4.5.1Core Organic Chemistry
Reactions of Haloalkanes — OCR A Level Chemistry A H432 Revision Notes & Topical Questions
5 min read 5 parts Lesson 20 of 29
Part 1 of 5Why Haloalkanes React
Polar bond
- Halogens are more electronegative than carbon, so the C–X bond is polar with a δ+ carbon and a δ− halogen.
Attack site
- The δ+ carbon attracts nucleophiles, which are species with a lone pair of electrons to donate.
Compared with alkanes
- Alkanes have nonpolar bonds and are much less reactive than haloalkanes of similar size.
Naming
- Haloalkanes are named with the prefixes fluoro-, chloro-, bromo- or iodo-, as in 2-chlorobutane.
Part 2 of 5Types of Reaction
Types of Reaction
Nucleophilic substitution
- A nucleophile replaces the halogen atom, which leaves as a halide ion.
Hydrolysis
- Water or hydroxide ions act as the nucleophile and the haloalkane becomes an alcohol.
Elimination
- With ethanolic hydroxide, hydrogen halide is removed to form an alkene.
Nitrile and amine formation
- Cyanide ions make nitriles and ammonia makes amines, both by substitution.
Part 3 of 5Reactivity of the Carbon-Halogen Bond
1
Bond enthalpy falls down the group, from about 467 kJ mol⁻¹ for C–F to about 228 kJ mol⁻¹ for C–I.
2
A lower bond enthalpy means the C–X bond breaks more easily during a reaction.
3
Therefore iodoalkanes react fastest and fluoroalkanes are the slowest, with the order of reactivity I > Br > Cl > F.
4
Polarity decreases down the group, but this does not decide the rate; bond strength is the key factor.
5
Rates can be compared by timing the formation of a silver halide precipitate when a haloalkane is warmed with silver nitrate.
Part 4 of 5Common Nucleophiles
Hydroxide, OH⁻
- Heating with aqueous sodium hydroxide gives an alcohol: C₂H₅Br + OH⁻ → C₂H₅OH + Br⁻.
Cyanide, CN⁻
- Heating with ethanolic potassium cyanide gives a nitrile, adding one carbon to the chain.
Ammonia, NH₃
- Heating with excess ammonia in ethanol in a sealed tube gives a primary amine.
Water, H₂O
- Water is a weak nucleophile, and hydrolysis with water alone is slow but can be used to compare rates.
| Bond | Approx. bond enthalpy / kJ mol⁻¹ |
|---|---|
| C–F | 467 |
| C–Cl | 346 |
| C–Br | 290 |
| C–I | 228 |
Part 5 of 5Exam Checklist & Tips
4.5.1 – Key Points for the Exam
- Haloalkane An alkane in which one or more hydrogen atoms have been replaced by halogen atoms.
- Nucleophile An electron pair donor that attacks a δ+ carbon atom.
- Nucleophilic substitution A reaction in which a nucleophile replaces an atom or group in a molecule.
- Bond enthalpy The energy needed to break one mole of a given bond in the gaseous state.
Common Mistakes
- Explaining the rate trend by polarity;
- the C–I bond is the least polar but weakest, and it is the weakness that matters.
- Forgetting that the nucleophile has a lone pair, and writing OH⁻ without a charge.
- Mixing up the conditions for substitution (aqueous) and elimination (ethanolic).
- Saying fluoroalkanes are the most reactive because fluorine is the most electronegative.
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