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NEET CHEMISTRYHydrocarbonsMedium

Question

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

A

CH≡CHCH \equiv CH

B

CH2=CH2CH_2 = CH_2

C

CH3−CH3CH_3 - CH_3

D

CH4CH_4

Step-by-Step Solution

  1. Analyze Reaction 1 (Substitution): The hydrocarbon (A) reacts with bromine via a substitution reaction. This indicates that (A) is a saturated hydrocarbon (Alkane), as unsaturated hydrocarbons (Alkenes/Alkynes) typically undergo addition reactions with halogens [NCERT 11th, Sec 9.2.3]. This eliminates Options A (CH≡CHCH \equiv CH) and B (CH2=CH2CH_2=CH_2).
  2. Analyze Reaction 2 (Wurtz Reaction): The alkyl bromide formed reacts with sodium (Wurtz reaction) to double its carbon chain. 2R−Br+2Na→dry etherR−R+2NaBr2R-Br + 2Na \xrightarrow{\text{dry ether}} R-R + 2NaBr
  3. Analyze Product Constraints: The final product is a gaseous hydrocarbon with less than four carbon atoms.
  • Case 1: If (A) is Methane (CH4CH_4):
  • Bromination: CH4+Br2→hνCH3BrCH_4 + Br_2 \xrightarrow{h\nu} CH_3Br (Methyl bromide).
  • Wurtz Reaction: 2CH3Br+2Na→CH3−CH32CH_3Br + 2Na \rightarrow CH_3-CH_3 (Ethane).
  • Ethane (C2H6C_2H_6) is a gas and has 2 carbon atoms (which is less than 4). This fits the criteria.
  • Case 2: If (A) is Ethane (CH3−CH3CH_3-CH_3):
  • Bromination: C2H6+Br2→C2H5BrC_2H_6 + Br_2 \rightarrow C_2H_5Br (Ethyl bromide).
  • Wurtz Reaction: 2C2H5Br+2Na→C4H102C_2H_5Br + 2Na \rightarrow C_4H_{10} (Butane).
  • Butane (C4H10C_4H_{10}) has 4 carbon atoms. The condition requires less than four. This does not fit.
  1. Conclusion: The starting hydrocarbon (A) must be Methane (CH4CH_4).

Exam Context & Concepts Covered

This question aligns with the NEET CHEMISTRY syllabus, specifically targeting concepts from Hydrocarbons. Mastering this topic is crucial for scoring well in the upcoming medical entrance examinations. Solving conceptually related problems will help you understand the nuances of these concepts and improve your problem-solving speed.

CHEMISTRYHydrocarbonshydrocarbonreactsbrominesubstitutionbromide

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