OxfordAQA International A Level Chemistry
OxfordAQA International A Level Chemistry 3.3.3 Halogenoalkanes Exam Questions
Free OxfordAQA International A Level Chemistry revision for 3.3.3 Halogenoalkanes: what you need to know, common mistakes and exam-style questions with answers. Covers nucleophilic substitution with hydroxide, cyanide and ammonia, the effect of carbon-halogen bond enthalpy on reactivity, and elimination using potassium hydroxide.
Practise 3.3.3 Halogenoalkanes with past papers
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About OxfordAQA International A Level Chemistry 3.3.3 Halogenoalkanes
Halogenoalkanes is a Unit 2 organic topic focused on two competing reaction types. You need to describe nucleophilic substitution of halogenoalkanes with hydroxide ions, cyanide ions and ammonia, drawing the mechanisms with curly arrows, and explain how the carbon-halogen bond enthalpy affects the rate of reaction. You also need to describe elimination, for example of 2-bromopropane with potassium hydroxide, and explain how conditions decide whether potassium hydroxide acts as a nucleophile or a base. This topic connects to alkenes, alcohols and amines, and the mechanisms are used again in organic synthesis at A2.
What you need to know
- Polar C-X bond The carbon-halogen bond is polar because the halogen is more electronegative, so the carbon is delta positive and attacked by nucleophiles. A nucleophile is an electron pair donor.
- Substitution with hydroxide Warm aqueous NaOH or KOH converts a halogenoalkane into an alcohol, for example CH3CH2Br + OH- → CH3CH2OH + Br-.
- Substitution with cyanide and ammonia Ethanolic KCN gives a nitrile, increasing the carbon chain length by one. Excess ethanolic ammonia under pressure gives a primary amine.
- Reactivity Rate depends on C-X bond enthalpy rather than polarity. C-I is the weakest and breaks most easily, so iodoalkanes react fastest and fluoroalkanes slowest.
- Elimination Hot ethanolic KOH causes elimination: OH- acts as a base, removing H+ from a carbon next to the C-X carbon, forming an alkene. 2-bromopropane gives propene.
- Conditions Aqueous, warm conditions favour substitution; ethanolic, hot conditions favour elimination.
Common mistakes in 3.3.3 Halogenoalkanes
- Explaining reactivity by bond polarity. C-F is the most polar but reacts slowest because C-F has the highest bond enthalpy.
- Starting the curly arrow from the negative charge rather than a lone pair on the nucleophile.
- Mixing up the conditions. Aqueous gives substitution; ethanolic gives elimination.
- Forgetting that excess ammonia is needed to make a primary amine. Otherwise further substitution gives secondary and tertiary amines.
Try three 3.3.3 Halogenoalkanes exam-style questions
1. Outline the mechanism for the reaction of bromoethane with aqueous hydroxide ions. (3 marks)
Show the mark scheme answer
Curly arrow from lone pair on OH- to the delta positive carbon; curly arrow from C-Br bond to Br; products CH3CH2OH and Br-.
2. Explain why 1-iodobutane reacts faster than 1-chlorobutane with hydroxide ions. (2 marks)
Show the mark scheme answer
C-I bond enthalpy is lower than C-Cl; so less energy is needed to break the bond.
3. State the conditions needed to convert 2-bromopropane into propene and name the role of the hydroxide ion. (2 marks)
Show the mark scheme answer
Hot KOH in ethanol (heat under reflux); hydroxide ion acts as a base.
3.3.3 Halogenoalkanes FAQs
What does OxfordAQA International A Level Chemistry 3.3.3 Halogenoalkanes cover?
It covers nucleophilic substitution with hydroxide, cyanide and ammonia, the effect of carbon-halogen bond enthalpy on reactivity, and elimination using potassium hydroxide.
Which unit is 3.3.3 Halogenoalkanes examined in?
It is examined in Unit 2, the second International AS paper, which covers organic and physical chemistry. The Unit 5 practical and synoptic paper at A2 can also assess it.
How do I decide between substitution and elimination?
Look at the conditions. Warm aqueous hydroxide favours nucleophilic substitution to give an alcohol, while hot ethanolic hydroxide favours elimination to give an alkene.
