Question

Which among the following molecules exhibits strong London forces?

• A. neo-pentane
• B. n-pentane
• C. isobutane
• D. isopentane

Hint:

For alkanes, remember this straightforward trend:
$$\text{Branching} \propto \frac{1}{\text{Surface Area}} \propto \frac{1}{\text{Strength of London Forces}} \propto \frac{1}{\text{Boiling Point}}$$
More linear chains mean greater surface area contact and stronger intermolecular forces. Hence, the straight-chain $n$-isomer will always win over branched versions.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks us to identify the hydrocarbon molecule among the given isomers that exhibits the strongest London dispersion forces.

Step 2: Key Formula or Approach:
London dispersion forces are weak intermolecular attractions present in non-polar molecules. The strength of these forces depends heavily on two structural parameters:
1. Molecular mass or size (more electrons mean greater polarizability).
2. Surface area of the molecule. For isomers sharing the exact same molecular formula, a straight, unbranched chain possesses a larger surface area than a branched, spherical isomer. A larger surface area allows for maximum intermolecular contact, thereby increasing the strength of London forces.

Step 3: Detailed Explanation:
Let's first compare the chemical compositions of the choices:
Isobutane ($\text{C}_4\text{H}_{10}$) has only 4 carbon atoms, so it inherently has weaker dispersion forces than the 5-carbon pentane isomers due to its lower molecular mass.
n-pentane, isopentane, and neo-pentane are structural isomers sharing the exact same formula ($\text{C}_5\text{H}_{12}$).
Let's examine their degrees of branching and spatial orientations:
n-pentane is a straight, unbranched continuous chain. It has a broad, open surface area that lets adjacent molecules pack closely together alongside each other.
Isopentane contains a single branch, making it more compact.
Neo-pentane is highly branched and adopts a nearly spherical, compact structure, which minimizes its outer surface area and keeps adjacent molecules further apart.
Since n-pentane provides the highest surface area of contact, its electron clouds are most easily polarized, resulting in the strongest London dispersion forces among the isomers.

Step 4: Final Answer:
The molecule with the strongest London forces is the straight-chain alkane, n-pentane, which corresponds to option (B).