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Organic Chemistry-II 1. Occurrence of methane gas : 1. Methane occurs as marsh gas in low wetlands. 2. 70% of natural gas contains methane. 3. 25% 40% of coal gas (found in coal mines) contains methane. 4. Gobar gas; sewage gas; biogas obtained by the microbial decomposition of cattle dung; human excreta; garbage, etc., contain a large amount of methane gas. 2. Laboratory preparation of methane gas : 1. It is prepared by heating a mixture of soda lime and sodium acetate. CaO CH3COONa + NaOH CH4 + Na2CO3 heat 2. It is prepared by the action of water on aluminium carbide. Al4C3 + 12H2O 4Al(OH)3 + 3CH4 3. Physical properties of methane : 1. It is a colourless; odourless and tasteless gas, which is insoluble in water. 2. It is lighter than air, its V.D. being 8 and that of air being 14.4. 3. The methane molecule reflects radiant heat 20 times more than carbon dioxide. Thus, it contributes a lot in the greenhouse effect. 4. Chemical properties of methane : 1. Methane burns in air or oxygen with a pale blue flame, forming carbon dioxide and water. However, if the supply of oxygen is limited, it forms carbon monoxide and water. 2. Methane reacts with chlorine in the presence of sunlight to form substitution products like CH3Cl; CH2Cl2; CHCl3 and CCl4. 3. When methane is heated to a temperature of 1000 C, it decomposes to form carbon and hydrogen. The process of decomposition of organic compound into elements, on heating is called pyrolysis. 1 Key Points 5. Laboratory preparation of ethane : It is prepared in the laboratory by heating a mixture of sodium propanoate with soda lime. CaO C2H5COONa + NaOH C2H6 + Na2CO3 6. Physical properties of ethane : It is a colourless, odourless, tasteless gas, insoluble in water and slightly heavier than air. 7. Chemical properties of ethane : 1. It burns in excess of oxygen or air with a pale blue flame, forming carbon dioxide, water and liberating large amount of heat. However, in the insufficient supply of air, it forms carbon monoxide. 2. Substitution reactions take place when ethane and chlorine are exposed to sunlight with the formation of compounds like ethane monochloride (C2H5Cl); ethane dichloride (C2H4Cl2); ethane trichloride (C2H3Cl3); ethane tetrachloride (C2H2Cl4); ethane pentachloride (C2HCl5) and ethane hexachloride (CCl6). 8. Laboratory preparation of ethene gas (ethylene gas) : It is prepared by dehydrating ethyl alcohol with conc. sulphuric acid. A mixture of one part of ethyl alcohol, two parts of conc. sulphuric acid and aluminium sulphate is heated to 160 C, when the ethylene gas is evolved. Aluminium sulphate prevents frothing of reaction mixture. CH3 CH2 OH + H2SO4 CH3 CH2 HSO4 + H2O 160 C CH3 CH2.HSO4 C2H4 + H2SO4 9. Physical properties of ethylene gas : (i) It is a colourless gas, having a peculiar smell. (ii) It is insoluble in water, but soluble in organic solvents such as chloroform, carbon tetra-chloride, ether, etc. (iii) It is slightly lighter than air, its V.D. being 14 and that of air being 14.4. 2 Key Points 10. Chemical properties of ethylene gas : (i) It burns in air with a sooty flame. However, in excess of air or oxygen it burns with a pale blue flame, forming large amount of heat, carbon dioxide gas and water. (ii) It being an unsaturated hydrocarbon, forms additive compounds with chlorine and bromine. H2C = CH2 + Cl2 ClH2C CH2Cl (ethylene dichloride) H2C = CH2 + Br2 BrH2C CH2Br (ethylene dibromide). (iii) Alkaline potassium permanganate solution oxidises ethylene to ethylene glycol. CH2OH CH2 + 2 MnO2 + 2KOH 3 | | + 2KMnO4 + 4H2O 3 | CH2 CH2OH (iv) Ethylene reacts with HCl to form ethane monochloride. CH2 = CH2 + HCl CH3 CH2 Cl (v) The process of forming a molecule of higher molecular mass, by combining similar molecules of low molecular mass is called polymerization. Ethylene forms polyethylene when treated catalytically. polymerization n(CH2 = CH2) ( CH2 CH2 )n 11. Laboratory preparation of acetylene gas (ethyne gas) : When calcium carbide is treated with water, it forms calcium hydroxide, with the liberation of acetylene. CaC2 + 2H2O Ca(OH)2 + C2H2 12. Physical properties of acetylene : (i) It is a colourless gas, and has an odour like garlic. (ii) It is insoluble in water, but soluble in industrial organic solvents. (iii) It is lighter than air, its V.D. is 13 and that of air is 14.4. 13. Chemical properties of acetylene : (i) It burns with a sooty flame in air. However, if there is excess of supply of air, it burns with brilliant white flame, producing large amount of light. 3 Key Points (ii) It forms additive compounds with chlorine; bromine and HCl gas. CH CH + Cl2 CHCl = CHCl CHCl = CHCl + Cl2 CHCl2 CHCl2 (iii) It is oxidised to oxalic acid by alkaline potassium permanganate solution. COOH Alkaline CH = CH + 4[O] | potassium permanganate COOH (iv) It forms reddish-brown ppt. of copper acetylide when treated with ammonical cuprous chloride solution. CH CCu CH CCu ||| + 2CuCl + 2NH 4 OH ||| + 2NH 4 Cl + 2H 2 O (v) When passed through red hot tube, it gets polymerized to benzene. red heat 3CH CH C6H6 4 Key Points

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