Question

Which one of the following pair belongs to same homologous series?

• A. \(\text{C}_2\text{H}_2 . \text{C}_2\text{H}_4\)
• B. \(\text{C}_2\text{H}_6 . \text{C}_3\text{H}_8\)
• C. \(\text{C}_2\text{H}_6 . \text{C}_2\text{H}_4\)
• D. \(\text{C}_2\text{H}_2 . \text{C}_6\text{H}_6\)

Answer 1

The Correct Option is B

To determine which pair belongs to the same homologous series, we need to understand the concept of homologous series. Homologous series are groups of organic compounds that have the same general formula and differ by a constant unit (usually $\text{CH}_2$) in their molecular structure.

1. Understanding Homologous Series:
- General Characteristics:
- Compounds in a homologous series have the same functional group.
- They differ by one or more $\text{CH}_2$ units in their molecular formula.
- The general formula for a homologous series depends on the type of compound (e.g., alkanes, alkenes, alkynes, etc.).

2. Analyzing Each Option:
We will analyze each pair to determine if they belong to the same homologous series.

(1) $\text{C}_2\text{H}_2$, $\text{C}_2\text{H}_4$:
- $\text{C}_2\text{H}_2$: This is ethyne (acetylene), an alkyne with the general formula $\text{C}_n\text{H}_{2n-2}$.
- $\text{C}_2\text{H}_4$: This is ethene, an alkene with the general formula $\text{C}_n\text{H}_{2n}$.
- These two compounds do not belong to the same homologous series because they have different functional groups (triple bond vs. double bond).

(2) $\text{C}_2\text{H}_6$, $\text{C}_3\text{H}_8$:
- $\text{C}_2\text{H}_6$: This is ethane, an alkane with the general formula $\text{C}_n\text{H}_{2n+2}$.
- $\text{C}_3\text{H}_8$: This is propane, also an alkane with the general formula $\text{C}_n\text{H}_{2n+2}$.
- Both compounds belong to the homologous series of alkanes because they have the same general formula and differ by one $\text{CH}_2$ unit.

(3) $\text{C}_2\text{H}_6$, $\text{C}_2\text{H}_4$:
- $\text{C}_2\text{H}_6$: This is ethane, an alkane with the general formula $\text{C}_n\text{H}_{2n+2}$.
- $\text{C}_2\text{H}_4$: This is ethene, an alkene with the general formula $\text{C}_n\text{H}_{2n}$.
- These two compounds do not belong to the same homologous series because they have different functional groups (single bonds vs. double bonds).

(4) $\text{C}_2\text{H}_2$, $\text{C}_6\text{H}_6$:
- $\text{C}_2\text{H}_2$: This is ethyne (acetylene), an alkyne with the general formula $\text{C}_n\text{H}_{2n-2}$.
- $\text{C}_6\text{H}_6$: This is benzene, an aromatic hydrocarbon with a cyclic structure and delocalized electrons. It does not fit into any simple homologous series like alkanes, alkenes, or alkynes.
- These two compounds do not belong to the same homologous series because they have fundamentally different structures and properties.

3. Conclusion:
The only pair that belongs to the same homologous series is $\text{C}_2\text{H}_6$ and $\text{C}_3\text{H}_8$, as both are alkanes and differ by one $\text{CH}_2$ unit.

Final Answer: $ \boxed{\text{(2) } \text{C}_2\text{H}_6, \text{C}_3\text{H}_8} $