OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2024: Question 18

14 marks · Medium difficulty · Structured Questions

Write the equation, mechanism, and limitations for the free radical substitution of alkanes, and complete a reaction flowchart for haloalkane conversions involving amines, alcohols, nitriles, and carboxylic acids.

Practise this question

Question

A chemistry exam question with two main sections. Part (a) asks about the synthesis of 2-bromo-2-methylpropane from an alkane and bromine, requiring an equation with displayed structures, the mechanism name, bond fission type, and two limitations of the reaction. Part (b) features a reaction flowchart starting from bromoethane in the center, with boxes for reagents and organic products branching out to form an amine, an alcohol, a nitrile, a carboxylic acid, and another amine.
Question text

18 This question is about haloalkanes.

(a) Haloalkanes can be synthesised by reacting alkanes with halogens in the presence of ultraviolet

radiation.

An alkane reacts with bromine to form 2-bromo-2-methylpropane.

(i) Write the equation for this reaction, showing the structures of the organic compounds.

Name the reaction mechanism and the type of bond fission that occurs.

Equation

Name of reaction mechanism …

Type of bond fission …

[3]

(ii) Describe two limitations of the synthesis of 2-bromo-2-methylpropane from an alkane and

bromine.

1 …

2 …

[2]

(b) Complete the flowchart by filling in each box.

amine

reagent(s)

reagent(s)

H

H3C C Br

H

bromoethane alcohol

reagent(s)

H

H3C C C N

H

nitrile

reagent(s) reagent(s)

carboxylic acid amine

[9]

Mark scheme

Show the mark scheme The mark scheme provides the answers for question 18. Part (a)(i) shows the equation for the radical substitution of methylpropane with bromine, naming radical substitution and homolytic fission. Part (a)(ii) lists acceptable limitations such as further substitution or substitution at different positions. Part (b) details the correct reagents and structures for the flowchart converting bromoethane to an amine, alcohol, and nitrile, and further converting the nitrile to a carboxylic acid and an amine.

Question Answer Marks Guidance

18 (a) (i) Equation 3 ALLOW any combination of skeletal OR structural OR

displayed formula as long as unambiguous

H H H H Br H

H C C C H + Br2 H C C C H + HBr IGNORE mechanism, need overall equation

H CH3 H H CH3 H

Name

Radical substitution

Bond fission

homolytic (fission)

18 (a) (ii) Further substitution/s 2 ALLOW dibromo/multibromo compounds formed

OR OR example of further substitution product e.g

Different termination products CH2BrCBr(CH3)2 / C4H8Br2 / 1,2-dibromo-2-methylpropane

OR OR example of different organic termination product

More than one termination step e.g.C8H18

ALLOW more than one H (atom) can be replaced

ALLOW radicals react with each other to form other

Substitution at different positions along (carbon) chain ✓ products

ALLOW a hydrogen (atom) on a different carbon (atom) can

be replaced

ALLOW Substitutions can occur at other carbons (along the

chain)

ALLOW example of substitution at different position on

chain e.g. CH2BrCH(CH3)2 / 1-bromo-2-methylpropane

IGNORE references to separation of products

IGNORE references to atom economy or yield

18 (b) 9 ALLOW any combination of skeletal OR structural OR

displayed formula as long as unambiguous

H

H3C C NH2 DO NOT ALLOW structure if H(s) are missing from ONE

structural/displayed formula…

H

amine BUT ALLOW any further omissions as ECF

Reagent(s) ALLOW any vertical bond to the OH OR NH2

NH3 AND ethanol Reagent(s)

OR excess NH3 –

OH (aq)

OR NaOH H

H

OR KOH

H C C OH DO NOT ALLOW OH–, OR NH2– but ALLOW ECF for

H C C Br 3

3 subsequent use in this part

H H

bromoethane alcohol ALLOW names of reagents e.g. ethanolic ammonia, if no

formulae given

Reagent(s) DO NOT ALLOW other additional reagents

CN– (/ethanol)

ORKCN (/ethanol) IGNORE Conditions

OR NaCN (/ethanol)

H For bromoethane to amine:

H3C C C N IF a secondary / tertiary amine is given ALLOW one mark for a

correct structure AND one mark for an appropriate reagent to

H produce the amine shown.

nitrile

Reagent(s)

Aqueous acid For bromoethane to alcohol:

+ Reagent(s) ALLOW H O

OR H /H2O 2

OR H+(aq) H AND Ni

2 IGNORE ethanol (as a solvent)

For bromoethane to nitrile:

DO NOT ALLOW HCN OR CN- / H+

H H H

H C C COOH DO NOT ALLOW H2O / (aq)

3 H C C C NH

H For nitrile to carboxylic acid:

H H ALLOW any mineral acid

carboxylic acid amine IGNORE dilute/concentrated

Check has 3C not 2C

For nitrile to amine:

ALLOW suitable non-specification alternative e.g. LiAlH4,

H2 with Pd or Pt

How to answer it

Haloalkanes: Synthesis, Limitations, and Organic Synthesis Pathways

What this question tests

This question assesses your knowledge of radical substitution mechanisms involving alkanes and halogens, the inherent limitations of free radical halogenation, and nucleophilic substitution/reduction pathways for converting haloalkanes into alcohols, amines, nitriles, and carboxylic acids.

Question 1 (a) (i)

Free Radical Halogenation Equation & Mechanism

Writing equations, naming mechanisms, and bond fission types

✅ Correct Answer

  • Equation: CH₃CH(CH₃)CH₃ + Br₂ → CH₃C(Br)(CH₃)CH₃ + HBr (or structural/displayed equivalent showing 2-methylpropane reacting to form 2-bromo-2-methylpropane).
  • Name of mechanism: Radical substitution (or free radical substitution).
  • Type of bond fission: Homolytic (fission).

💡 Key Knowledge

  • Homolytic fission: Each bonding atom receives one electron from the shared pair, forming free radicals.
  • Alkanes require UV light to provide energy to break the halogen bond homolytically.

🧠 Exam Technique

  • Make sure you draw out unambiguous structural or displayed formulas when requested.
  • Do not confuse homolytic fission with heterolytic fission (which occurs in polar nucleophilic substitutions).

❌ Common Errors

  • Writing "homolytic substitution" instead of "radical substitution".
  • Writing "heterolytic" due to mixing up free radical mechanisms with nucleophilic substitution.
Mark breakdown: 3 marks total (1 for overall balanced equation, 1 for mechanism name, 1 for bond fission type).
Question 1 (a) (ii)

Limitations of Alkane Halogenation

Explaining why free radical substitution is poor for specific synthesis

✅ Correct Answers (Any two)

  • Further substitution (formation of dibromo- or multibromo-compounds).
  • Substitution occurring at different positions along the carbon chain (yielding structural isomers like 1-bromo-2-methylpropane).
  • Different termination products / side products formed when radicals combine (e.g., C₈H₁₈).

💡 Key Knowledge

  • Free radical substitution is notoriously unselective because propagation steps can happen repeatedly on the same molecule or at different carbon environments.
  • This leads to low atom economy and difficult separation processes.

🧠 Exam Technique

  • Keep your bullet points concise and chemically precise. Use terms like "further substitution" or "isomers".
  • Examiners ignore vague mentions of "poor yield" unless backed up by a chemical reason like side-products or isomer formation.

❌ Common Errors

  • Stating "low atom economy" — halogenation involves substitution, but the key limitation tested here is the lack of control over position and extent of substitution.
Mark breakdown: 2 marks total (1 mark for each valid limitation).
Question 1 (b)

Organic Synthesis Flowchart: Haloalkane Reactions

Reagents and structural formulas for functional group interconversions

✅ Correct Pathway Answers

  • Bromoethane to Amine: Reagents: NH₃ and ethanol (or excess NH₃). Structure: CH₃CH₂NH₂ (ethylamine).
  • Bromoethane to Alcohol: Reagents: OH⁻(aq) or NaOH(aq) / KOH(aq) . Structure: CH₃CH₂OH (ethanol).
  • Bromoethane to Nitrile: Reagents: CN⁻ / ethanol or KCN / ethanol .
  • Nitrile to Carboxylic Acid: Reagents: Aqueous acid or H⁺/H₂O . Structure: CH₃CH₂COOH (propanoic acid - note the 3-carbon chain!).
  • Nitrile to Amine (Reduction): Reagents: H₂ and Ni (or LiAlH₄). Structure: CH₃CH₂CH₂NH₂ (propylamine - 3 carbons).

💡 Key Knowledge & Traps

  • Chain lengthening: Converting bromoethane (2 carbons) to a nitrile introduces an extra carbon, yielding a 3-carbon chain (propanoic acid and propylamine downstream).
  • Amine synthesis conditions: Requires ethanolic ammonia under pressure to prevent further substitution into secondary/tertiary amines, though the primary amine structure is expected here.

🧠 Exam Technique

  • Always double-check carbon chain lengths when nitriles are involved! Examiners specifically penalise students who accidentally drop or add carbons during hydrolysis or reduction steps.
  • Check if specific state symbols or solvent conditions are requested (e.g., aqueous vs. ethanolic).

❌ Common Errors

  • Using aqueous ammonia instead of ethanolic ammonia for making amines from haloalkanes.
  • Using HCN instead of KCN/ethanol for nitrile formation.
  • Forgetting that nitrile reduction or hydrolysis products retain the extra carbon from the nitrile group ( -C≡N ).
Mark breakdown: 9 marks total across all reagent boxes and structure boxes in the reaction web.

Topics

Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · 6.2 Nitrogen compounds, polymers and synthesis · 4.1 Basic concepts and hydrocarbons · 4.2 Alcohols, haloalkanes and analysis

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.