Introduction to the Chemistry of Alkyl Halides
Physical Properties of Alkyl Halides
The physical properties of alkyl halides considerably differ from those of the corresponding alkanes. The strength and length of the carbon-halogen bond (C-X bond), the dipole moment and the boiling point of an alkyl halide are determined by the polarity of the bond as well as by the size of the various halogen atoms.
- The C-X bond strength decreases with an increase of the size of the halogen (X) because the size of the p orbital of the halogen increases as well. Thus, the p orbital becomes more diffused and the overlap with the orbital of the carbon deteriorates. As a result, the C-X bond is weakend and elongated.
- Tab.1
- Bond lengths, dipole moments and dissociation energies of methyl halides ()
| 3D representation |
|
|
|
|
|---|---|---|---|---|
| Controls for 3D representation | Zurücksetzen | Zurücksetzen | Zurücksetzen | Zurücksetzen |
| Methyl halide (halomethane) | ||||
| Bond length (pm) | 138.5 | 178.4 | 192.9 | 213.9 |
| Dipole moment (D) | 1.85 | 1.87 | 1.81 | 1.62 |
| Dissociation energy (kJ/mol) | 461 | 356 | 297 | 239 |
- Halogens (F, Cl and Br) are more electronegative than carbon. Consequently, carbon atoms that carry halogens are charged partially positive while the halogen is charged partially negative. The polarity of the C-X bond causes a measureable dipole moment. As a result of the partial positive charge, the carbon atom displays an electrophilic character. The chemical behavior of alkyl halides is determined mainly by the electrophilicity of the carbon.
- The boiling points of alkyl halides are considerably higher than those of the corresponding alkanes. The main reason for this is the dipole moment of alkyl halides, which leads to attractive dipole-dipole interactions in liquid alkyl halides. Furthermore, the higher molar mass and the stronger London forces (Cl, Br and I) lead to higher boiling points. The main reason of stronger London forces between alkyl halides is the fact that the electron shell of halogens is larger than that of hydrogen and carbon. In larger electron shells, the electrons are not as strongly attracted by the nucleus as is the case in small electron shells. Consequently, the interactions between the electron shells of larger atoms are stronger.
