Explain the oxidation of acetylene.

 

  1. No.4. Explain the oxidation of acetylene.

Answer. Acetylene is oxidized by alkaline KMn04. Hydroxyl groups add to the triple bond to form an intermediate product.

HC≡CH   +   2KMnO4   +   2H20   OH OH   +   2MnO2   +   2KOH

H-C—C-H

OH OH

 

O O

OH OH                         -2H2O        ║ ║

H-C—C-H                        H-C-C-H

OH OH                             Glyoxal

 

This intermediate product eliminates water molecules to from glyoxal, which on oxidation forms oxalic acid

O O        2[O]                 O O

OH OH                            -2H2O         ║ ║       KMnO4              ║ ║

H-C—C-H                             H-C-C-H                    HO-C-C-OH

OH OH                                  Glyoxal                     Oxalic acid

 

This reaction is also used to test the unsaturation in an organic compound.

 

  1. No.5. Write balanced chemical equations for the following reactions. Also name the products that formed.

 

(i) A mixture of ethyne and hydrogen is passed over heated nickel

Answer. When mixture of ethyne and hydrogen is passed over heated nickel at 250-300 °C ethane is formed.

                          

                        

                          Ni

      HC≡CH    +   H2   250-300 C     H2C=CH2

                               Ni

      H2C=CH2    +   H2   250-300 C     H3C-CH3

 

 (ii) Ethene is treated with chlorine

Answer. When ethene is treated with chlorine tetrachloroethane is formed.

      2CH≡CH + 2C12             Cl  Cl

                                 H-C―C-H

                                   Cl  Cl

 

(iii) Ethene is burnt in air

Answer. When ethene is burnt in the presence of excess of air or oxygen carbon dioxide, water and a lot of heat are produced.

 

2CH=CH+ 5O2 ———> 4CO2+ 2H20 + heat

 

 (iv). Ethene is passed through bromine water

Answer.     When    ethene    is    passed    through    bromine    water tetrabromoethane is formed.

 

 

Br    Br

HC≡CH +   2Br2   →   H-C — C-H

Br    Br

                         Tetrabromoethane

 

  1. No.6. Explain briefly.

(i) Why butane undergoes substitution reactions?

Answer.   Alkanes   are   least   reactive   compound   being   saturated hydrocarbons they undergo substitution reactions.

 

diffused sunlight

CH3-CH2-CH2-CH3 + C12                   CH3-CH2-CH2-CH2-C1 + HC1

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