Haloalkanes and Haloarenes

After studying this Unit, you will be able to

Halogenated compounds persist in the environment due to their resistance to breakdown by soil bacteria.

The replacement of hydrogen atom(s) in an aliphatic or aromatic hydrocarbon by halogen atom(s) results in the formation of alkyl halide (haloalkane) and aryl halide (haloarene), respectively. Haloalkanes contain halogen atom(s) attached to the $s{p}^3$ hybridised carbon atom of an alkyl group whereas haloarenes contain halogen atom(s) attached to $s{p}^2$ hybridised carbon atom(s) of an aryl group. Many halogen containing organic compounds occur in nature and some of these are clinically useful. These classes of compounds find wide applications in industry as well as in day-to-day life. They are used as solvents for relatively non-polar compounds and as starting materials for the synthesis of wide range of organic compounds. Chlorine containing antibiotic, chloramphenicol, produced by microorganisms is very effective for the treatment of typhoid fever. Our body produces iodine containing hormone, thyroxine, the deficiency of which causes a disease called goiter. Synthetic halogen compounds, viz. chloroquine is used for the treatment of malaria; halothane is used as an anaesthetic during surgery. Certain fully fluorinated compounds are being considered as potential blood substitutes in surgery. In this Unit, you will study the important methods of preparation, physical and chemical properties and uses of organohalogen compounds.

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 (\ce{R-X})

In alkyl halides, the halogen atom is bonded to an alkyl group (R). They form a homologous series represented by $\text{ce}{C}_n{H}_{2n+1}X$. 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 $1^{\circ }$ alkyl halide. Similarly, if halogen is attached to secondary or tertiary carbon atom, the alkyl halide is called secondary alkyl halide ($2^{\circ }$) and tertiary ($3^{\circ }$) alkyl halide, respectively.

(b) Allylic halides

Allylic carbon

Allylic carbon

(c) Benzylic halides

These are the compounds in which the halogen atom is bonded to an $s{p}^3$-hybridised carbon atom attached to an aromatic ring.

This class includes:

(a) Vinylic halides

These are the compounds in which the halogen atom is bonded to a $s{p}^2$-hybridised carbon atom of a carbon-carbon double bond (\ce{C=C}).

(b) Aryl halides

These are the compounds in which the halogen atom is directly bonded to the $s{p}^2$-hybridised carbon atom of an aromatic ring.