The compounds obtained by the substitution of hydrogen atom of alkanes by halogen atom are called haloalkane, while the compounds obtained by the substitution of hydrogen atoms of arenes by halogen atom are called haloarenes.
Modern classification of halo compounds is based on the halogen with carbon possessing sp3 hybridisation. In these compounds the electronegativity of halogen is more than that of carbon, hence Cδ+ – Xδ- bond becomes polar.
Haloalkanes are prepared from alkanes, alkenes or alcohols. The boiling points of haloalkane are higher than that of corresponding hydrocarbons.
Haloalkane undergoes nucleophilic substitution and elimination reactions. Primary alkyl halides undergo SN2 mechanism. If the reactant is chiral, the product formed exhibits inversion of stereo chemical configuration Tertiary alkyl halide undergoes SN1 mechanism, via carbonium ion formation. If the reactant is chiral, the product formed is optically inactive due to racemisation.
Organo metallic compound
Haloalkane reacts with metal to form organometallic compounds like Grignard reagent. It is represented as Rδ- - Mgδ+X. Grignard reagent reacts with variety of substances to give almost all class of organic compounds like alcohols, aldehydes, ketones, acids etc.
Haloarenes are prepared from arenes or by decomposition of benzene diazonium chloride. Haloarenes are more stable than haloalkane. C – X bond in halo arenes is short and strong.
Under normal conditions halo arenes do not undergo nucleophilic substitution but takes part in electrophilic substitution. Electron withdrawing inductive effect of halogen atom deactivates the benzene ring whereas resonating structure control o, p directing nature of halo arenes.
Poly halogen compounds
Organic compounds having two or more halogen atoms are called poly halogen compounds. These compounds are useful in our day to day life but pose environmental threat.
Chloroform is used as an anasthetic, but because of its toxic nature it has been replaced by less toxic and safer anaesthetic like ethers.
Iodoform is used as an antiseptic, due to the liberation of free iodine. But it has been replaced by other formulation containing iodine, due to its objectional smell.
Carbon tetrachloride is used in fire extinguishers. Freons are used as refrigerant. But both these compounds lead to adverse environmental effect.
DDT is used an effective insecticide. Now a days it is banned because of it's long term toxic effect.
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