Counting σ Bonds in HC≡CCH₂CH₃

Steps:

• Identify the structure: HC≡C-CH₂-CH₃ (1-butyne, C₄H₆) with a triple bond between C1-C2, single bonds C2-C3 and C3-C4.
• Count C-C σ bonds: 3 (one per C-C connection, regardless of multiplicity).
• Count C-H σ bonds: 6 (1 on C1, 2 on C3, 3 on C4).
• Total σ bonds: 3 (C-C) + 6 (C-H) = 9.

Why D is correct:

• In organic molecules, each atom-atom connection contributes one σ bond; for C₄H₆, 3 C-C + 6 C-H = 9 σ bonds by the definition of σ bonds as the skeletal framework.

Why the others are wrong:

• A: Overcounts, perhaps including π bonds or assuming a larger alkane like pentane (16 σ).
• B: Underestimates for this molecule; matches σ in propene (C₃H₆) but not here.
• C: Too high, typical for saturated C₅ (16 σ) or miscounting hydrogens.

Final answer: D