Which of the following pairs of compounds belongs to the same homologous series?

Step 1: Analyze the question A homologous series is a family of organic compounds that share the same general formula and the same functional group. Successive members differ by a CH₂ unit.

Step 2: Recall the general formulas

• Alkanes: CₙH₂ₙ₊₂ (e.g., CH₄, C₂H₆, C₃H₈, C₄H₁₀)
• Alkenes: CₙH₂ₙ (e.g., C₂H₄, C₃H₆, C₄H₈, C₅H₁₀)
• Alkynes: CₙH₂ₙ₋₂

Step 3: Classify each pair

• A. C₃H₈ and C₃H₆: C₃H₈ is an alkane (2(3)+2 = 8) and C₃H₆ is an alkene (2(3) = 6). Different series. ✗
• B. C₄H₁₀ and C₅H₁₀: C₄H₁₀ is an alkane and C₅H₁₀ is an alkene. Different series. ✗
• C. C₂H₄ and C₄H₁₀: C₂H₄ is an alkene and C₄H₁₀ is an alkane. Different series. ✗
• D. C₂H₆ and C₄H₁₀: C₂H₆ (n=2: 2(2)+2 = 6) and C₄H₁₀ (n=4: 2(4)+2 = 10) both fit CₙH₂ₙ₊₂. Both are alkanes, and they differ by C₂H₄ (two CH₂ units). ✓

**Answer: D.

Tip: Quickly test each formula with the rule "H = 2C + 2" (alkane) or "H = 2C" (alkene). If both compounds in a pair pass the same test, they belong to the same homologous series. Ethane and butane both pass the alkane test, so they match.