OCR A-Level Chemistry AS Depth in chemistry (02), November 2021: Question 2

6 marks · Medium difficulty · Structured Questions

Define a radical, identify the type of bond breaking in radical substitution, draw two monohalogenated products of alkane A, and write an equation for its further substitution with bromine.

Practise this question

Question

The question presents alkane A, which is 2-methylpropane (shown with a central carbon bonded to three methyl groups and one hydrogen atom). Part (a) asks for the definition of a radical. Part (b) asks to name the type of bond breaking that occurs. Part (c) asks to show the structures of two monosubstituted organic products formed when alkane A reacts with bromine. Part (d) asks to write a balanced equation using structures for the reaction of alkane A with excess bromine to produce 1,3-dibromo-2-methylpropane.
Question text

2 Alkane A, shown below, reacts with bromine in a radical substitution reaction.

CH3

H3C C H

CH3

Alkane A

(a) What is meant by a radical?

… [1]

(b) Name the type of bond breaking that occurs in a radical substitution reaction.

… [1]

(c) In this reaction with bromine, monosubstitution of alkane A forms a mixture of organic

products.

Show the structures of two monosubstituted organic products that are formed.

[2]

(d) With excess bromine, further substitution takes place.

Write an equation for the reaction of alkane A with excess bromine to produce

1,3-dibromo-2-methylpropane.

Use structures for the organic compounds.

[2]

Mark scheme

Show the mark scheme The mark scheme gives the answers for question 2: (a) 'A species with an unpaired electron', (b) 'Homolytic (fission)', (c) structures showing bromination at different positions of methylpropane, and (d) a balanced chemical equation with correct structures for the reaction producing 1,3-dibromo-2-methylpropane.

How to answer it

Radical Substitution of Alkanes (Alkane A)

What this question tests

This question assesses your understanding of free radical chemistry involving alkanes. Key topics include defining a radical, identifying types of bond fission, predicting monoisomer structures resulting from substitution, and writing balanced stoichiometric equations for further halogenation reactions using structural representations.

Question Part (a) - [1 Mark]

Definition of a Radical

✅ Correct Answer

A species with an unpaired electron.

❌ Common Errors

Writing "a species with one electron" instead of an unpaired electron. Examiners strictly penalise this omission because species like hydride ions have single electrons or pairs that don't match the radical definition.

Mark scheme note: 1 mark awarded for specifying an unpaired electron. Do not allow "species with one electron".
Question Part (b) - [1 Mark]

Type of Bond Breaking

✅ Correct Answer

Homolytic (or homolytic fission )

💡 Key Knowledge

Homolytic fission occurs when a covalent bond breaks evenly, with each bonded atom taking one of the shared electrons to form radicals. Contrast this with heterolytic fission, where one atom takes both electrons to form ions.

Mark scheme note: 1 mark for stating homolytic (fission).
Question Part (c) - [2 Marks]

Monosubstituted Organic Products

✅ Correct Answer

Alkane A is 2-methylpropane ( (CH₃)₃CH ). Monosubstitution can occur on the methyl groups or the central carbon:

  • Product 1: 2-bromo-2-methylpropane ( (CH₃)₃CBr )
  • Product 2: 1-bromo-2-methylpropane ( (CH₃)₂CHCH₂Br )

🧠 Exam Technique

You can use structural, displayed, or skeletal formulas as long as they are completely unambiguous. Make sure carbon valencies are respected (C=4, H=1, Br=1).

Mark scheme note: 1 mark per correct structure (total 2 marks). Any combination of structural, displayed, or skeletal formulas accepted.
Question Part (d) - [2 Marks]

Equation for Further Substitution

✅ Correct Answer

Reacting alkane A with excess bromine to form 1,3-dibromo-2-methylpropane:

(CH₃)₂CHCH₃ + 2Br₂ → BrCH₂CH(CH₃)CH₂Br + 2HBr

(Or using displayed/skeletal structures in place of molecular formulas for the organic compounds as requested by the stem).

❌ Common Errors

Forgetting to balance the equation! Because it specifies 1,3-dibromo, two bromine atoms substitute into the molecule, requiring 2Br₂ on the reactant side and producing 2HBr as a byproduct.

Mark scheme note: 2 marks total: 1 mark for the correct structure of the organic product, and 1 mark for a complete, balanced equation (including correct stoichiometry for Br₂ and HBr).

Topics

Module 4: Core organic chemistry · 4.1 Basic concepts and hydrocarbons

Question and mark scheme from the OCR A-Level Chemistry examination, AS Depth in chemistry (02), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.