AQA A-Level Chemistry AS Paper 2, June 2025: Question 1

14 marks · Medium difficulty · State/Explain/Describe

Explain the reactions and mechanisms of halogenoalkanes, including nucleophilic substitution with ammonia, elimination with hydroxide ions to form isomeric alkenes, and bonding in carbocation intermediates.

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Question text

01 This question is about halogenoalkanes.

01.1 Halogenoalkanes undergo substitution reactions with nucleophiles such as

CN–, NH and OH–

Explain why halogenoalkanes react with nucleophiles.

[2 marks]

01.2 2-Bromobutane reacts with ammonia to form CH3CH(NH2)CH2CH3

Complete the mechanism for this reaction.

[3 marks]

2-Bromobutane reacts with KOH to give a mixture of products.

01.3 Identify the solvent that should be used to ensure that butan-2-ol is the

main organic product when 2-brombutane reacts with KOH

[1 mark]

01.4 Under different conditions, 2-bromobutane reacts with KOH to form but-1-ene and

two other alkenes.

Name and complete the mechanism to form but-1-ene from 2-bromobutane.

Identify the other two alkenes formed.

[5 marks]

Name of mechanism

Mechanism

Other alkene 1

Other alkene 2

01.5 The carbocation (CH ) C+ is formed as an intermediate in the reaction of

2-bromo-2-methylpropane with KCN

Suggest the value of the bond angle around the central C atom in the

(CH ) C+ carbocation.

[1 mark]

01.6 A co-ordinate bond is formed when the (CH ) C+ carbocation reacts with CN– to

form a nitrile.

Explain how this co-ordinate bond is formed during this reaction.

[2 marks]

Mark scheme

Show the mark scheme

Question Marking guidance Additional Comments/Guidelines Mark

M1 C of C-halogen is δ+ Ignore drawn mechanism (but could score M1 for

δ+ on mechanism)

M2 lone pair on nucleophile attacks/attracted to C of C-halogen

Alternative based on SN1

01.1

M1 carbocation formed (2 x AO1)

M2 lone pair on nucleophile attacks/attracted to

positive C

– AS CHEMISTRY – 7404/2 –

M1 Attack by NH3 with curly arrow from lone pair on N to the correct Ignore use of sticks

Penalise M1 for formal charge on C and / or Br of

M2 Structure of intermediate C–Br or incorrect partial charges on C–Br; ignore

+ other partial charges on uncharged atoms

M3 Loss of H with curly arrow from N–H bond to N

Penalise M1 for any additional arrow or charge on

NH3 or missing lone pair

Penalise M1 for any additional arrow(s) to / from

the Br to / from anything else

For M2, the + should be on the N

01.2 For M3 there is no need to show attack by a (1 x AO1,

second NH3 molecule, but if it is shown, it must be 2 x AO2)

correct (but, if the NH3 is charged or the lone pair is

missing and this has been penalised in M1 then do

not penalise the same error again in M3)

For M3, NOT removal of H by attack with Br

For SN1:

M1 structure of correct carbocation 11

M2 attack by NH3 with curly arrow from lone pair on

N to the +C of their carbocation

Allow aqueous KOH/NaOH

NOT ethanol / alcohol 1

01.3 water / aqueous

(1 x AO1)

Ignore temperature

– AS CHEMISTRY – 7404/2 –

M1 (basic) elimination Ignore use of sticks but H must be shown for H that

– is lost by C–H breaking

M2 curly arrow from lone pair of OH to an H on C1

Penalise M2 for any additional arrow or charge on

M3 curly arrow from C–H bond on C1 to bond between C1 and C2 –

Penalise M3 for formal charge on C and / or Br of

C–Br or incorrect partial charges on C–Br; ignore

other partial charges on uncharged atoms

Penalise M3 for any additional arrow(s) to / from

the Br to / from anything else

If H on C3 attacked, NOT M2: but allow M3 for 5

correct arrows only for curly arrow from C–H bond

M4 E-but-2-ene (name or structure) (1 x AO1,

01.4 on C3 to bond between C3 and C2

2 x AO2,

M5 Z-but-2-ene (name or structure)

Cannot score M2/3 if attacking H on C2 or C4 2 x AO3)

For E1:

M2 curly arrow from lone pair of OH– to an H on C1

of correct carbocation

M3 curly arrow from C–H bond on C1 to bond

between C1 and C2 (allow ECF for making but-2-

ene)

M4 and M5 in either order–ASCHEMISTRY; allow 1 mark for–7404M4/5/2–

for but-2-ene (even if there is a second incorrect

product named)

120(°) 1

01.5

(1 x AO3)

M1 (idea of covalent bond where or shared electrons where) both – +

Allow structure showing (:)→ from CN( ) to C( ), but

electrons come from the same species

with a straight arrow 2

M2 lone pair donated from C of CN– to C+ or central C (of

01.6 Allow curly arrow mechanism but need charges (1 x AO1,

carbocation) 1 x AO2)

and lone pair

M2 also scores M1

How to answer it

Halogenoalkanes: Substitution, Elimination & Mechanisms

Topic Overview

What This Question Tests

A comprehensive assessment of nucleophilic substitution, elimination reactions, stereoisomerism, carbocation structure, and bonding fundamentals from AQA A-Level Chemistry (Section 3.3.3 Halogenoalkanes).

Bond polarity: Explaining reactivity via electronegativity differences (Cδ+–Xδ-).
Curly arrow mechanisms: Drawing precise nucleophilic substitution with ammonia and base elimination with hydroxide.
Regiochemistry and isomerism: Identifying minor/major alkene products and recognizing E/Z stereoisomerism.
Shapes of intermediates: Deducing the geometry and bond angle (120°) of a trigonal planar carbocation.
Dative bonding: Describing coordinate bond formation using lone pairs.

Question 01.1

Why Halogenoalkanes React with Nucleophiles

2 Marks • Explanation

✅ Model Answer

M1: The carbon atom in the C–halogen bond is partially positive / electron deficient ( Cδ+ ).
M2: A lone pair of electrons on the nucleophile is attracted to (or attacks) this partially positive carbon ( Cδ+ ).

Award 1 mark for each point. (Alternatively, explaining via an S_N1 route: M1 for carbocation formation, M2 for nucleophile attacking the C+).

❌ Common Errors & Pitfalls

Stating merely "the bond is polar" without specifying that the carbon atom is δ+ .
Saying the nucleophile is attracted to the "halogen" or the "molecule" rather than specifically to the carbon atom.
Forgetting to mention the lone pair on the nucleophile.

Question 01.2

Nucleophilic Substitution with Ammonia

3 Marks • Mechanism Completion

Examiner Diagram Description:
• The starting material is 2-bromobutane with the C–Br bond breaking arrow already provided.
• Arrow 1 (M1): Start a curly arrow at the lone pair on the nitrogen of an :NH₃ molecule, pointing directly to C2 (the carbon atom bonded to Br).
• Intermediate (M2): Draw the protonated amine intermediate: CH₃–CH(N⁺H₃)–CH₂–CH₃ . The positive charge must be placed on the N atom.
• Arrow 2 (M3): A curly arrow starting from an N–H bond pointing directly onto the N⁺ atom (loss of H⁺). Alternatively, show another :NH₃ lone pair attacking this H atom, with the N–H bond pair moving onto the N.

🧠 Exam Technique

Do not redraw the C–Br arrow: The exam paper already printed this curly arrow. Drawing an extra arrow or contradicting it loses M1.
Charge Placement: Ensure the formal positive charge is clearly situated on the nitrogen atom, never floating or on a hydrogen/carbon.

❌ Mark-Loss Traps

Starting the arrow from the letter "N" rather than from the lone pair.
Showing Br⁻ removing the proton (examiners do not accept attack by Br⁻ for M3; ammonia acts as the base in practice).
Missing out the intermediate entirely and trying to form the amine in a single step.

Question 01.3

Solvent Condition for Substitution (Alcohol Formation)

1 Mark • Reaction Conditions

✅ Correct Answer

Water or aqueous (e.g., aqueous KOH / aqueous ethanol with high water content).

💡 Solvent Deciding Factor

Aqueous KOH (warm/reflux): Favours nucleophilic substitution where OH⁻ acts as a nucleophile → Alcohol formed.
Ethanolic KOH (hot/reflux): Favours elimination where OH⁻ acts as a base → Alkene formed.

Question 01.4

Elimination Mechanism & Stereoisomer Identification

5 Marks • Mechanism, Nomenclature & Isomerism

✅ Model Answers

Name of mechanism (M1): Elimination (or basic elimination).
Other alkene 1 (M4): E-but-2-ene (or trans-but-2-ene).
Other alkene 2 (M5): Z-but-2-ene (or cis-but-2-ene).

Names or correct displayed/structural formulas are accepted. "But-2-ene" alone scores only 1 mark across M4/M5 because it fails to distinguish the two stereoisomers.

🧠 Completing the Elimination Mechanism (M2, M3)

• Note: The curly arrow from C–Br to Br is already printed.
• Arrow 1 (M2): From the lone pair on :OH⁻ to one of the H atoms on C1 (the methyl group next to C–Br).
• Arrow 2 (M3): From the C1–H bond to the middle of the C1–C2 single bond (forming the double bond of but-1-ene).

❌ Critical Examiner Watch-outs

Attacking the wrong carbon: To make but-1-ene, the base must attack a proton on C1. If you show the proton on C3 being removed, you produce but-2-ene and forfeit M2.
Stereoisomers required: The prompt explicitly states "but-1-ene and two other alkenes". Since but-2-ene exhibits geometric isomerism due to restricted rotation around the planar C=C double bond and two different groups on each carbon, you must specify E and Z (or trans and cis).

Question 01.5

Bond Angle of the (CH₃)₃C⁺ Intermediate

1 Mark • Structure & Bonding

✅ Correct Answer

120°

💡 VSEPR Explanation

The central positively charged carbon has 3 bonding pairs of electrons and 0 lone pairs. The electron pairs repel equally to adopt a trigonal planar geometry with bond angles of 120°.

Question 01.6

Formation of a Co-ordinate (Dative) Bond

2 Marks • Bonding Concept

✅ Model Answer

M1: A covalent bond in which both electrons come from the same species / atom (or shared pair donated by one species).
M2: The lone pair of electrons is donated from the carbon atom of CN⁻ to the electron-deficient positive carbon ( C⁺ ) of the carbocation.

🧠 Precision in Dative Bonding

Saying "the cyanide ion donates a lone pair" is good, but specifying that the carbon (not nitrogen) donates to the carbocation's central carbon ensures full credit.

Drawing an arrow showing (:)C≡N⁻ → C⁺ correctly scores both marks if lone pair donation is unambiguous.

Topics

Organic Chemistry · Physical Chemistry · 3.3.3 Halogenoalkanes · 3.3.1 Introduction to Organic Chemistry · 3.3.4 Alkenes · 3.1.3 Bonding

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.