- No.7. How can you prepare ethene from alcohol and ethyl bromide?
Answer. Ethene is prepared by heating a mixture of ethanol and excess of concentrated sulphuric acid at 180°C. In first step, ethyl hydrogensulphate is formed which decomposes on heating to produce ethene, which is collected over water.
CH3-CH2OH + H2S04 ——> CH3CH2-OSO3H + H20
CH3-CH2OSO3H ———> H2C=CH2 + H2SO4
On heating ethyl bromide with alcoholic KOH, ethene is formed. Removal of hydrogen and halogen from adjacent carbon atoms forms double bond. This method is called dehydrohalogenation, of alkyl halides.
H3C-CH2-Br + KOH (alcoholic) ———> H2OCH2 + KBr + H2O
- No.8. Distinguish between propane and propene with a chemical test
Answer. Propene does not react with alkaline KMnO4 solution.
CH3-CH=CH2 + KMnO4 + H2O ———> No reaction
While alkenes decolorize the pink colour of acidified dilute solution of potassium permanganate. Propene decolorizes the pink colour of KMnO4 solution and forms propene glycol that is colorless compound.
3CH3-CH=CH2+ 2KMn04 + 4H2O
3GH3-HC (OH)-CH2 (OH) + 2MnO2 + 2KOH
This reaction is also used to test the unsaturation of an organic compound.
- No.9. Why the alkenes are called ‘olefins’?
Answer. Olefin is derived from Latin word meaning oil forming. Lower members of alkenes form oily products when react with halogens therefore, alkenes are called olefins.
- No.10. Why alkane can’t be oxidized with KMnO4 solution?
Answer. Alkane has no double or triple bond. It has only sigma bond that is not broken by alkaline KMnO4 solution. So alkane cannot be oxidized by KMnO4 solution.
- No.11. What are the addition reactions? Explain with an example.
Answer. Reactions in which two or more substances combine to form a big molecule are called addition reactions. Addition of hydrogen in alkenes and alkynes is called addition reaction. Alkenes and alkynes are unsaturated compound, so they have the capacity to add up atoms in them. This reaction is carried out in the presence of nickel catalyst at 250 °C to 300 °C. However, in the presence of catalyst platinum or palladium, the reaction takes place at room temperature,
H2C=CH2 + H2 250-300 C H3C-CH3