the products obtained by addition reactions of HBr to hex- -ene (i) in the absence of peroxide and (ii) in the presence of peroxide. . Addition of sulphuric acid : Cold concentrated sulphuric acid adds to alkenes in accordance with Markovnikov rule to form alkyl hydrogen sulphate by the electrophilic addition reaction. (i) ( .
) ( . ) . Addition of water : In the presence of a few drops of concentrated sulphuric acid alkenes react with water to form alcohols, in accordance with the Markovnikov rule. ( .
) . Oxidation: Alkenes on reaction with cold, dilute, aqueous solution of potassium permanganate (Baeyer’s reagent) produce vicinal glycols. Decolorisation of KMnO solution is used as a test for unsaturation. ( .
) ( . ) b) Acidic potassium permanganate or acidic potassium dichromate oxidises alkenes to ketones and/or acids depending upon the nature of the alkene and the experimental conditions ( . ) CH – CH=CH–CH KMnO /H 2CH COOH But- -ene Ethanoic acid ( . ) .
Ozonolysis : Ozonolysis of alkenes involves the addition of ozone molecule to alkene to form ozonide, and then cleavage of the ozonide by Zn-H O to smaller molecules. This reaction is highly useful in detecting the position of the double bond in alkenes or other unsaturated compounds. ( . ) ( .
) . Polymerisation: You are familiar with polythene bags and polythene sheets. Polythene is obtained by the combination of large number of ethene molecules at high temperature, high pressure and in the presence of a catalyst. The large molecules thus obtained are called polymers.
This reaction is known as polymerisation. The simple compounds from which polymers are made are called monomers . Other alkenes also undergo polymerisation. n(CH =CH ) High temp./pressure Catalyst —( CH –CH )— Polythene ( .
) n(CH –CH=CH ) High temp./pressure Catalyst —( CH–CH )— n Polypropene ( . ) Polymers are used for the manufacture of plastic bags, squeeze bottles, refrigerator dishes, toys, pipes, radio and T.V. cabinets etc. Polypropene is used for the manufacture of milk crates, plastic buckets and other moulded articles.
Though these materials have now become common, excessive