A compound ' X′ absorbs 2 moles of hydrogen and ' X ' upon oxidation with KMnO₄ | H⁺ gives
The total number of σ bonds present in the compound ' X′ ' is __________.

Solution

Chemistry ma’am · 4 steps

Join the oxidation fragments back at their C=O ends, then count every single bond line in the molecule.

1. Read the fragments. Hot acidic KMnO₄ cuts each C=C. An R₂C= end becomes a ketone and an RCH= end becomes a carboxylic acid. Absorbing 2 mol of H₂ means X has two C=C bonds, so there are two cut points.
2. Rebuild X. Acetone came from (CH₃)₂C= and acetic acid from CH₃CH=. The middle piece HOOC-CH₂-CH₂-CO-CH₃ carries the other ends of both double bonds. Joining the pieces gives a C₁₀H₁₈ diene, for example:

   (CH₃)₂C=CH-CH₂-CH₂-C(CH₃)=CH-CH₃

3. Count σ bonds. X is an open chain with 10 C and 18 H, which is 28 atoms. An open-chain molecule has one σ bond for each link, so it has 28 - 1 = 27 σ bonds: 9 C–C σ bonds plus 18 C–H bonds. Each C=C adds a π bond on top, and that π bond is not counted.
4. So. Both ways of joining the middle fragment give the same C₁₀H₁₈ chain, so X has 27 σ bonds.

The common slip: 29. Counting each C=C as two bonds gives 29. A double bond has one σ bond and one π bond, so only one of the two counts.

The idea to keep. In an acyclic molecule, the number of σ bonds is the number of atoms minus 1. Each ring adds one more σ bond. Double and triple bonds still count as only one σ bond each.

Answer from the NTA final answer key. Solution by PrepWiser.