UNIT 10 HALOALKENES AND HALOARENES
- Alkyl halides 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.
- Since halogen atoms are more electronegative then carbon, the carbon-halogen bond of alkyl halide is polarized; the carbon atom bears a partial positive charge, and the halogen atom bears a partial negative charge.
- Alkyl halides are prepared by the free radical halogenation of alkenes, addition of halogen acids to alkenes, replacement of –OH group of alcohols with halogens using phosphorus halides, thionyl chloride or halogen acids.
- Aryl halides are prepared by electrophilic substitution to arenes. Fluorides and iodides are best prepared by halogen exchange method.
- The boiling points of organohalogen compounds are comparatively higher than the corresponding hydrocarbons because of strong dipole-dipole and van der Waals forces of attraction. These are slightly soluble in water but completely soluble in organic solvents.
- The polarity of carbon-halogen bond of alkyl halides is responsible for their nucleophilic substitution, elimination and their reaction with metal atoms to form organometallic compounds.
- Nucleophilic substitution reactions are categorized into SN1 and SN2 on the basis of their kinetic properties. Chirality has a profound role in understanding the reaction mechanisms of SN1 and SN2 reactions. SN2 reactions of chiral alkyl halides are characterized by the inversion of configuration while SN1 reactions are characterized by racemization.
- A number of polyhalogen compounds e.g., dichloromethane, chloroform, iodoform, carbon tetrachloride, freon and DDT have many industrial applications. However, some of these compounds cannot be easily decomposed and even cause depletion of ozone layer and are posing serious risks to the environment.