Which of the following could not be alkane?

Analyze the question: We need to find the formula that cannot belong to an alkane.

Concept: Alkanes are saturated hydrocarbons with only single bonds. Their general formula is CₙH₂ₙ₊₂.

Solution: Apply the formula to each option:

• A. C₄H₁₀: n = 4, so 2(4) + 2 = 10 H. ✔ Alkane (butane)
• B. C₅H₁₂: n = 5, so 2(5) + 2 = 12 H. ✔ Alkane (pentane)
• C. C₇H₁₄: n = 7, so 2(7) + 2 = 16 H, but the formula has only 14 H. ✘ Not an alkane. It fits CₙH₂ₙ, the general formula for alkenes (heptene) or cycloalkanes (cycloheptane).
• D. C₈H₁₈: n = 8, so 2(8) + 2 = 18 H. ✔ Alkane (octane)

Answer: C

Tip: For an alkane, the number of hydrogens must be twice the carbons plus 2. If H = 2n, think alkene or cycloalkane. If H = 2n − 2, think alkyne. You can check any option in a few seconds this way.