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

0%read

Part 1 of 5Polarity of the C–X Bond

Electronegativity decreases down Group 7.

Dipole

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

Down the group Polarity falls from fluorine to iodine because the halogen atoms attract the bonding pair less strongly.

Prediction

Prediction Polarity alone would suggest fluoroalkanes react fastest, but experiments show they are the least reactive.

Conclusion

Conclusion Polarity is therefore not the controlling factor for the rate of nucleophilic substitution.

Breaking the C–X bond is the key step, so its strength decides the rate.

Bond Enthalpy and Reactivity

Strength order

Strength order The bond enthalpy decreases in the order C–F > C–Cl > C–Br > C–I as the halogen atom gets larger.

Reason

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 order Reactivity in substitution increases in the order RF < RCl < RBr < RI, the reverse of bond strength.

Evidence

Evidence Silver nitrate tests give the fastest precipitate with iodoalkanes and no reaction at all with fluoroalkanes.

Intermolecular forces explain boiling point trends.

Halogenoalkanes are polar, but the dipole–dipole forces are weak compared with the London forces in larger molecules.

Boiling point rises as the halogen gets larger because more electrons give stronger London dispersion forces between molecules.

For ethyl halides the boiling points rise from about 12 °C for chloroethane to 38 °C for bromoethane and 72 °C for iodoethane.

Boiling point also rises with chain length for the same halogen, since longer molecules have more surface contact and electrons.

Halogenoalkanes are poorly soluble in water because they cannot form hydrogen bonds with water molecules.

Choosing a reagent

Choosing a reagent An iodoalkane is the best choice when a fast substitution is needed, though it is more expensive than the chloroalkane.

Stability

Stability Fluoroalkanes persist in the environment because the very strong C–F bond resists attack by most reagents.

Comparing tests

Comparing tests If three unlabelled halogenoalkanes are tested with silver nitrate, the order of precipitate appearance identifies them.

Prediction

Prediction A bromoalkane will always react faster than the chloroalkane with the same carbon skeleton under identical conditions.

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.

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.

Last updated: 27 October 2026