2.8.5Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Trends in Halogenoalkanes — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 50 of 53 Unit quiz
Section 1 of 5Polarity of the C–X Bond
Electronegativity decreases down Group 7.
Dipole
- Every C–X bond is polar, with δ+ on carbon and δ– on the halogen, because all halogens are more electronegative than carbon.
Down the group
- Polarity falls from fluorine to iodine because the halogen atoms attract the bonding pair less strongly.
Prediction
- Polarity alone would suggest fluoroalkanes react fastest, but experiments show they are the least reactive.
Conclusion
- Polarity is therefore not the controlling factor for the rate of nucleophilic substitution.
Section 2 of 5Bond Enthalpy and Reactivity
Breaking the C–X bond is the key step, so its strength decides the rate.
Bond Enthalpy and Reactivity
Strength order
- The bond enthalpy decreases in the order C–F > C–Cl > C–Br > C–I as the halogen atom gets larger.
Reason
- A larger halogen atom has a longer, more diffuse bond to carbon, with poorer overlap, so less energy is needed to break it.
Reactivity order
- Reactivity in substitution increases in the order RF < RCl < RBr < RI, the reverse of bond strength.
Evidence
- Silver nitrate tests give the fastest precipitate with iodoalkanes and no reaction at all with fluoroalkanes.
Section 3 of 5Physical Properties
Intermolecular forces explain boiling point trends.
1
Halogenoalkanes are polar, but the dipole–dipole forces are weak compared with the London forces in larger molecules.
2
Boiling point rises as the halogen gets larger because more electrons give stronger London dispersion forces between molecules.
3
For ethyl halides the boiling points rise from about 12 °C for chloroethane to 38 °C for bromoethane and 72 °C for iodoethane.
4
Boiling point also rises with chain length for the same halogen, since longer molecules have more surface contact and electrons.
5
Halogenoalkanes are poorly soluble in water because they cannot form hydrogen bonds with water molecules.
| Compound | Boiling point / °C |
|---|---|
| Chloroethane | 12 |
| Bromoethane | 38 |
| Iodoethane | 72 |
Section 4 of 5Applying the Trends
Choosing a reagent
- An iodoalkane is the best choice when a fast substitution is needed, though it is more expensive than the chloroalkane.
Stability
- Fluoroalkanes persist in the environment because the very strong C–F bond resists attack by most reagents.
Comparing tests
- If three unlabelled halogenoalkanes are tested with silver nitrate, the order of precipitate appearance identifies them.
Prediction
- A bromoalkane will always react faster than the chloroalkane with the same carbon skeleton under identical conditions.
Section 5 of 5Exam Checklist & Tips
2.8.5 – Key Points for the Exam
- Electronegativity The ability of an atom to attract the bonding pair of electrons in a covalent bond.
- Bond enthalpy The energy required to break one mole of a particular covalent bond in the gas phase.
- Dipole A separation of partial positive and partial negative charge across a bond.
- London forces Weak attractions between temporary dipoles in neighbouring molecules.
Common Mistakes
- Claiming fluoroalkanes are most reactive because the C–F bond is most polar;
- bond strength matters more than polarity.
- Saying the C–I bond is weak because iodine is less electronegative;
- the reason is its larger atomic size and poorer orbital overlap.
- Describing the boiling point trend using covalent bond strength rather than intermolecular forces.
- Stating that halogenoalkanes dissolve in water because they are polar.
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