enes Halogenated compounds persist in the environment due to their resistance to breakdown by soil bacteria. Haloalkanes and haloarenes may be classified as follows: These may be classified as mono, di, or polyhalogen (tri-,tetra-, etc.) compounds depending on whether they contain one, two or more halogen atoms in their structures. For example, Monohalocompounds may further be classified according to the hybridisation of the carbon atom to which the halogen is bonded, as discussed below. This class includes (a) Alkyl halides or haloalkanes (R—X) In alkyl halides, the halogen atom is bonded to an alkyl group (R).
They form a homologous series represented by C n H 2n+ X. They are further classified as primary, secondary or tertiary according to the nature of carbon to which halogen is attached. If halogen is attached to a primary carbon atom in an alkyl halide, the alkyl halide is called primary alkyl halide or ° alkyl halide. Similarly, if halogen is attached to secondary or tertiary carbon atom, the alkyl halide is called secondary alkyl halide ( °) and tertiary ( °) alkyl halide, respectively.
(b) Allylic halides These are the compounds in which the halogen atom is bonded to an sp -hybridised carbon atom adjacent to carbon-carbon double bond (C=C) i.e. to an allylic carbon. (c) Benzylic halides These are the compounds in which the halogen atom is bonded to an sp -hybridised carbon atom attached to an aromatic ring.