The total number of C-C sigma bond/s in mesityl oxide (C₆H₁₀O) is __________. (Round off to the Nearest Integer).

Solution

Chemistry ma’am · 5 steps

Draw mesityl oxide first, then count each carbon-carbon link once, even the double one.

1. Write the structure. Mesityl oxide is 4-methylpent-3-en-2-one. It is an α,β-unsaturated ketone with a five-carbon chain and a methyl branch on C4.

   (CH₃)₂C=CH-CO-CH₃

2. List the C-C links. The links are C1–C2, C2–C3, C3=C4, C4–C5 and C4–CH₃ (the branch). That makes 5 carbon-carbon connections.
3. One sigma each. Every single bond is one σ bond. The C3=C4 double bond is one σ and one π, so it also adds just one σ. The C=O bond is not a C–C bond, so it does not count.
4. Quick check. An open-chain skeleton of n carbons always has n - 1 C–C σ bonds, whether it is straight or branched. Here n = 6, so the count is 6 - 1 = 5.
5. So. Mesityl oxide has 5 C–C σ bonds.

The common slip: 6. Counting the C=C double bond as two σ bonds gives 6. Its second bond is a π bond, so it adds only one σ.

The idea to keep. Any double or triple bond between two atoms holds exactly one σ bond. In an open-chain molecule with n carbons, the number of C–C σ bonds is n - 1.

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