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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)|
Halogenoalkanes are more reactive than alkanes. Iodoalkanes are the most reactive because of the relatively small bond enthalpy of the C-I
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–
- 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.
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