Edexcel A-Level Chemistry AS Paper 2, June 2023: Question 4

16 marks · Medium difficulty · Open Response

Examine halogenoalkane reactions including hydrolysis rates, nucleophilic substitution mechanisms with ammonia, infrared spectroscopy, and synthetic routes to increase chain length or form alkenes.

Practise this question

Question

Question text

4 This question is about halogenoalkanes and some of their reactions.

(a) X, Y and Z are three different halogenoalkanes.

X is 1‑chloropropane

Y is 1‑bromopropane

Z is 1‑iodopropane

An experiment is carried out to compare the rates of hydrolysis of

these compounds.

Outline procedure:

1 cm3 of each of the three halogenoalkanes, X, Y and Z, is added to separate

test tubes, each containing 5 cm3 of ethanol and 5 cm3 of aqueous silver nitrate

solution, in a water bath at 50°C.

The time taken for a precipitate to form in each test tube is measured.

(i) Give three reasons why these reaction conditions are specified.

(3)

(ii) Explain why a precipitate forms.

(2)

(iii)The three halogenoalkanes were placed in order of*P71927A01228*increasingrate

of reaction.

Which is the correct sequence?

(1)

A X, Y, Z

B X, Z, Y

C Y, X, Z

D Z, Y, X

(b) The results table shows the time taken to produce a precipitate when

three bromoalkanes react with aqueous ethanolic silver nitrate solution.

Time to produce a precipitate

Halogenoalkane

/ s

1‑bromobutane 58

2‑bromobutane 33

2‑bromo‑2‑methylpropane 2

Give a reason why the times taken to produce a precipitate for these

isomeric bromoalkanes are different.

(1)

1‑bromopropane with ammonia?

(1)

δ+ δ–

CH3 CH2 CH2 Br CH3 CH2 CH2 NH2 + HBr

••

NH3

δ+ δ– + –

CH3 CH2 CH2 Br CH3 CH2 CH2 NH3 + Br

•• 13

NH3

*P71927A01328*

δ+ δ– + –

CH3 CH2 CH2 Br CH3 CH2 CH2 NH2 + Br

••

NH3

δ+ δ–

CH3 CH2 CH2 Br CH3 CH2 CH2 NH3 + Br

••

NH3

(ii) Write the formula of the final inorganic product of the reaction of

1‑bromopropane with excess ammonia.

(1)

(iii) The infrared spectrum of ethylamine is shown.

Transmittance

*P71927A01428* S

P Q

4000 3000 2000 1500 1000 500

Wavenumber / cm–1

Which absorption peak confirms this as an amine?

(1)

A absorption P

B absorption Q

C absorption R

D absorption S

(d) 1‑bromopropane undergoes reactions when heated with different reagents.

(i) Give two reasons why organic reactions are often heated for a long time but

the yield is frequently low.

(2)

(ii) Give the name or formula of the reagent and the condition, other than heat,

used to increase the carbon chain length by one carbon atom, starting from

1‑bromopropane.

(2)

… 15

(iii) 1‑bromopropane, CH*P71927A01528*CHCHCHBr, can be converted into

32 2 2

but‑1‑ene, CH3CH2CH CH2.

Give the name or formula of the reagent and the condition, other than heat,

used for this reaction.

(2)

(Total for Question 4 = 16 marks)

Mark scheme

Show the mark scheme

4(a)(i) An answer that makes reference to the following points: (3)

• ethanol / ethanol & water mixture is used to ensure all Allow ethanol enables halogenoalkane to be soluble

reactants mix efficiently / are in same phase (1)

• same volumes (of each reactant) ensures a fair test (1) Allow use the same volume of silver nitrate

Allow use the same volume of halogenoalkane

Allow alternative wording to ‘fair test’ such as

improves reliability

• warm water bath used to speed up reaction

Or Allow because the hydrolysis / reaction of

water bath ensures all reactions carried out at same chloroalkanes / some halogenoalkanes is (very) slow

temperature (fair test) (1)

Question

Acceptable Answer Additional Guidance Mark

Number

4(a)(ii) An explanation that makes reference any two of the following (2)

three points:

Allow a correct equation with / without states

• (hydrolysis by water) releases halide ions (1) Example of equation

R−X + H O R−OH + X− + H+

If shown, states must be correct but allow ethanol as

solvent

• halide ions react with silver (ions) (1) Do not award use of halogen for halide

• to give an insoluble silver halide/ product / ppt (1)

A correct equation with states to score (2)

Example of equation

X−(aq) + Ag+(aq) AgX(s)

Question

Answer Mark

Number

4(a)(iii) The only correct answer is A (X, Y, Z) (1)

B is not correct because Z hydrolyses faster than Y

C is not correct because X is the slowest to hydrolyse

D is not correct because Z is the fastest to hydrolyse / X is the slowest to hydrolyse and this is in order of decreasing rate

Question

Acceptable Answer Additional Guidance Mark

Number

4(b) An answer that makes reference to the following point: (1)

• tertiary bromoalkanes react fastest Allow names as identifiers in explaining reason e.g.

OR Allow answers in terms of bond strength, ie C-Br bond is

primary bromoalkanes react slowest weakest in 2-bromo-2-methylpropane

strongest in 1-bromobutane

Allow primary bromoalkanes are more stable

Ignore reference to secondary bromoalkane

Question

Answer Mark

Number

4(c)(i) (1)

A is not correct because the nitrogen in the intermediate should be positively charged with three hydrogens and the other

product should be the bromide ion

C is not correct because there is a hydrogen atom missing on the N of NH2 in the product cation

D is not correct because the charges on the intermediate must be shown

Question

Acceptable Answer Additional Guidance Mark

Number

4(c)(ii) (1)

• NH (+)Br(−) Ignore HBr

Ignore ammonium bromide

Question

Answer Mark

Number

4(c)(iii) The only correct answer is A (absorption P) (1)

B is not correct because this is an alkane absorption

C is not correct because this is the C=O of an amide absorption

D is not correct because this is an unassigned peak that happens to appear in the IR spectrum

Question

Acceptable Answer Additional Guidance Mark

Number

4(d)(i) An answer that makes reference to two of the following points: (2)

• many organic reactions are equilibria (with a significant Allow reverse reactions lower yield

equilibrium constant) (1)

Allow eqm not achieved, takes too long / slow reaction /

high activation energy

Reaction stopped before eqm achieved

Allow reactions are incomplete

e.g. substitution v elimination

• side reactions may also take place (1) Allow reference to by-products/minor products

e.g. loss of liquid during transfer between containers,

• a specific handling loss (1) volatility

Question

Acceptable Answer Additional Guidance Mark

Number

4(d)(ii) An answer that makes reference to the following points: (2)

Reagent:

• KCN / potassium cyanide (1) Allow NaCN / sodium cyanide

Do not award just ‘cyanide’

Do not allow nitrile in place of cyanide

If the name and formula are given, both must be correct

Conditions:

• aqueous ethanolic / ethanolic solution (1) Allow just ethanol/alcohol

Ignore heat

Mark independently

Question

Acceptable Answer Additional Guidance Mark

Number

4(d)(iii) An answer that makes reference to the following points: (2)

Reagent:

• (concentrated) potassium hydroxide / KOH (1) Accept (conc.) sodium hydroxide / NaOH

Do not award contradictory reagents, e.g. acidified KOH

Condition:

• alcoholic / ethanolic solution (1) Allow just ethanol/alcohol

Ignore heat

Mark independently

(Total for Question 4 = 16 marks)

How to answer it

Halogenoalkanes and Their Reactions

What this question tests

This comprehensive question assesses your understanding of halogenoalkane chemistry, including nucleophilic substitution mechanisms, rates of hydrolysis experiments, carbon chain lengthening, elimination reactions, infrared spectroscopy interpretation, and practical synthetic yields.

Question 4(a) - Rates of Hydrolysis

Parts (i), (ii), and (iii)

💡 Key Knowledge (a)(i)

When comparing rates of hydrolysis using aqueous silver nitrate in ethanol:

Ethanol: Acts as a common solvent so the insoluble halogenoalkane and aqueous reagent can mix into a single phase.
Same volumes: Ensures a fair test by controlling reactant concentrations.
Water bath: Speeds up the slow reaction and keeps temperature constant for a fair test.

✅ Correct Answers & Mark Schemes

(a)(i) (3 marks): Any three valid points from solvent miscibility, control variables (volumes), and temperature regulation.

(a)(ii) (2 marks): Halogenoalkane undergoes hydrolysis releasing halide ions ( X⁻ ), which react with silver ions ( Ag⁺ ) to form an insoluble silver halide precipitate ( AgX ).

(a)(iii) (1 mark): A (X, Y, Z) - 1-chloropropane is slowest, 1-iodopropane is fastest due to decreasing carbon-halogen bond enthalpy.

❌ Common Errors

In (a)(ii), students often lose marks by stating "silver reacts with halogen" instead of specifying that halide ions react with silver ions.

Question 4(b) - Isomeric Bromoalkanes

✅ Correct Answer

Tertiary bromoalkanes react fastest and primary bromoalkanes react slowest (or reference to C-Br bond strength being weakest in tertiary/tertiary forming more stable carbocations via an S_N1 pathway).

🧠 Exam Technique

Clearly reference the structure (primary vs tertiary) or the C–Br bond strength. Avoid vague statements like "tertiary has more branching" without linking it to reactivity or stability.

Question 4(c) - Mechanisms and IR Spectroscopy

Parts (i), (ii), and (iii)

✅ Correct Answers

(c)(i): B — Shows correct curly arrows from the nitrogen lone pair to the electron-deficient carbon, heterolytic fission of the C–Br bond, and correct charges on the resulting alkylammonium cation and bromide ion.

(c)(ii): NH₄⁺Br⁻ (Ammonium bromide).

(c)(iii): A (absorption P) — Represents the N–H stretch characteristic of primary amines in the 3300–3500 cm⁻¹ region.

❌ Common Errors

In mechanism questions like (c)(i), option D is a common trap because it omits the formal charges on the intermediate/products. Always double-check that your nitrogen has a positive charge when it forms a fourth bond!

Question 4(d) - Synthetic Routes & Yields

Parts (i), (ii), and (iii)

💡 Key Knowledge & Conditions

(d)(i) Reasons for low yield despite long heating: Reversible reactions reaching equilibrium, side reactions/by-products (e.g., substitution competing with elimination), and practical handling/volatility losses.

(d)(ii) Chain lengthening (1 more carbon): Reagent: Potassium cyanide ( KCN ) or sodium cyanide ( NaCN ). Condition: Aqueous ethanolic solution.

(d)(iii) Elimination to form an alkene: Reagent: Potassium hydroxide ( KOH ) or sodium hydroxide ( NaOH ). Condition: Alcoholic / ethanolic solution (and concentrated/heated).

🧠 Exam Technique

Always give both the chemical name (or correct formula) and the precise solvent condition (ethanolic vs aqueous) for organic synthesis questions. Aqueous favours substitution (alcohols), while alcoholic/ethanolic favours elimination (alkenes).

Topics

Organic Chemistry · Core Practicals · Topic 6: Organic Chemistry I · Topic 7: Modern Analytical Techniques I · Topic 18: Organic Chemistry III

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.