AQA A Level Chemistry
AQA A Level Chemistry 3.3.3 Halogenoalkanes Exam Questions
Free AQA A Level Chemistry exam questions for 3.3.3 Halogenoalkanes, with a mark scheme for every pack. Covers nucleophilic substitution with hydroxide, cyanide and ammonia, elimination to alkenes, carbon-halogen bond enthalpy and ozone depletion.
3.3.3 Halogenoalkanes exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About AQA A Level Chemistry 3.3.3 Halogenoalkanes
Halogenoalkanes are versatile intermediates in AQA A Level organic chemistry. You need to explain why the carbon bonded to the halogen is attacked by nucleophiles, draw nucleophilic substitution mechanisms with hydroxide ions, cyanide ions and ammonia, and describe elimination with hydroxide ions in ethanol to form alkenes. You also explain why reactivity depends on carbon-halogen bond enthalpy and how CFCs damage the ozone layer. These reactions appear again in synthesis routes, amines and chain-lengthening steps. Examiners expect precise curly arrows, correct reagents and conditions, and the ability to choose between substitution and elimination.
What you need to know
- Polarity and reactivity The C-X bond is polar with a δ+ carbon that attracts nucleophiles. Reactivity increases from C-F to C-I because bond enthalpy decreases, so C-I is broken most easily. Bond enthalpy, not polarity, explains the trend.
- Substitution with hydroxide Warm aqueous NaOH or KOH under reflux. The OH- lone pair attacks the δ+ carbon and the C-X bond breaks to release X-, forming an alcohol.
- Substitution with cyanide KCN in aqueous ethanol under reflux gives a nitrile, adding one carbon atom to the chain. For example, bromoethane gives propanenitrile.
- Substitution with ammonia Excess ethanolic ammonia, heated under pressure, gives a primary amine. The mechanism has two steps: substitution, then removal of H+ from the nitrogen by a second ammonia molecule.
- Elimination Hot KOH in ethanol (the hydroxide acts as a base) removes H and X from adjacent carbons to form an alkene. Unsymmetrical halogenoalkanes can give a mixture of alkenes, including E/Z isomers.
- Ozone depletion CFCs release chlorine radicals in UV light, which catalyse the breakdown of ozone. Legislation banned CFCs, and chemists developed alternatives such as HFCs that do not contain chlorine.
Common mistakes in 3.3.3 Halogenoalkanes
- Explaining the reactivity trend using bond polarity. The C-I bond reacts fastest because it has the lowest bond enthalpy.
- Mixing up conditions: aqueous hydroxide gives substitution, ethanolic hydroxide and heat gives elimination.
- Starting the curly arrow from the negative charge instead of a lone pair on the nucleophile, or forgetting the arrow from the C-X bond to X.
- Showing only one alkene from 2-bromobutane. Elimination gives but-1-ene and both E and Z but-2-ene.
- Leaving out excess ammonia. Without excess, the amine product reacts further to give secondary and tertiary amines.
Try three 3.3.3 Halogenoalkanes exam-style questions
1. Outline the mechanism for the reaction of bromoethane with aqueous sodium hydroxide. (3 marks)
Show the mark scheme answer
Curly arrow from a lone pair on O of OH- to the δ+ carbon; curly arrow from the C-Br bond to Br; products ethanol and Br- shown (with δ+/δ- on the C-Br bond).
2. Give the reagent and conditions for converting 2-bromopropane into propene, and name the type of reaction. (3 marks)
Show the mark scheme answer
KOH (or NaOH); dissolved in ethanol, heated (reflux); elimination.
3. Explain why iodoethane is hydrolysed faster than chloroethane. (2 marks)
Show the mark scheme answer
The C-I bond enthalpy is lower than the C-Cl bond enthalpy; so less energy is needed to break the C-I bond and the reaction is faster.
3.3.3 Halogenoalkanes FAQs
What does AQA A Level Chemistry 3.3.3 Halogenoalkanes cover?
It covers nucleophilic substitution with hydroxide, cyanide and ammonia, elimination to form alkenes, the effect of bond enthalpy on reactivity and the impact of CFCs on ozone.
Which paper is 3.3.3 Halogenoalkanes examined in?
It is examined in Paper 2, and Paper 3 can also include it.
How do I remember when substitution or elimination happens?
Link the solvent to the role of hydroxide: in water it acts as a nucleophile and substitutes; in ethanol with heat it acts as a base and eliminates. Write both sets of conditions on a flashcard with an example of each.
