Edexcel International A Level Chemistry
Edexcel International A Level Chemistry Halogenoalkanes Exam Questions
Free Edexcel International A Level Chemistry exam questions for Halogenoalkanes, with a mark scheme for every pack. Covers classifying halogenoalkanes, nucleophilic substitution with hydroxide, water, cyanide and ammonia, elimination, SN1 and SN2 mechanisms and relative rates of hydrolysis.
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 Edexcel International A Level Chemistry Halogenoalkanes
Halogenoalkanes are a key Unit 2 organic topic in Edexcel International A Level Chemistry because they react with many nucleophiles and so are useful in synthesis. You study nucleophilic substitution with aqueous hydroxide, water, potassium cyanide and ammonia, and elimination with ethanolic hydroxide to form alkenes. You compare the rates of hydrolysis of chloro, bromo and iodoalkanes using silver nitrate, and of primary, secondary and tertiary halogenoalkanes, explaining them through bond enthalpy and the SN1 and SN2 mechanisms. Examiners regularly test the conditions that favour substitution or elimination, the mechanisms, and practical rate comparisons.
What you need to know
- Classification and polarity The C to X bond is polar because the halogen is more electronegative, so the carbon is delta positive and attacked by nucleophiles. Halogenoalkanes are classified as primary, secondary or tertiary.
- Nucleophilic substitution With aqueous KOH under reflux, they form alcohols. With KCN in ethanol, nitriles form (adding a carbon). With excess ethanolic ammonia under pressure, primary amines form.
- Elimination Heating with KOH dissolved in ethanol eliminates HX to give an alkene. Here OH- acts as a base, not a nucleophile.
- SN2 mechanism Typical of primary halogenoalkanes: the nucleophile attacks the delta positive carbon as the C to X bond breaks, via a single transition state with five groups around carbon.
- SN1 mechanism Typical of tertiary halogenoalkanes: the C to X bond breaks first to form a tertiary carbocation, which is then attacked by the nucleophile. Tertiary carbocations are stabilised by alkyl groups.
- Rates of hydrolysis Add ethanolic silver nitrate and time the formation of a precipitate. Rate increases from C to Cl to C to Br to C to I because C to I has the lowest bond enthalpy; tertiary hydrolyse faster than primary.
Common mistakes in Halogenoalkanes
- Explaining the rate of hydrolysis using bond polarity. The C to I bond is the least polar but reacts fastest; bond enthalpy is the deciding factor.
- Mixing up the solvents: aqueous hydroxide gives substitution, ethanolic hydroxide gives elimination.
- Drawing the SN2 curly arrow from the negative charge on OH- rather than from a lone pair.
- Forgetting that the halide ion leaves with both electrons from the C to X bond.
- Writing that a nitrile forms with aqueous KCN. Ethanolic KCN is used, heated under reflux.
Try three Halogenoalkanes exam-style questions
1. Draw the mechanism for the reaction of bromoethane with aqueous hydroxide ions. Name the mechanism. (4 marks)
Show the mark scheme answer
Dipole on C to Br bond; curly arrow from lone pair on OH- to delta positive carbon; curly arrow from C to Br bond to Br; products ethanol and Br-; nucleophilic substitution (SN2).
2. 1-chlorobutane, 1-bromobutane and 1-iodobutane are each warmed with ethanolic silver nitrate. State the order in which precipitates appear and explain the order. (3 marks)
Show the mark scheme answer
1-iodobutane first, then 1-bromobutane, then 1-chlorobutane; C to I bond has the lowest bond enthalpy; so it breaks most easily and hydrolysis is fastest.
3. 2-bromopropane is heated with potassium hydroxide dissolved in ethanol. Name the organic product, the type of reaction and the role of the hydroxide ion. (3 marks)
Show the mark scheme answer
Propene; elimination; hydroxide ion acts as a base (removing H+).
Halogenoalkanes FAQs
What does Edexcel International A Level Chemistry Halogenoalkanes cover?
It covers the structure and classification of halogenoalkanes, nucleophilic substitution with hydroxide, water, cyanide and ammonia, elimination to alkenes, SN1 and SN2 mechanisms and comparing rates of hydrolysis.
Which unit is Halogenoalkanes examined in?
Halogenoalkanes is part of Unit 2, and its reactions are used again in Unit 5 organic synthesis questions.
What conditions do I need to learn for Halogenoalkanes?
Learn aqueous KOH under reflux for alcohols, ethanolic KCN under reflux for nitriles, excess ethanolic ammonia heated in a sealed tube for amines and ethanolic KOH under reflux for alkenes.
