Hydrocarbons (Organic Chemistry)

Total Questions: 23

21. Consider the following statements about acetylene: [I.A.S. (Pre) 1998]

  1. It is used in welding industry.
  2. It is a raw material for preparing plastics.
  3. It is easily obtained by mixing silicon carbide and water.

Of these statements:

Correct Answer: (a) 1 and 2 are correct
Solution:Acetylene is also called as Ethyne (C₂H₂). It is used in welding. Chloroethane is produced by acetylene which is used as raw material in the plastic industry. The mixture of calcium carbide (not silicon carbide) and water produces acetylene.

22. For benzene which of the following statements is true? [Chhattisgarh P.C.S. (Pre) 2014]

Correct Answer: (c) It has twelve sigma and three pi bonds
Solution:Benzene () has a cyclic structure with alternating single and double bonds (though it's delocalized).
  • Sigma () bonds: There are 6 C-C single bonds within the ring and 6 C-H single bonds (one attached to each carbon). Each single bond is a sigma bond. So, sigma bonds.
  • Pi () bonds: The three double bonds in the resonance structure contribute one pi bond each. Due to delocalization, these three pi bonds form a delocalized pi system above and below the ring. So, there are 3 pi bonds. Therefore, benzene has 12 sigma bonds and 3 pi bonds.

23. Numbers of sigma and pi bonds in benzene are: [66th B.P.S.C. (Pre) 2020]

Correct Answer: (c) 12, 3
Solution:Benzene () has a cyclic structure with alternating single and double bonds (though it's delocalized).
  • Sigma () bonds: There are 6 C-C single bonds within the ring and 6 C-H single bonds (one attached to each carbon). Each single bond is a sigma bond. So, sigma bonds.
  • Pi () bonds: The three double bonds in the resonance structure contribute one pi bond each. Due to delocalization, these three pi bonds form a delocalized pi system above and below the ring. So, there are 3 pi bonds. Therefore, benzene has 12 sigma bonds and 3 pi bonds.