Question

Which of the following set of compounds does not belong to same homologous series?

• A. CH$_4$ and C$_4$H$_{10}$
• B. C$_2$H$_6$ and C$_3$H$_8$
• C. C$_3$H$_8$ and C$_5$H$_{12}$
• D. C$_4$H$_8$ and C$_5$H$_{12}$

Hint:

Same homologous series → Same general formula. Alkanes: C$_n$H$_{2n+2}$, Alkenes: C$_n$H$_{2n}$.

Answer 1

The Correct Option is D

To determine which set of compounds does not belong to the same homologous series, we need to understand what constitutes a homologous series in organic chemistry. A homologous series is a group of organic compounds that have a similar general formula, exhibit similar chemical properties, and differ by a CH₂ unit. These compounds have a regular stepwise increment in their molecular formula.

1. Analyzing CH₄ and C₄H₁₀:
   • Methane (CH₄) and Butane (C₄H₁₀) both belong to the alkane series, which follows the general formula C_nH_2n+2.
   • The difference between them is three CH₂ units, which keeps them in the same homologous series.
2. Analyzing C₂H₆ and C₃H₈:
   • Ethane (C₂H₆) and Propane (C₃H₈) also belong to the alkane series with the general formula C_nH_2n+2.
   • The difference between them is one CH₂ unit, indicating they are in the same homologous series.
3. Analyzing C₃H₈ and C₅H₁₂:
   • Propane (C₃H₈) and Pentane (C₅H₁₂) are both alkanes, fitting the general formula C_nH_2n+2.
   • They differ by two CH₂ units, confirming they belong to the same homologous series.
4. Analyzing C₄H₈ and C₅H₁₂:
   • Cyclobutane or Butene (C₄H₈) typically belongs to the alkene series with the general formula C_nH_2n, exhibiting a double bond or a ring structure.
   • In contrast, Pentane (C₅H₁₂) is an alkane (C_nH_2n+2), indicating that C₄H₈ and C₅H₁₂ do not belong to the same homologous series.

Thus, the correct answer is C₄H₈ and C₅H₁₂, as they do not belong to the same homologous series.