Explain the nature of the C-X bond. How do the boiling points of haloalkanes depend on (i) the halogen, (ii) the size of the alkyl group and (iii) branching?

1. 1 · State the principle. The halogen is more electronegative than carbon, so the C-X bond is polar: Cδ⁺-Xδ⁻.
2. 2 · Bond length and strength. Atom size grows from F to I, so the C-X bond gets longer and weaker.
3. 3 · Effect of the halogen. RI > RBr > RCl > RF: a bigger halogen has more electrons, so the van der Waals forces are stronger.
4. 4 · Effect of size and branching. Boiling point rises with more carbon atoms. Branching lowers it (less surface area): 1-bromobutane 375 K > 2-bromobutane 364 K > 2-bromo-2-methylpropane 346 K.
5. 5 · Close in one line. So the boiling point follows the strength of the forces between the molecules.

Scoring tip: Always name the force (dipole-dipole or van der Waals) when you explain a boiling point.