Edexcel A-Level Chemistry AS Paper 2, June 2018: Question 4

8 marks · Medium difficulty · Short Open Response

Analyze reactions of 1-chloropropane and compare the rates of hydrolysis of various bromoalkanes.

Practise this question

Question

Question text

4 This question concerns halogenoalkanes.

(a) 1-chloropropane can react to form organic products as shown in the reaction scheme:

CH3 CH CH2 CH3 CH2 CH2 C N

Reaction 1 Reaction 2

CH3 CH2 CH2 Cl

Reaction 3

concentrated ammonia solution

heated in a sealed tube

CH3 CH2 CH2 NH2

(i) State the reagent and conditions used in Reaction 1.

(2)

(ii) Identify a suitable reagent for Reaction 2 and include a reason why this is a

particularly useful type of reaction in organic chemistry.

(2)

Reagent …

Reason …

(iii) Explain why, in Reaction 3, the reactants are heated in a sealed container.

(2)

(iv) Write the structural formula of the product that will be formed if

1-chloropropane is refluxed with aqueous potassium hydroxide solution.

(1)

(b) The bromoalkanes, X, Y and Z, were each added to a mixture of aqueous silver nitrate

and ethanol at 50°C. The rate of hydrolysis was compared by measuring the time

for a precipitate to appear.

CH3 Br

X CH3 CH CH CH3

10 CH

Y CH3 CH CH*P51460RA01024*2CH2Br

Z CH3 CH2 C CH3

CH3

The relative rates of hydrolysis are in the order (fastest first)

(1)

A X, Y, Z

B Z, X, Y

C Z, Y, X

D X, Z, Y

(Total for Question 4 = 8 marks)

Mark scheme

Show the mark scheme

Question

Acceptable Answer Additional Guidance Mark

Number

4(a)(i) Reagent

(concentrated) NaOH/KOH (1) do not award OH- or just ‘hydroxide’

do not award M1 if ‘acidified’

Conditions

ethanol (solvent) and heat/warm (1) allow reflux

M2 is dependent on M1 except for a near miss

e.g. OH- (2)

Question Acceptable Answer Additional Guidance Mark

Number

4(a)(ii) Reagent:

KCN/NaCN /potassium cyanide / sodium cyanide (1) ignore any mention of the solvent (aq ethanol)

and conditions (reflux)

do not award just CN-/cyanide/HCN

Reason:

increases the number of carbon atoms in the carbon

chain/ length of carbon chain (1) (2)

Question Acceptable Answer Additional Guidance Mark

Number

4(a)(iii) An explanation that makes reference to the

following:

ignore reference to activation energy/ starting

heating increases rate (of reaction) (1) the reaction/ reaction is endothermic

ignore toxicity of reactants

no sealed tube would result in loss of ammonia

(gas)/ reactants / gas (1) (2)

Question Acceptable Answer Additional Guidance Mark

Number

4(a)(iv)

allow displayed/structural/skeletal formula

ignore name

do not award just C3H7OH (1)

Question Acceptable Answer Mark

Number

4(b) The only correct answer is B

A is not correct because Z (3rd) is tertiary (fastest)

C is not correct because Y (2nd) is primary (slower than X, secondary)

D is not correct because X (1st) is secondary (slower than Z, tertiary) (1)

(Total for Question 4 = 8 marks)

How to answer it

Reactions and Hydrolysis Rates of Halogenoalkanes

What this question tests

This question assesses your understanding of halogenoalkane reaction pathways (elimination, nucleophilic substitution), required reagents and conditions, practical precautions for volatile reactants, organic functional group products, and the relative rates of hydrolysis linked to carbon skeleton classification (primary vs. tertiary bromoalkanes).

Question 4(a)(i)

Elimination Reaction: Reagents and Conditions

✅ Correct Answer

Reagent: Potassium hydroxide ( KOH ) or sodium hydroxide ( NaOH ).

Conditions: Ethanolic (ethanol as solvent) and heat / reflux.

❌ Common Errors

Writing just OH⁻ or "hydroxide" instead of the full compound formula/name.
Forgetting the solvent and writing aqueous conditions (which leads to substitution/alcohol formation instead of elimination/alkene formation).

Mark Breakdown: 2 marks total (1 mark for correct reagent, 1 mark for ethanolic solvent and heat). Note: M2 is dependent on M1.

Question 4(a)(ii)

Nucleophilic Substitution with Cyanide

✅ Correct Answer

Reagent: Potassium cyanide ( KCN ) or sodium cyanide ( NaCN ).

Reason: Increases the number of carbon atoms in the carbon chain (extends the carbon skeleton).

🧠 Exam Technique

When asked why a reaction is useful in organic synthesis, always look for structural changes. Reactions involving cyanide are fundamental because they form a new carbon-carbon (C-C) bond.

Mark Breakdown: 2 marks total (1 mark for naming KCN / NaCN , 1 mark for stating it increases chain length / carbon atoms).

Question 4(a)(iii)

Practical Safety and Technique in Sealed Tubes

💡 Key Knowledge

Reaction 3 uses concentrated ammonia solution to form an amine. Because ammonia is a volatile gas at room temperature and heating increases reaction rates, a sealed container is required.

✅ Correct Answer

1. Heating increases the rate of reaction.

2. A sealed container prevents the escape of volatile ammonia gas / reactants.

Mark Breakdown: 2 marks total (1 mark for linking heating to rate, 1 mark for explaining that a sealed tube prevents loss of gaseous ammonia/reactants).

Question 4(a)(iv)

Nucleophilic Substitution to Form an Alcohol

✅ Correct Answer

CH₃—CH₂—CH₂—OH

(Allow structural, displayed, or skeletal formulas).

❌ Common Errors

Writing a molecular formula such as C₃H₇OH . The question specifically asks for the structural formula, so bonds or clear carbon groupings must be shown.

Mark Breakdown: 1 mark for the correct structural formula of propan-1-ol.

Question 4(b)

Relative Rates of Hydrolysis (Multiple Choice)

✅ Correct Answer

B (Z, X, Y)

💡 Explanation & Mechanism

Z is tertiary (3rd): Forms a tertiary carbocation intermediate which is the most stable due to the positive inductive effect of three alkyl groups. This reacts the fastest.
X is secondary (2nd): Intermediate stability, reacts at an intermediate rate.
Y is primary (1st): Forms a primary carbocation which is the least stable, reacting the slowest.

Mark Breakdown: 1 mark for selecting option B.

Topics

Organic Chemistry · Core Practicals · Topic 6: Organic Chemistry I · Core Practical 4: Investigate the hydrolysis of halogenoalkanes

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.