Animated Solution for Chemistry - Organic Chemistry: Isomers of hexane, based on their branching, can be divided into three distinct classes as shown in the figure. The correct order of their boiling point is

Boiling Point and Intermolecular Forces

To master questions about the physical properties of organic molecules, we must first understand what happens at the microscopic level when a liquid boils. Boiling is the process of overcoming the intermolecular forces that hold molecules together in the liquid phase. The stronger these forces, the more thermal energy (higher temperature) is required to break them apart.

For non-polar molecules like alkanes (hydrocarbons with only single bonds), the only intermolecular forces present are Van der Waals dispersion forces. These are weak, temporary forces that arise from momentary fluctuations in electron distribution.

The strength of Van der Waals forces is directly proportional to the surface area of the molecule. Think of it like Velcro: a long strip of Velcro will have a much stronger grip than a tiny square of Velcro.

When a carbon chain is straight and unbranched, it has a large, extended surface area, allowing for maximum contact with neighboring molecules. However, as we introduce branches to the carbon chain, the molecule folds in on itself. It becomes more compact and adopts a more spherical shape.

Mathematically, a sphere has the minimum possible surface area for a given volume. Therefore, more branching leads to a smaller surface area, which in turn results in weaker Van der Waals forces and a lower boiling point.

Let's apply this logic to the three classes of hexane ($C_6{H}_{14}$) isomers provided in the problem:

1. Class III (n-hexane): This is a straight, unbranched chain. It has the maximum extended surface area among all the isomers. Consequently, it will exhibit the strongest Van der Waals forces. 2. Class II (2-methylpentane & 3-methylpentane): These molecules possess a single methyl branch (mono-branched). This single branch makes the molecules slightly more compact than the straight chain, reducing their surface area. 3. Class I (2,2-dimethylbutane & 2,3-dimethylbutane): These molecules have two branches (di-branched). They are the most compact and spherical among the given isomers. Thus, they have the minimum surface area and the weakest Van der Waals forces.

Based on our analysis, the order of surface area is:

Since the boiling point is directly proportional to the surface area, the boiling point will follow the exact same trend:

Matching this derived order with the given options, we find that Option (B) is the correct answer.

Melting Point Anomalies

While the rule for boiling point is straightforward (more branching = lower boiling point), you must be careful not to blindly apply this to melting points.

Melting point depends not only on intermolecular forces but also heavily on packing efficiency within a solid crystal lattice. Highly symmetrical molecules, even if they are highly branched (like 2,2,3,3-tetramethylbutane), can pack incredibly well into a crystal lattice, leading to unusually high melting points. Always keep this distinction in mind for advanced organic chemistry problems!