Welcome back to Beyond’s Science Blog! This A Level entry explores the Homologous Series: Halogenoalkanes.
You can also subscribe to Beyond Secondary Resources for access to thousands of worksheets and revision tools. Our site was created with teachers in mind and includes lots of teacher instructions, however, it also contains content for students that will be particularly useful when revising! You can sign up for a free account here and take a look around at our free resources before you subscribe too.
Halogenoalkanes are alkanes in which one or more hydrogen atoms have been substituted by a halogen atom (F, Cl, Br or I). The prefixes fluoro-, chloro-, bromo- and iodo- are used to represent the halogens within the nomenclature of halogenoalkanes.
| Functional Group | General Formula |
|---|---|
| R – X | CnH2n+1X (X = F, Cl, Br or I) |
| fluoromethane | chloroethane | 2-bromopropane | 1-iodobutane |
|---|---|---|---|
 |  |  |  |
Chemical Properties
Halogenoalkanes are more reactive than alkanes. Iodoalkanes are the most reactive because of the relatively small bond enthalpy of the C-I
bond.
Halogenoalkanes can be identified by the addition of silver nitrate and nitric acid. A silver halide, AgX, precipitate will form, the colour of which will indicate the halogen which is present (Cl = white, Br = cream, I = yellow). No precipitate will form when a fluoroalkane is reacted with silver nitrate and nitric acid.
Reactions of halogenoalkanes include:
- Nucleophilic Substitution
Nucleophiles (electron-pair donors) can attack the slightly positively charged carbon atom of the polar C-X bonds within halogenoalkanes. The products of the reaction depend on the nucleophile.
halogenoalkane + OH– → alcohol + X–
halogenoalkane + CN– → nitrile + X–
halogenoalkane + NH3 → amine + NH4X - Under anhydrous conditions, hydroxide ions act as a base and remove a hydrogen ion from the halogenoalkane. A double bond forms and the halogen atom is eliminated.
halogenoalkane + OH– → alkene + water + X–
Other Information
- The C-X bond in halogenoalkanes is polar due to the highly electronegative halogen atom drawing electron density towards that side of the covalent bond.
- Halogenoalkanes are used as refrigerants, solvents and in the production of pharmaceutical compounds.
- The use of chlorofluorocarbons (CFCs) is restricted due to their damaging effect on the ozone layer.
Found this blog post on the Homologous Series: Halogenoalkanes useful? You can read our other Science revision blogs here.