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NEET CHEMISTRYMedium

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.
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