Distillation

With solubility-based methods covered, the second family of separation techniques in MCAT organic chemistry is distillation — separating liquids based on differences in their boiling points, rather than differences in solubility. Distillation is especially useful when a mixture contains two or more liquids and one needs to be isolated by taking advantage of how readily it vaporizes.

Key Takeaways

• Distillation separates liquids by boiling-point differences, via evaporation then condensation; the collected liquid is the distillate.

• Simple distillation: boiling points under 150°C, with at least a 25°C gap between the two liquids.

• Vacuum distillation: target boiling point over 150°C — lowering pressure lowers boiling point, avoiding thermal degradation.

• Fractional distillation: boiling points less than 25°C apart — uses a packed fractionating column for repeated vaporization-condensation cycling to resolve close boiling points.

How Distillation Works

Distillation involves two key steps: evaporation followed by condensation.

The process begins by heating the liquid mixture. The component with the lowest boiling point vaporizes first, turning into a gas while the other components remain liquid. That vapor travels up the distillation column and reaches a water-cooled condenser. As the vapor cools in the condenser, it condenses back into a liquid, which drips down into a separate container. This collected liquid is called the distillate.

Depending on the boiling points of the liquids being separated, one of three types of distillation is used.

Simple Distillation

Simple distillation is used when both of the following conditions are met:

• The boiling points of the liquids are less than 150°C.

• The difference in boiling points between the two liquids is at least 25°C.

If the boiling points are far enough apart, and the liquids aren't too thermally sensitive, good separation is achievable without a more complex setup.

Vacuum Distillation

Vacuum distillation is used when the compound being isolated has a boiling point over 150°C. That high a boiling point risks thermal degradation of the compound if it were heated under normal atmospheric pressure.

Instead, the pressure in the system is lowered using a vacuum. This works because of a key principle: as pressure decreases, boiling point also decreases. Under reduced pressure, the liquid boils at a lower temperature, making the separation both gentler and safer for heat-sensitive compounds.

Fractional Distillation

Fractional distillation is the go-to method when the boiling points of the two liquids are less than 25°C apart — too narrow a gap for simple distillation to resolve well.

Fractional distillation uses a fractionating column, packed with materials like glass beads or steel wool to increase surface area. As vapors rise through this column, they repeatedly condense and re-vaporize. Each cycle enriches the vapor in the compound with the lower boiling point, so by the time it reaches the top of the column and passes into the condenser, it's mostly that single component. This repeated vaporization-condensation cycling is what allows for better resolution between liquids with similar boiling points.

A standard fractional distillation setup consists of a round-bottom flask holding the liquid mixture, heated from below (e.g., by a Bunsen burner). The fractionating column sits on top of the flask, enabling the repeated condensation cycle described above. Vapors that make it through the column enter the condenser, which is cooled by water flowing in and out; as the vapors cool there, they turn back into liquid and are collected in a separate receiving flask.

Common MCAT Mistakes

• Forgetting the 25°C/150°C cutoffs. Simple distillation needs boiling points under 150°C and at least a 25°C gap between the two liquids — miss either condition and a different method is needed.

• Mixing up why vacuum distillation is used. It's not to speed things up — it's to lower the boiling point (by lowering pressure) so high-boiling, heat-sensitive compounds don't thermally degrade.

• Confusing simple and fractional distillation. Simple distillation works when boiling points are far apart (≥25°C); fractional distillation is needed when they're close together (<25°C) and a single vaporization-condensation cycle can't resolve them.

• Overlooking what the fractionating column actually does. It isn't just a taller path for vapor — the packing material forces repeated condensation/re-vaporization cycles, each one further enriching the vapor in the lower-boiling component.

MCAT-Style Concept Check

Question: A chemist needs to separate two liquids with boiling points of 110°C and 128°C. Which distillation method is most appropriate?

• A) Simple distillation

• B) Vacuum distillation

• C) Fractional distillation

• D) Recrystallization

Answer: C

Explanation: Both boiling points are under 150°C, so vacuum distillation isn't needed. But the gap between them is only 18°C — under the 25°C threshold for simple distillation — so fractional distillation's packed column and repeated vaporization-condensation cycling is needed to resolve the two liquids. Recrystallization is a solubility-based method for purifying solids, not liquids.

FAQ

What's the difference between evaporation and condensation in distillation?

Evaporation happens when the heated liquid mixture's lowest-boiling component turns into a vapor. Condensation happens when that vapor reaches the water-cooled condenser and cools back into a liquid — the collected liquid is called the distillate.

When is vacuum distillation used instead of simple distillation?

Vacuum distillation is used when the compound being isolated has a boiling point over 150°C. Lowering the system pressure lowers the boiling point, letting the liquid boil at a gentler temperature and avoiding thermal degradation.

Why does fractional distillation work better than simple distillation for close boiling points?

The fractionating column is packed with materials like glass beads or steel wool, which force vapor to repeatedly condense and re-vaporize as it rises. Each cycle further enriches the vapor in the lower-boiling component, giving much better separation than a single vaporization-condensation pass.

What is the distillate?

The distillate is the liquid collected after vapor passes through the condenser and condenses back into liquid form — it's the separated, purified component being isolated from the original mixture.