Understanding Homologous Series in Organic Chemistry

In organic chemistry, a homologous series is a series of compounds with the same general formula, usually varying by a single parameter such as the length of a carbon chain.

Members of a homologous series generally have similar chemical properties due to having the same functional group. However, their physical properties show a gradual change with increasing molecular weight.

Characteristics of a Homologous Series

Let's examine the characteristics mentioned in the options:

• General formula is same: This is a defining characteristic of a homologous series. All members can be represented by a common general formula. For example, the general formula for alkanes is $\text{C}_n\text{H}_{2n+2}$, where 'n' is the number of carbon atoms. For alkenes, it is $\text{C}_n\text{H}_{2n}$ (for cyclic or one double bond), and for alkynes, it is $\text{C}_n\text{H}_{2n-2}$ (for cyclic or one triple bond). Each series has its unique general formula.
• Molecular formula is same: This is incorrect. The molecular formula changes from one member to the next in a homologous series. Successive members differ by a $\text{CH}_2$ group. For example, in the alkane series, methane ($\text{CH}_4$) is the first member, ethane ($\text{C}_2\text{H}_6$) is the second, differing by $\text{CH}_2$. Propane ($\text{C}_3\text{H}_8$) is the third, differing from ethane by another $\text{CH}_2$.
• Structural formula is same: This is also incorrect. While the basic structural pattern involving the functional group is similar, the overall structural formula differs due to the increasing number of $\text{CH}_2$ units in the carbon chain. The structures get larger and more complex as you move up the series.
• Physical properties are same: This is incorrect. Physical properties like melting point, boiling point, density, and solubility show a gradual change (increase or decrease) as the molecular mass increases within a homologous series. For example, the boiling points of alkanes increase steadily with the number of carbon atoms.

Comparing Properties in a Homologous Series

Here's a summary comparing the properties:

Based on the characteristics, the general formula is indeed the same for all members of a homologous series.