Vacuum Distillation

Vacuum distillation applies reduced pressure to the distillation system, lowering the boiling points of all components. This allows you to distill high-boiling compounds at lower temperatures, preventing thermal decomposition.

Why Reduced Pressure Lowers Boiling Points

At any temperature, a liquid has a vapor pressure. Boiling occurs when the vapor pressure equals the external (atmospheric) pressure. Under vacuum, the external pressure is lower, so boiling can occur at a lower temperature.

Rule of thumb: dropping the pressure by a factor of 10 lowers the boiling point by roughly 25-40°C. Going from 760 torr (atmospheric) to 76 torr (mild vacuum) lowers most boiling points by ~25°C. Going to 7.6 torr (stronger vacuum) lowers another ~25°C.

When to Use Vacuum Distillation

1. High-boiling compounds that decompose before reaching their atmospheric bp. Vacuum distillation lets them distill intact.
2. Heat-sensitive compounds (many natural products, peptides, drugs) that need to avoid high temperatures.
3. Isolating trace impurities that would otherwise be lost.

Many industrial distillations (petroleum refining, pharmaceutical manufacture) are done under vacuum for efficiency and product stability.

Setup

A vacuum distillation setup adds:

• Vacuum pump (water aspirator for mild vacuum; oil pump for stronger vacuum).
• Vacuum gauge to monitor pressure.
• Trap (cold trap with liquid N₂ or dry ice/acetone) to protect the pump from solvent vapors.
• Vacuum-tight fittings and joints (grease the ground-glass joints to ensure seal).

Clausius-Clapeyron in Action

The relationship between vapor pressure and temperature is given by the Clausius-Clapeyron equation:

ln(P₂/P₁) = -Δ$H_{\text{vap}}$/R × (1/T₂ - 1/T₁)

This tells you how much the boiling point drops as you reduce pressure. For a compound with Δ$H_{\text{vap}}$ = 40 kJ/mol:

• Atmospheric (760 torr): normal bp.
• $\frac{760}{10}$ = 76 torr: bp about 30-40°C lower.
• $\frac{76}{10}$ = 7.6 torr: bp about 60-80°C lower total.

On the MCAT, you will not need to do this calculation, but recognize the principle: lower pressure = lower bp.

Example: Glycerol Distillation

Glycerol (propane-1,2,3-triol) has atmospheric bp 290°C. Heating it to 290°C causes significant decomposition. Under vacuum (10 torr), glycerol boils at ~180°C - easily distillable without decomposition.

Practical Tips

• Always check for leaks in the system before starting.
• Start heating gently; sudden boiling under reduced pressure causes severe bumping.
• Use a cooling bath in the receiving flask to prevent loss of volatile distillate.
• Record the vapor temperature AND the pressure. Without both, boiling point data are meaningless.