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Question Bank, Organic Chemistry II, 1. What are saturated and unsaturated hydrocarbons. Classify the, following as saturated and unsaturated hydrocarbons., CH4 , C2H2 , C2H6 , C3H6 , C3H4, Ans. Compounds of hydrogen and carbon in which all the valencies of, carbon atoms in the main chain are satisfied by a single covalent, bond are called saturated hydrocarbons., Compounds of hydrogen and carbon in which there are double or, triple covalent bond between two carbon atoms in the main carbon, chain are called unsaturated hydrocarbons., Saturated hydrocarbons : CH4 and C2H6, Unsaturated hydrocarbons : C2H4, C2H2, C3H6 and C4H8, 2. (a) How is methane gas prepared in the laboratory?, [3], (b) What do you understand by the term substitution reaction?, [1], (c) How does chlorine in excess react with methane?, [4], Ans. (a) Laboratory preparation of methane gas :, 1. It is prepared by heating a mixture of soda lime and sodium, acetate., CaO, → CH4 + Na2CO3, CH3COONa + NaOH ⎯⎯⎯, heat, 2. It is prepared by the action of water on aluminium carbide., Al4C3 + 12H2O —→ 4Al(OH)3 + 3CH4, (b) A reaction in which hydrogen atoms of a hydrocarbon are, replaced by more reactive elements such as chlorine, bromine,, etc., is called a substitution reaction., (c) Reaction of chlorine with methane., Sunlight, HCl + CH3Cl (Monochloromethane), CH4 + Cl2 ⎯⎯⎯→, Sunlight, HCl + CH2Cl2 (Dichloromethane), CH3Cl + Cl2 ⎯⎯⎯→, Sunlight, HCl + CHCl3 (Chloroform), CH2Cl2 + Cl2 ⎯⎯⎯→, Sunlight, CHCl3 + Cl2 ⎯⎯⎯→ HCl + CCl4 (Carbon tetrachloride), Chemistry Class-X, , 1, , Question Bank
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8. How will you bring about the following conversions?, (i) Methane to methyl alcohol (ii) Methane to formic acid, (iii) Ethane to ethyl alcohol (iv) Ethane to acetaldehyde., [2 each], Ans. (i) Conversion of methane into methyl alcohol :, (a) When equal volumes of methane and chlorine are exposed to, sunlight, then methyl chloride is formed., Sunlight, CH3Cl + HCl, CH4 + Cl2 ⎯⎯⎯→, (b) When methyl chloride is treated with potassium hydroxide, solution, hydrolysis takes place, leading to the formation of, methyl alcohol., CH3Cl + KOH —→ CH3OH + KCl, (ii) Conversion of methane into formic acid : First of all, methane, is converted to methyl alcohol, as described above. When, methyl alcohol is treated with acidified potassium dichromate, solution, it is oxidised to formic acid., Acidified, → HCOOH + H2O, CH3OH + 2[O] ⎯⎯⎯⎯, K 2Cr2O7, (iii) Conversion of ethane to ethyl alcohol :, (i) Ethane gas treated with chlorine in the presence of, sunlight to form monochloro ethane., Sunlight, C2H5Cl + HCl, C2H6 + Cl2 ⎯⎯⎯→, (ii) Monochloroethane is hydrolysed with KOH to form, ethyl alcohol., C2H5Cl + KOH —→ C2H5OH + KCl, (iv) Conversion of ethane into acetaldehyde :, (i) Ethane is converted into ethyl alcohol as described above., (ii) When the vapours of ethyl alcohol are passed over copper, oxide at 300°C, then they are oxidised to acetaldehyde., C2H5OH + CuO —→ CH3CHO + H2O + Cu, , Chemistry Class-X, , 3, , Question Bank
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9. Give two uses of (i) methane and (ii) ethane., [2], Ans. (i) Uses of methane :, 1. It is used as a household fuel. In liquefied form, it is used, as a fuel for automobiles., 2. It is used in the preparation of hydrogen on industrial scale, by the process of pyrolysis., 3. It is used in the manufacture of industrial compounds such, as methyl alcohol, formic acid, chloroform, carbon, tetrachloride, etc., (ii) Uses of ethane :, (i) It is used as a gaseous fuel. Liquefied ethane is used as a, fuel in automobiles., (ii) It is used in the manufacture of hydrogen by pyrolysis., (iii) It is used in the manufacture of important organic, compounds such as ethyl alcohol, acetaldehyde, acetic, acid, etc., 10. How will you prepare ethene gas in the laboratory? Support your, answer by chemical equations., [3], Ans. 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 (conc.) —→ CH3—CH2 —HSO4 + H2O, 160ºC, CH3—CH2.HSO4 ⎯⎯⎯, → C2H4 + H2SO4, 11. What do you understand by the term pyrolysis. Write chemical, equations for the pyrolysis of, (i) methane (ii) ethane., Ans. Thermal or catalytic decomposition of alkanes in the absence of air is, called pyrolysis., 1500ºC, → HC ≡ CH + 3H2, (i) 2CH4 ⎯⎯⎯, Al2CO3 – 500ºC, (ii) C2H6 ⎯⎯⎯⎯⎯, → H2C = CH2 + H2, , Chemistry Class-X, , 4, , Question Bank
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17. How will you distinguish between alkanes, alkenes and alkynes?, Ans. Distinguishing Tests for Alkanes, Alkenes and Alkynes :, S. No., , Test, , 1., , Bromine Test. Add a few, drops of sol. of bromine in, carbon tetra-chloride to the, hydrocarbon, Alkaline Potassium, Permanganate Test. Add, a few drops of alkaline pot., permanganate sol. To the, hydrocarbon, Ammoniacal Cuprous, Chloride Test., Add a few drops of, ammoniacal, cuprous chloride sol. to, the hydrocarbon., , 2., , 3., , Alkanes, No change, takes place., , No, change, takes, place., No change, takes place., , Alkenes, , [3], , Alkynes, , The red colour, of bromine is, decolorised., , The red, colour of, bromine is, decolorised., The purple, The purple, colour of, colour of, potassium, Potassium, permanganate is permanganate, is decolorised., decolorised., No change takes A red ppt. of, place., copper, acetylide is, formed., , 18. How does ethane gas react with the following :, (i) Hydrogen, (ii) Halogen acid, (iii) sulphuric acid, (iv) Bromine., Write chemical equation and experimental conditions., [8], Ans. (i) When a mixture of ethene and hydrogen are passed over heated, catalyst (Ni, Pd or Pt), an addition reaction takes place with the, formation of ethane, a saturated hydrocarbon., , (ii) When vapours of ethene and hydrobromic acid are mixed at room, temperature, they react to form addition product, bromoethane., CH2 H, CH3, →|, || + | ⎯⎯, CH2 Br, CH2 Br, , Chemistry Class-X, , 7, , Question Bank
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(iii) When ethene is passed through conc. sulphuric acid, an addition, reaction takes place at room temperature with the formation of, ethyl hydrogen sulphate., CH2 H, CH3, || + | ⎯⎯, →|, CH2 HSO4 CH2 HSO4, , (iv) When bromine is passed through the inert solution of ethene,, an addition reaction takes place with the formation of 1, 2,, dibromoethane., CH2 Br, CH2 Br, CCl4, |, ||, + | ⎯⎯⎯→, inert sol, CH2 Br, CH2 Br, , 19., , What do you understand by the term polymerization? Write, chamical equation in support of your answer., [3], Ans. The process by which simple organic molecules, under high, temperature and pressure and in the presence of a catalyst form a, large molecule, the process is called polymerization. Ethene under, high temperature and pressure and in the presence of sulphuric acid, vapour polymerizes to form polyethene., high temp and pressure, → – [H2 C– CH2]2 –, n[ CH2 = CH2] ⎯⎯⎯⎯⎯⎯⎯, catalyst, 20. How can you obtain ethyne gas from 1, 2 dibromoethane? Explain,, why this reaction is called dehydro halogenation reaction?, Ans. When 1, 2 - dibromoethane is boiled with conc. alcoholic potassium, hydroxide, ethyne gas is evolved., , The above reaction is called dehydro halogenation reaction, because, the atoms of hydrogen as well as bromine are removed from its, composition., , Chemistry Class-X, , 8, , Question Bank
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21. How does ethyne reacts with the following., (i) Hydrogen (ii) Bromine (iii) Hydrobromic acid, (iv) ammonical silver nitrate., Write equations and experimental conditions., [8], Ans. (i) When a mixture of ethyne and hydrogen is passed over heated, catalyst [Ni, Pb or Pt], at 300°C, substitution reaction takes, place with the formation of ethane., , (ii) When bromine is passed through neutral solution of ethyne, in carbon tetrachloride, addition reaction takes place with the, formation 1, 1, 2, 2 tetrabromoethane., , (iii) When ethyne gas is passed through hydrobromic acid in the, presence of sunlight, addition reaction takes place with the, formation of 1, 2, dibromoethane., , (iv) When ethyne gas is passed through ammonical silver nitrate,, it forms white ppt. of silver acetylide., , Chemistry Class-X, , 9, , Question Bank
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22. State four uses of acetylene gas., Ans. (i) In the form of oxy-acetylene flame, it is used for cutting and, welding purposes., (ii) It is used in the artificial ripening of fruits., (iii) It is used in the manufacture of important organic compounds,, such as acetic acid, acetaldehyde, ethyl alcohol etc., (iv) It is used in the manufacture of synthetic polymers, [polyvinyl acetate], synthetic rubber and fibres., 23. Name the gas evolved and write chemical equations when ethanoic, acid is treated wtih (i) Zinc metal (ii) sodium carbonate., [2 + 1], Ans. (i) Hydrogen gas is evolved., → (CH3COO)2Zn + H2, 2CH3COOH (dil) + Zn ⎯⎯, (ii) Carbon dioxide gas is evolved., → 2CH3COONa + H2O + CO2, 2CH3COOH (dil.) + Na2CO3 ⎯⎯, , Chemistry Class-X, , 10, , Question Bank
