Animated Solution for Chemistry - Organic Chemistry: A flask contains a mixture of isohexane and 3-methylpentane. One of the liquids boils at $63^{\circ }\text{C}$ while the other boils at $60^{\circ }\text{C}$. What is the best way to separate the two liquids and which one will be distilled out first?

Fractional Distillation

When dealing with a mixture of miscible liquids, the choice of separation technique heavily relies on the difference in their boiling points. In this problem, we are presented with a mixture of isohexane and 3-methylpentane.

The boiling points are given as $60^{\circ }\text{C}$ and $63^{\circ }\text{C}$ respectively. The first thing we must notice is the incredibly small difference between these two temperatures:

Simple distillation is a fantastic tool, but it has a major limitation: it is only effective when the boiling points of the components differ by at least $25^{\circ }\text{C}$. If we were to heat this flask using simple distillation, both liquids would vaporize almost simultaneously. Their vapors would mix, and the condensed liquid (the distillate) would still be a mixture, defeating the entire purpose of the separation.

To overcome this, we must use fractional distillation. A fractionating column provides a massive surface area for repeated cycles of vaporization and condensation. This allows the more volatile component to progressively enrich the vapor phase as it travels up the column, leading to a clean separation even when the boiling point difference is just $3^{\circ }\text{C}$.

Who Distills First?

In any distillation process, the liquid with the lower boiling point is inherently more volatile. It requires less thermal energy to overcome its intermolecular forces and escape into the vapor phase.

Since isohexane boils at $60^{\circ }\text{C}$, it will vaporize more readily than 3-methylpentane, which boils at $63^{\circ }\text{C}$. Therefore, isohexane will be the first to distill out and collect in the receiving flask.

Why the Difference?

You might wonder why two isomers with the exact same molecular formula (${\text{C}}_6{\text{H}}_{14}$) have different boiling points. The secret lies in their molecular geometry.

3-methylpentane has a more symmetrical structure compared to isohexane (which is 2-methylpentane). This symmetry allows the molecules of 3-methylpentane to pack more closely together. Better packing means a larger surface area of contact between adjacent molecules, which directly strengthens the van der Waals dispersion forces. Stronger intermolecular forces require more heat energy to break, resulting in a slightly higher boiling point for 3-methylpentane.

By understanding both the macroscopic technique and the microscopic molecular interactions, we can confidently conclude that fractional distillation is the correct method, and isohexane will be the first to emerge.