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.
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
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- recall the reactions (reagents and conditions) by which halogenoalkanes can be produced: (a) the free-radical substitution of alkanes by Cl2 or Br2 in the presence of ultraviolet light, as exemplified by the reactions of ethane (b) electrophilic addition of an alkene with a halogen, X2, or hydrogen halide, HX(g), at room temperature (c) substitution of an alcohol, e.g. by reaction with HX(g); or with KCl and concentrated H2SO4 or concentrated H3PO4; or with PCl 3 and heat; or with PCl 5; or with SOCl 2
- classify halogenoalkanes into primary, secondary and tertiary
- describe the following nucleophilic substitution reactions: (a) the reaction with NaOH(aq) and heat to produce an alcohol (b) the reaction with KCN in ethanol and heat to produce a nitrile (c) the reaction with NH3 in ethanol heated under pressure to produce an amine (d) the reaction with aqueous silver nitrate in ethanol as a method of identifying the halogen present as exemplified by bromoethane
- describe the elimination reaction with NaOH in ethanol and heat to produce an alkene as exemplified by bromoethane
- describe the SN1 and SN2 mechanisms of nucleophilic substitution in halogenoalkanes including the inductive effects of alkyl groups
- recall that primary halogenoalkanes tend to react via the SN2 mechanism; tertiary halogenoalkanes via the SN1 mechanism; and secondary halogenoalkanes by a mixture of the two, depending on structure
- describe and explain the different reactivities of halogenoalkanes (with particular reference to the relative strengths of the C–X bonds as exemplified by the reactions of halogenoalkanes with aqueous silver nitrates)
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.
Tips to avoid common mistakes in Halogenoalkanes
- › Recall the relative oxidizing strengths of halogens; HCl cannot oxidize iodide, and silver chloride is soluble in aqueous ammonia.
- › For the formation of NaClO, specify that sodium hydroxide must be cold and in aqueous solution.
- › Recall that primary halogenoalkanes typically undergo SN2 mechanisms while tertiary halogenoalkanes proceed via the SN1 mechanism due to carbocation stability.
Give the reagents and conditions for the free-radical substitution of ethane to produce chloroethane.
Reagents: Chlorine gas (Cl₂). Conditions: Ultraviolet (UV) light. This reaction is a chain reaction involving initiation, propagation, and termination steps.
What reagents and conditions are used for the electrophilic addition of bromine to ethene?
Reagents: Bromine (Br₂). Conditions: Room temperature. The double bond of the alkene breaks, and a bromine atom adds to each carbon, forming a dibromoalkane.
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.
How would you classify 2-chlorobutane as a primary, secondary or tertiary halogenoalkane?
Secondary. The carbon atom bonded to the halogen (chlorine) is bonded to two other carbon atoms.
Describe the reaction of a halogenoalkane (e.g. bromoethane) with NaOH(aq) and heat.
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.
What products are formed when bromoethane reacts with KCN in ethanol and heat?
The bromoethane undergoes nucleophilic substitution with cyanide ion (CN⁻) to produce ethanenitrile (CH₃CH₂CN) and potassium bromide (KBr).
Outline the elimination reaction of bromoethane with NaOH in ethanol and heat.
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.
Describe the SN1 mechanism, including the inductive effects of alkyl groups on carbocation stability.
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.
Explain why the C-I bond in iodoalkanes is typically more reactive than the C-Cl bond in chloroalkanes in nucleophilic substitution reactions.
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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