OCR A-Level Chemistry AS Breadth in chemistry (01), November 2021: Question 24

10 marks · Medium difficulty · Structured Questions

Outline the mechanism for the reaction of methylpropene with hydrogen bromide, define structural isomerism, draw an isomer, complete a substitution reaction equation, and draw a labelled reflux apparatus.

Practise this question

Question

Exam question showing the reaction of methylpropene with hydrogen bromide to form compound A, followed by parts testing the electrophilic addition mechanism, definition and structure of structural isomers, and the conversion of compound A to alcohol C via reflux.
Question text

24 A student reacts methylpropene with hydrogen bromide, HBr, as shown in Reaction 1.

H C H CH3 H

Reaction 1

C C + HBr H3C C C H

H3C H Br H

Methylpropene Compound A

(a) Outline the reaction mechanism for Reaction 1.

The structures of methylpropene and compound A have been provided.

Include curly arrows and relevant dipoles.

H C H CH3 H

C C H3C C C H

H3C H Br H

name of mechanism … [4]

(b) When reacting methylpropene with HBr, a small amount of compound B also forms.

Compound B is a structural isomer of compound A.

(i) Explain the term structural isomer.

… [1]

(ii) Show the structure for compound B.

[1]

(c) Compound A can be refluxed with a reagent to make alcohol C.

(i) Choose a reagent for this reaction and complete the equation for this reaction.

Your equation should show the structure of alcohol C.

CH3 H

H3C C C H + … + …

Reagent

Br H

Compound A Alcohol C

[2]

(ii) Draw a labelled diagram to show how you would set up apparatus for reflux.

[2]

Mark scheme

Show the mark scheme Mark scheme detailing the step-by-step marks for the curly arrows and dipole in the electrophilic addition mechanism, the definition of structural isomerism, the structure of the isomer, the reagents and products for the substitution reaction, and a labelled diagram of a reflux setup.

AO

Question Answer Marks Guidance

element

24 (a) Curly arrows can be straight, snake-like, etc. 4 1st curly arrow must

but NOT double headed or half headed arrows • go to the H atom of H–Br

AND

1. Curly arrow from C=C to HBr and H–Br 2 • start from, OR be traced back to

marks any point across width of C=C

DO NOT ALLOW

partial charge on C=C

2nd curly arrow must

• start from, OR be traced back to

AO1.2 any part of δ+H–Brδ– bond

Curly arrow from C=C bond to H of H–Br

AND

• go to Brδ–

Correct dipole shown on H–Br

AND curly arrow that breaks H–Br bond AO1.2

2. Curly arrow from Br– to carbocation 1 mark

DO NOT ALLOW

δ+ on C of 3rd curly arrow must

carbocation • go to the C+ of carbocation

AND

• start from, OR be traced back to any point

Correct carbocation AND curly arrow from across width of lone pair on :Br–

Br– to C+ of CORRECT carbocation –

AO2.5 • OR start from – charge of Br ion

3. Name of mechanism 1 mark

Electrophilic addition AO1.1

AO

element

(Lone pair NOT needed if curly arrow shown from

– charge of Br– ion)

IF Br2 is used instead of HBr contact your Team

Leader

DO NOT ALLOW incorrect carbocation, i.e.

(b) (i) Same molecular formula 1 AO1.1 Same formula is not sufficient (no reference to

AND molecular)

Different structural formulae

Different arrangement of atoms is not sufficient

(no reference to structure/structural)

For structural formulae,

ALLOW structure/displayed/skeletal formulae

(b) (ii) 1 AO2.5 ALLOW any combination of skeletal OR

structural OR displayed formula as long as

unambiguous

AO

element

(c) (i) 2 AO2.5

Alcohol C Reagent AND product ×2

NaOH AND NaBr ALLOW Reagent: H2O/water

OR AND Product: HBr

KOH AND KBr

OR

OH– AND Br–

(c) (ii) 2 AO3.3

1st mark: ×2 For condenser label,

Labelled ALLOW ‘condenser’

condenser above OR

a flask water in AND water out (May be implied by

connection to tap and sink).

2nd mark:

Only available if 1st

mark has been

awarded

Flask

AND

heat labelled

Total 10

How to answer it

Alkene Reactions, Isomerism, and Reflux Apparatus

What this question tests

This question assesses your understanding of alkene chemistry mechanisms (electrophilic addition), carbocation stability, structural isomerism definition, nucleophilic substitution reactions of halogenoalkanes to form alcohols, and practical laboratory techniques (setting up reflux apparatus).

Question 24 (a)

Electrophilic Addition Mechanism

✅ Correct Answer

Name of mechanism: Electrophilic addition

Mechanism details:

  • Curly arrow starting from the C=C double bond pointing to the H atom of H-Br.
  • Partial charges correctly shown on H-Br ( H(δ+) and Br(δ-) ) with a curly arrow breaking the H-Br bond towards Br.
  • Formation of the correct major carbocation ( (CH₃)₂C⁺-CH₂CH₃ or equivalent structure with C⁺ on the tertiary carbon).
  • Curly arrow from the lone pair (or negative charge) of the bromide ion ( :Br⁻ ) pointing to the C⁺ of the carbocation.

💡 Key Knowledge

  • Alkenes act as nucleophiles because the electron-rich C=C double bond attacks electrophiles.
  • Electrophilic addition proceeds via a carbocation intermediate. Tertiary carbocations are more stable than primary ones due to positive inductive effects from alkyl groups.

🧠 Exam Technique

  • Start your first curly arrow precisely: the tail must touch either the carbon-carbon double bond or the middle of the C=C bond, and the head must point directly at the hydrogen atom.
  • Ensure the carbocation intermediate shows the positive charge clearly located on the correct carbon atom.

❌ Common Errors

  • Placing a partial charge ( δ+ ) directly on the carbon atoms of the double bond (this loses mark 1).
  • Drawing the first curly arrow originating from the hydrogen atom instead of the H-Br bond.
  • Drawing an incorrect primary carbocation rather than the stable tertiary carbocation.
Total for (a): 4 marks
Question 24 (b)

Structural Isomerism & Minor Product

✅ Correct Answers

(i) Definition of structural isomer: Molecules with the same molecular formula, but with different structural formulae (or different arrangement of atoms).

(ii) Structure of compound B: 1-bromo-2-methylpropane ( H₃C-CH(CH₃)-CH₂-Br or equivalent unambiguous displayed/structural formula showing attachment via the primary carbon).

❌ Common Errors & Mark Scheme Traps

  • In part (i), writing "same formula" without specifying molecular formula loses the mark. Similarly, omitting "structure/structural" when describing the difference is penalised.
  • In part (ii), drawing a halogenoalkane with the bromine attached to the wrong carbon atom.
Total for (b): 2 marks (1 mark per part)
Question 24 (c)

Nucleophilic Substitution & Reflux Technique

✅ Correct Answers

(i) Reagent & Equation:

  • Reagent: Aqueous NaOH (or KOH , or OH⁻ ions, or H₂O / water).

    • Equation products: Alcohol C ( 2-methylpropan-2-ol : (CH₃)₃COH ) plus NaBr (or KBr / Br⁻ depending on reagent choice).

(ii) Reflux Apparatus Requirements:

  • 1st mark: Labelled vertical condenser positioned above a flask. Water entering at the bottom and exiting at the top.
  • 2nd mark: A round-bottom or pear-shaped flask containing the reaction mixture, with a heat source labelled at the base.

🧠 Exam Technique & Practical Tips

  • When drawing reflux apparatus, ensure the condenser is open at the top (never seal a reflux system, as pressure will build up and cause an explosion!).
  • Cooling water must always enter at the bottom jacket of the condenser and leave at the top to ensure the water jacket remains completely full.
Total for (c): 4 marks (2 marks for (i), 2 marks for (ii))

Topics

Module 1: Development of practical skills in chemistry · Module 4: Core organic chemistry · Practical Activity Groups · 1.2 Practical skills assessed in the practical endorsement · 4.1 Basic concepts and hydrocarbons · 4.2 Alcohols, haloalkanes and analysis · PAG 5: Synthesis of an organic liquid

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.