Properties of alkenes
The first three alkenes namely ethene, propene and butene are gases at ordinary temperature. The alkenes containing 5 to 18 carbon atoms are liquids while the higher members of the family are solids. All the alkenes are colourless and odourless except ethene. Ethene is a colourless gas with a faint sweet smell. Alkenes consist of pi-electrons forming an electron cloud, which lies above and below the plane of sigma–bonded carbon atoms. Alkenes show addition reactions, oxidation reactions and polymerisation reactions. Hydrogenation of edible vegetable oils in the presence of finely divided nickel is used in the manufacturing of vanaspati ghee. Addition reactions of HBr to symmetrical alkenes take place by electrophilic addition mechanism while addition reaction of HBr to unsymmetrical alkenes takes place by Markovnikov Rule. Markovnikov rule states that in case of addition of an unsymmetrical reagent of the type HX to an unsymmetrical alkene, its negative part goes to the carbon atom carrying smaller number of hydrogen atoms and its positive part gets attached to the carbon atom carrying larger number of hydrogen atoms.
In the presence of peroxide, addition of HBr to unsymmetrical alkenes like propene takes place contrary to the Markovnikov rule. It is also known as peroxide effect or Kharash effect. When alkenes are treated with cold and dilute aqueous or alkaline solution of potassium permanganate, two hydroxyl groups are added across double bond and 1,2 diols known as glycols are obtained. It is known as Baeyer’s test. Ethene, Chloroethene and Tetrafluoroethene undergo polymerisation reaction to form polythene, polyvinyl chloride (PVC) and Teflon respectively. Polythene thus obtained is widely used as a packaging material and as insulation for electric wires and cables.
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