Question

The number of different substitution products formed when ethane is allowed to react with bromine in sunlight is:
1. 9
2. 6
3. 8
4. 5

Answer 1

Correct Answer - Option 2 : 6

Concept:

• Alkanes react with halogens in presence of Ultraviolet light or at temperatures between 520 - 670K.
• For example, when methane is treated with chlorine in diffused sunlight, a mixture of four different derivatives is obtained.
• The reaction takes place as follows:
  CH4 + Cl2 → CH3Cl + HCl
  CH3Cl + Cl2 → CH2Cl2 + HCl
  CH2Cl2 + Cl2 → CHCl3 + HCl
  CHCl3 + Cl2 → CCl4 + HCl
• Similarly, Fluorination and Bromination take place.

Explanation:

• ​The reaction of bromine with ethane takes place as follows:
  C₂H₆ + Br₂ → C₂H₅Br + HBr
  C2H5Br + Br2 → C₂H₄Br₂ + HBr
  C2H4Br₂ + Br2 → C2H₃Br₃ + HBr
  C2H3Br3 +  Br2 → C2H2Br₄ + HBr
  C2H2Br4 + Br2 → C2HBr5 + HBr
  C2HBr5 + Br2 → C₂Br₆ + HBr
• The number of products formed are thus Six.

• The reaction takes place via a free radical mechanism.
• The rate of reactivity of alkanes towards halogenation re tertiary > secondary> primary.
• The reactivity of halogens towards the same alkane is: Fluorination > chlorination > bromination.