Nature of the C-X bond and physical properties
Halogens are more electronegative than carbon, so the C-X bond is polar. Haloalkanes boil higher than alkanes of similar mass; boiling point rises with halogen size and falls with branching.
Why the C-X bond is polar
Each halogen is more electronegative than carbon: it pulls the shared pair of electrons towards itself. So carbon gets a partial positive charge (δ⁺) and the halogen a partial negative charge (δ⁻). Down the group from F to I the halogen atom gets bigger, so the C-X bond gets longer and weaker. The δ⁺ carbon is the spot that electron-rich reagents (nucleophiles) attack in the reactions of this chapter.
C-X bond data for methyl halides
| Bond | Bond length (pm) | Bond enthalpy (kJ mol⁻¹) | Dipole moment (D) |
|---|---|---|---|
| CH₃-F | 139 | 452 | 1.847 |
| CH₃-Cl | 178 | 351 | 1.860 |
| CH₃-Br | 193 | 293 | 1.830 |
| CH₃-I | 214 | 234 | 1.636 |
Boiling points
- Methyl chloride, methyl bromide and ethyl chloride are gases at room temperature; higher members are liquids or solids.
- Haloalkanes boil higher than hydrocarbons of similar molar mass. Their polar molecules attract each other by dipole-dipole forces as well as van der Waals forces.
- For the same alkyl group: R-I > R-Br > R-Cl > R-F. A bigger, heavier halogen has more electrons, so the van der Waals forces are stronger.
- Boiling point rises as the alkyl group gets bigger.
- Isomers: more branching, lower boiling point. 1-Bromobutane (375 K) > 2-bromobutane (364 K) > 2-bromo-2-methylpropane (346 K). A branched molecule is more nearly spherical, so it has less surface area for contact.
Dichlorobenzenes: boiling point vs melting point
Solubility, density and other properties
- Only slightly soluble in water. To dissolve, a haloalkane must break the strong hydrogen bonds between water molecules. The new attractions it forms with water are weaker, so too little energy is released.
- Soluble in organic solvents, because the new attractions are about as strong as the ones that are broken.
- Density: bromo, iodo and polychloro derivatives are heavier than water. Density rises with the number of halogen atoms and the atomic mass of the halogen: CH₂Cl₂ (1.336 g mL⁻¹) < CHCl₃ (1.489) < CCl₄ (1.595).
- Pure haloalkanes are colourless; bromides and iodides become coloured in light. Many volatile ones smell sweet.
- CH₃Cl has the highest dipole moment. The C-F bond is more polar, but it is so short that charge × distance ends up a little smaller.
The white toilet-freshener cubes and some moth balls sold in India are p-dichlorobenzene. Its symmetrical molecules make it a solid that slowly turns into vapour at room temperature.
2 marks: “Haloalkanes have higher boiling points than hydrocarbons of comparable molecular mass. Explain.” The C-X bond is polar, so the molecules attract by dipole-dipole forces as well as van der Waals forces. The halogen also adds mass and electrons. More energy is needed to separate the molecules.
2 marks: “The three dichlorobenzenes boil within 7 K of each other (446 to 453 K), yet the para isomer melts at 323 K and the other two below 260 K. Explain.” The para isomer is symmetrical and fits closely in the crystal lattice. So the forces in the solid are stronger, and more energy is needed to melt it. (The boiling points are nearly equal.)
2 marks: “1-Chloropropane has a polar C-Cl bond, yet it forms a separate layer when shaken with water. Explain.” Energy is needed to break the hydrogen bonds in water and the attractions between haloalkane molecules. Less energy is released when the weaker haloalkane-water attractions form.
Big X, big boil; branch it, it falls. Para packs best, so para melts highest.