as C-1.
  • An alcohol group is present at C-3, denoted by the prefix 3-hydroxy .

❌ Common Errors

  • Omiting hyphens or adding unauthorized commas ( 3 hydroxybutanal ).
  • Misnumbering the carbon chain to give higher locants.
Question 6(a)(ii) - Reaction Type

Identifying the Reaction Type

✅ Correct Answer

Addition

Mark: 1 / 1

🧠 Exam Technique

Keep your answer concise. Do not over-qualify with terms like "nucleophilic addition" unless specified, though simple addition is strictly required here. Avoid writing "addition-elimination".

Question 6(a)(iii) - Equilibria & Mechanism Steps

Step-by-Step Equilibria and Catalytic Action

✅ Correct Answers & Mark Scheme

Step 1 Equilibrium:

CH₃CHO + OH⁻ ⇌ CH₂CHO⁻ + H₂O

Conjugate pairs: CH₃CHO (A1) & CH₂CHO⁻ (B1), and OH⁻ (B2) & H₂O (A2) [or vice versa].

Step 2 Equation:

CH₃CHO + ⁻CH₂CHO + H₂O → CH₃CH(OH)CH₂CHO + OH⁻

Marks: 3 / 3

💡 Key Knowledge

Catalysts are consumed in an early step and regenerated in a later step. Here, OH⁻ is used in step 1 and regenerated in step 2, proving its role as a catalyst.

Question 6(a)(iv) - Structural Drawing

Product from Propanone

✅ Correct Structure

The product formed when propanone reacts under similar base-catalysed conditions is 4-hydroxy-4-methylpentan-2-one (diacetone alcohol).

Structure features a central chain with a tertiary alcohol and a ketone carbonyl group.

Mark: 1 / 1

❌ Common Errors

Incorrect connectivity of the methyl groups or placing the alcohol group on the wrong carbon atom. Ensure valencies of carbon (4 bonds) are strictly maintained.

Question 6(b)* - Extended Response (Level of Response)

The Role of Electrophiles in Organic Chemistry

💡 Core Definition

An electrophile is defined as an electron pair acceptor.

🧠 Structuring a Level 3 Response (5–6 Marks)

  • Part 1: Define electrophile clearly and explain its role (electron-deficient species seeking areas of high electron density).
  • Part 2 (Aliphatic): Outline an electrophilic addition reaction, such as ethene + Br₂ . Include curly arrows showing attack on the electrophilic bromine molecule, carbocation intermediate, and bromide ion attack to form 1,2-dibromoethane.
  • Part 3 (Aromatic): Outline an electrophilic substitution reaction, such as nitration of benzene ( HNO₃ / H₂SO₄ ) or halogenation ( Cl₂ / AlCl₃ ). Show generation of the electrophile (e.g. NO₂⁺ ), curly arrows from the benzene ring delocalised system, the intermediate ring, and loss of H⁺ to regenerate the catalyst.

❌ Examiner Pitfalls

Examiners frequently penalise scripts where curly arrows originate from a positive charge or point away from electrons. Ensure arrows start precisely from a bond or lone pair and point directly to electron-deficient sites or atoms accepting electrons.

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

Question and mark scheme from the OCR A-Level Chemistry examination, Unified chemistry (03), June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.