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NEET CHEMISTRYHydrocarbonsMedium

Question

In the following reaction, CH3−C≡CH→873KRed hot iron tubeACH_3-C \equiv CH \xrightarrow[873 K]{\text{Red hot iron tube}} A The number of sigma (σ\sigma) bonds present in the product AA is:

A

21

B

9

C

24

D

18

Step-by-Step Solution

  1. Identify the Reaction: The reaction of an alkyne passed through a red-hot iron tube at 873 K undergoes cyclic polymerisation. While NCERT explicitly details the polymerisation of ethyne to benzene [NCERT 11th, Ch 13, Sec 13.5.4; Source 118, 300], the polymerisation of propyne (CH3−C≡CHCH_3-C \equiv CH) follows the same pattern to form a trisubstituted benzene derivative.
  2. Identify Product A: Three molecules of propyne polymerise to form 1,3,5-trimethylbenzene (commonly known as Mesitylene). 3CH3−C≡CH→Red hot Fe tubeMesitylene3 CH_3-C \equiv CH \xrightarrow{\text{Red hot Fe tube}} \text{Mesitylene}
  3. Structure of Mesitylene (C9H12C_9H_{12}):
  • It consists of a benzene ring (C6C_6) with three methyl groups (−CH3-CH_3) attached at positions 1, 3, and 5.
  • Carbon atoms: 6 in the ring + 3 in methyl groups = 9 carbons.
  • Hydrogen atoms: 3 on the ring + 9 in methyl groups = 12 hydrogens.
  1. Count Sigma (σ\sigma) Bonds:
  • C-C sigma bonds: 6 within the benzene ring + 3 connecting methyl groups to the ring = 9.
  • C-H sigma bonds: 3 on the benzene ring (positions 2, 4, 6) + 9 on the three methyl groups (3×33 \times 3) = 12.
  • Total Sigma Bonds: 9+12=219 + 12 = 21.
  1. Conclusion: The product contains 21 sigma bonds.

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.

CHEMISTRYHydrocarbonsfollowingreactionxrightarrowktextrednumber

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