15.1 A2 Level

Halogenoalkanes

Cambridge A-Level Chemistry (9701)  · Unit 15: Halogen compounds  · 9 flashcards

Halogenoalkanes is topic 15.1 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 15 — Halogen compounds .  In one line: 1. HX(g) (hydrogen halide gas) 2. PCl₅ (phosphorus pentachloride) 3. SOCl₂ (thionyl chloride). Different reagents may require heat or other specific conditions.

Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).

The deck below contains 9 flashcards — 3 definitions and 6 key concepts — covering the precise wording mark schemes reward.  Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.

Key definition

Name three reagents that can be used to convert an alcohol (R-OH) to a halogenoalkane (R-X)

1. HX(g) (hydrogen halide gas) 2. PCl₅ (phosphorus pentachloride) 3. SOCl₂ (thionyl chloride). Different reagents may require heat or other specific conditions.

What the Cambridge 9701 syllabus says

2025-2027 syllabus, in our words · A2 Level

What the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).

  1. Recall how halogenoalkanes can be made, with reagents and conditions: reaction of an alkane (e.g. ethane) with Cl₂ or Br₂ under ultraviolet light by free-radical substitution; electrophilic addition to an alkene of X₂ (a halogen) or HX(g) (a hydrogen halide) at room temperature; replacing the OH group of an alcohol, for example using HX(g), KCl with concentrated H₂SO₄ or concentrated H₃PO₄, PCl₃ with heat, PCl₅, or SOCl₂.
  2. Classify a halogenoalkane as primary, secondary or tertiary.
  3. Describe these nucleophilic substitutions of halogenoalkanes: (a) heating with NaOH(aq) to give an alcohol; (b) heating with KCN dissolved in ethanol to give a nitrile; (c) heating under pressure with NH₃ in ethanol to give an amine; (d) reaction with AgNO₃(aq) in ethanol to identify which halogen is present, using bromoethane as the example.
  4. Describe how heating a halogenoalkane with NaOH in ethanol causes elimination to form an alkene, using bromoethane as the example.
  5. Describe how halogenoalkanes undergo nucleophilic substitution by SN2 and SN1 mechanisms, including the part played by the inductive effect of alkyl groups.
  6. Recall that primary halogenoalkanes usually react by SN2, tertiary ones by SN1, and secondary ones by a mix of both, depending on their structure.
  7. Describe and explain why halogenoalkanes differ in reactivity, especially in terms of the relative C-X bond strengths, as shown by their reactions with aqueous silver nitrate.

Cambridge syllabus keywords to use in your answers

These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.

halogenoalkanes nucleophilic substitution nitrile amine SN1 mechanism SN2 mechanism inductive effects

Tips to avoid common mistakes in Halogenoalkanes

Key Concept Flip

Give the reagents and conditions for the free-radical substitution of ethane to produce chloroethane.

Answer Flip

Reagents: Chlorine gas (Cl₂). Conditions: Ultraviolet (UV) light. This reaction is a chain reaction involving initiation, propagation, and termination steps.

Key Concept Flip

What reagents and conditions are used for the electrophilic addition of bromine to ethene?

Answer Flip

Reagents: Bromine (Br₂). Conditions: Room temperature. The double bond of the alkene breaks, and a bromine atom adds to each carbon, forming a dibromoalkane.

Definition Flip

Name three reagents that can be used to convert an alcohol (R-OH) to a halogenoalkane (R-X).

Answer Flip

1. HX(g) (hydrogen halide gas) 2. PCl₅ (phosphorus pentachloride) 3. SOCl₂ (thionyl chloride). Different reagents may require heat or other specific conditions.

Definition Flip

How would you classify 2-chlorobutane as a primary, secondary or tertiary halogenoalkane?

Answer Flip

Secondary. The carbon atom bonded to the halogen (chlorine) is bonded to two other carbon atoms.

Key Concept Flip

Describe the reaction of a halogenoalkane (e.g. bromoethane) with NaOH(aq) and heat.

Answer Flip

This is a nucleophilic substitution reaction (SN2). The hydroxide ion (OH⁻) acts as a nucleophile, replacing the halogen to form an alcohol and a halide ion.

Example: Bromoethane + NaOH(aq) → Ethanol + NaBr.
Definition Flip

What products are formed when bromoethane reacts with KCN in ethanol and heat?

Answer Flip

The bromoethane undergoes nucleophilic substitution with the cyanide ion (CN⁻) to produce propanenitrile (CH₃CH₂CN) and potassium bromide (KBr). The carbon chain is lengthened by one carbon atom.

Key Concept Flip

Outline the elimination reaction of bromoethane with NaOH in ethanol and heat.

Answer Flip

The hydroxide ion (OH⁻) acts as a base, removing a proton from the halogenoalkane and causing the formation of a C=C double bond and elimination of the halogen.

Example: Bromoethane + NaOH(alc) → Ethene + H₂O + NaBr.
Key Concept Flip

Describe the SN1 mechanism, including the inductive effects of alkyl groups on carbocation stability.

Answer Flip

SN1 (Substitution Nucleophilic Unimolecular) involves two steps: (1) Halogenoalkane ionizes forming a carbocation intermediate. (2) Nucleophile attacks the carbocation. Alkyl groups are electron-donating (positive inductive effect), stabilizing the carbocation. More alkyl groups = more stable carbocation.

Key Concept Flip

Explain why the C-I bond in iodoalkanes is typically more reactive than the C-Cl bond in chloroalkanes in nucleophilic substitution reactions.

Answer Flip

The C-I bond is weaker than the C-Cl bond. This is due to the larger atomic radius of iodine, which results in a longer and weaker bond that requires less energy to break during the reaction.

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Key terms covered in this Halogenoalkanes deck

Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.

Name three reagents that can be used to convert an alcohol (R-OH) to a halogenoalkane (R-X)
How would you classify 2-chlorobutane as a primary, secondary or tertiary halogenoalkane
What products are formed when bromoethane reacts with KCN in ethanol and heat

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