Question

Hydrocarbon (A) reacts with bromine by substitution to form an alkyl bromide which by Wurtz reaction is converted to gaseous hydrocarbon containing less than four carbon atoms. (A) is

• A. CH ≡ CH
• B. CH₃ - CH₃
• C. CH₂ = CH₂
• D. CH₄

Answer 1

The Correct Option is D

To determine which hydrocarbon (A) reacts with bromine by substitution to form an alkyl bromide and subsequently undergoes a Wurtz reaction to form a gaseous hydrocarbon with less than four carbon atoms, let's analyze each option step-by-step.

1. We have four hydrocarbon options:
   • CH ≡ CH (Ethyne)
   • CH₃ - CH₃ (Ethane)
   • CH₂ = CH₂ (Ethene)
   • CH₄ (Methane)
2. The reaction with bromine by substitution generally occurs with saturated hydrocarbons (alkanes) and not unsaturated hydrocarbons (alkenes or alkynes). This narrows our possible candidates to ethane (CH₃-CH₃) and methane (CH₄).
3. In the Wurtz reaction, two molecules of an alkyl halide are reacted with sodium metal to form a higher alkane. This reaction is primarily applicable to alkyl halides derived from simple alkanes.
4. Let's evaluate the substitution and Wurtz reactions for ethane and methane:
   • Ethane (CH₃-CH₃):
     • Substitution: CH₃-CH₃ + Br₂ ➞ CH₃-CH₂Br + HBr
     • Wurtz Reaction: CH₃-CH₂Br + 2Na + BrCH₂-CH₃ ➞ CH₃-CH₂-CH₂-CH₃ + 2NaBr
     • The product is butane, which does not fit the criteria of less than four carbon atoms, ruling out ethane.
   • Methane (CH₄):
     • Substitution: CH₄ + Br₂ ➞ CH₃Br + HBr
     • Wurtz Reaction: CH₃Br + 2Na + BrCH₃ ➞ CH₃-CH₃ + 2NaBr
     • The product is ethane, a gaseous hydrocarbon with less than four carbon atoms.
5. Following the above analysis, the hydrocarbon (A) is methane (CH₄). This is because methane can undergo substitution to form bromoethane, which during the Wurtz reaction forms ethane, meeting the criteria of a gaseous hydrocarbon with less than four carbon atoms.

The correct answer is therefore CH₄ (Methane).