Which of the following pairs of compounds are members of the same homologous series?  

The correct answer is Option 1.

Key Points

• C₉H₂₀ and C₇H₁₆
  • C₉H₂₀ (nonane): The general formula for alkanes is C_nH_2n+2. For n = 9, the number of hydrogen atoms would be 2(9) + 2 = 18 + 2 = 20.
  • C₇H₁₆ (heptane): The same general formula for alkanes, C_nH_2n+2, applies. For n = 7, the number of hydrogen atoms would be 2(7) + 2 = 14 + 2 = 16.
  • Since both follow the formula C_nH_2n+2, they are both alkanes and thus members of the same homologous series.
• C₉H₁₆ and C₇H₁₆
  • C₉H₁₆ (nonene): The general formula for alkenes is C_nH_2n. For n = 9, the number of hydrogen atoms would be 2(9) = 18.
  • C₇H₁₆ (heptane): As above, heptane is an alkane with the formula C_nH_2n+2. For n = 7, the number of hydrogen atoms would be 2(7) + 2 = 16.
  • These compound classes (alkenes vs. alkanes) do not follow the same general formula, hence they are not members of the same homologous series.
• C₉H₁₈ and C₇H₁₆
  • C₉H₁₈ (nonene): The general formula for alkenes is C_nH_2n. For n = 9, the number of hydrogen atoms would be 2(9) = 18.
  • C₇H₁₆ (heptane): As above, heptane is an alkane with the formula C_nH_2n+2. For n = 7, the number of hydrogen atoms would be 2(7) + 2 = 16.
  • Similar to the previous comparison, these compound classes (alkenes vs. alkanes) do not follow the same general formula, hence they are not members of the same homologous series.
• C₉H₂₀ and C₇H₁₄
  • C₉H₂₀ (nonane): As mentioned, nonane is an alkane with the formula C_nH_2n+2. For n = 9, the number of hydrogen atoms would be 2(9) + 2 = 20.
  • C₇H₁₄ (heptene): The general formula for alkenes is C_nH_2n. For n = 7, the number of hydrogen atoms would be 2(7) = 14.
  • These compound classes (alkanes vs. alkenes) do not follow the same general formula, hence they are not members of the same homologous series.

Therefore, the correct pair that belongs to the same homologous series is C₉H₂₀ and C₇H₁₆ (Option 1), as they both adhere to the general formula for alkanes (C_nH_2n+2).

Additional Information

• Homologous Series
  • A homologous series is a group of organic compounds with the same functional group and similar chemical properties.
  • Each successive member differs by a CH₂ unit.
• Alkenes
  • Alkenes are hydrocarbons with at least one carbon-carbon double bond.
  • The general formula for alkenes is C_nH_2n.
• Alkynes
  • Alkynes are hydrocarbons with at least one carbon-carbon triple bond.
  • The general formula for alkynes is C_nH_2n-2.