Counting σ bonds in the alkane analog of the 6-carbon chain Steps:

• Determine the number of carbons: the formula HCCCH2CHCH3 indicates a 6-carbon chain.
• Calculate C-C σ bonds: a straight-chain molecule has 5 C-C σ bonds.
• Determine C-H σ bonds: the saturated alkane C6H14 has 14 C-H σ bonds.
• Add them: total σ bonds = 5 (C-C) + 14 (C-H) = 19.

Why D is correct:

• For an n-carbon alkane, the formula for σ bonds is 3n + 1; for n=6, 3(6) + 1 = 19, accounting for all single bonds in the skeleton.

Why the others are wrong:

• A: 17σ counts C-H bonds for an alkene (C6H12) with one double bond, underestimating hydrogens.
• B: 15σ counts C-H bonds for an alkyne (C6H10) with one triple bond, further underestimating hydrogens.
• C: 18σ miscounts by adding an extra bond without basis in the structure.

Final answer: D