In the following reaction, $$ CH_3-C \equiv CH \xrightarrow[873 K]{ ext{Red hot iron tube}} A $$ The number o

Question

In the following reaction, $$ CH_3-C \equiv CH \xrightarrow[873 K]{	ext{Red hot iron tube}} A $$ The number of sigma ($\sigma$) bonds present in the product $A$ is:

Accepted Answer

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 ($CH_3-C \equiv CH$) 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**).
   $$ 3 CH_3-C \equiv CH \xrightarrow{	ext{Red hot Fe tube}} 	ext{Mesitylene} $$
3. **Structure of Mesitylene ($C_9H_{12}$):**
   - It consists of a benzene ring ($C_6$) with three methyl groups ($-CH_3$) 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.
4. **Count Sigma ($\sigma$) 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 ($3 	imes 3$) = **12**.
   - **Total Sigma Bonds:** $9 + 12 = 21$.
5. **Conclusion:** The product contains 21 sigma bonds.

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