Q.Assertion (A): Among isomeric pentanes, 2, 2-dimethylpentane has highest boiling point.
Reason (R): Branching does not affect the boiling point.

The key idea here is the effect of chain isomerism (specifically, branching) on the boiling points of alkanes.

1. Evaluating Assertion (A): Isomeric pentanes have the molecular formula $C_5H_{12}$. The isomers are n-pentane, 2-methylbutane (isopentane), and 2,2-dimethylpropane (neopentane). Among these, n-pentane (the straight-chain isomer) has the highest boiling point ($36^\circ C$), while 2,2-dimethylpropane (the most branched isomer) has the lowest ($9.5^\circ C$). The molecule "2, 2-dimethylpentane" is an isomer of heptane ($C_7H_{16}$), not pentane. Therefore, Assertion (A) is incorrect. …

When considering the boiling points of organic compounds, especially hydrocarbons, the primary factor to examine is the strength of intermolecular forces. For non-polar molecules like alkanes, these forces are predominantly London dispersion forces (a type of Van der Waals force). The strength of these forces depends significantly on the surface area available for contact between molecules.

Concept: Effect of Branching on Boiling Point

Imagine molecules as objects trying to stick to each other.

• Linear molecules (like n-pentane) have a larger, more extended surface area. This allows for more points of contact between adjacent molecules, leading to stronger London dispersion forces. More energy is required to overcome these stronger forces, resulting in a higher boiling point.
• Branched molecules (like isopentane or neopentane) are more compact and spherical. A spherical shape minimises the surface area available for intermolecular contact. With fewer points of contact, the London dispersion forces are weaker. Less energy is needed to separate these molecules, leading to a lower boiling point.

Therefore, as branching increases in isomeric alkanes, the molecules become more spherical, the surface area for intermolecular contact decreases, the London dispersion forces weaken, and the boiling point decreases.

Let's apply this understanding to the given assertion and reason.

1. Identify Isomeric Pentanes: Pentane refers to alkanes with the molecular formula $C_5H_{12}$. There are three structural isomers of pentane:
   • n-pentane: A straight-chain alkane.

*   **Isopentane (2-methylbutane):** A branched alkane with one methyl group branching off the main chain.

*   **Neopentane (2,2-dimethylpropane):** A highly branched alkane with two methyl groups branching off the central carbon.

Their approximate boiling points are:
*   n-pentane: $36^\circ C$
*   Isopentane: $28^\circ C$
*   Neopentane: $9.5^\circ C$

Notice that as branching increases (n-pentane $\rightarrow$ isopentane $\rightarrow$ neopentane), the boiling point decreases.

2. Evaluate Assertion (A): "Among isomeric pentanes, 2, 2-dimethylpentane has highest boiling point."

* First, let's check if 2,2-dimethylpentane is an isomer of pentane.

* Pentane has 5 carbon atoms.

* 2,2-dimethylpentane has a 5-carbon main chain and two methyl groups ($CH_3$) attached to the second carbon. This means it has a total of $5 + 2 = 7$ carbon atoms. Its molecular formula is $C_7H_{16}$.

* Since 2,2-dimethylpentane has 7 carbon atoms and pentane has 5 carbon atoms, 2,2-dimethylpentane is not an isomer of pentane. Isomers must have the same molecular formula. …