Vacuum distillation

Draw and describe the key components of a vacuum distillation setup.

Components (in order):

1. Round-bottom flask — holds the liquid to be distilled.

2. Heating mantle / heat source — gently heats the mixture.

3. Distillation head — connects boiling flask to condenser.

4. Thermometer — measures vapor temperature.

5. Condenser — cools vapor back to liquid (water in/out).

6. Vacuum adapter — creates reduced pressure environment.

7. Receiving flask — collects distillate.

8. Vacuum source — water aspirator or vacuum pump.

Function:
Vacuum lowers pressure → lowers boiling point → allows high-boiling or heat-sensitive compounds to distill without decomposition.

How is vacuum distillation different from fractional distillation?

• Fractional distillation:

  • Performed at atmospheric pressure

  • Uses a fractionating column for multiple “theoretical plates”

  • Best for separating liquids with close boiling points (10–25°C).

• Vacuum distillation:

  • Uses reduced pressure (vacuum)

  • NO fractionating column required

  • Used for very high boiling compounds or heat-sensitive substances

  • Lowers the boiling point significantly.

Shortcut:
Fractional = better separation. Vacuum = lower temperature.

How do you use a pressure–temperature nomograph?

• A nomograph allows you to estimate how a compound’s boiling point changes with pressure.

• To use:

  1. Find the compound’s normal boiling point at 760 torr.

  2. Draw a straight line to the new pressure (vacuum pressure).

  3. Where the line intersects the temperature scale → reduced-pressure boiling point.

• Lower pressure → lower boiling point (almost always).

Shortcut:
Draw a straight line between known BP (760 torr) and new pressure → read new BP.

What does a polarimeter do, and how do you interpret its data?

• A polarimeter measures optical rotation of chiral compounds.

• It determines whether a sample is:

  • Optically active (rotates light)

  • Dex (+) or Levo (–)

  • Racemic (no net rotation)

Interpretation:

• If rotation = 0° → racemic mixture or achiral compound.

• If rotation is positive → dextrorotatory (“+”).

• If rotation is negative → levorotatory (“–”).

• Specific rotation equation:

[α]=αobsl⋅c[\alpha] = \frac{\alpha_{\text{obs}}}{l \cdot c}[α]=l⋅cαobs​​

(l = path length in dm, c = concentration)

Shortcut:
Polarimeter = chiral purity + sign of rotation.

Why do we use vacuum distillation instead of heating until it boils at atmospheric pressure?

• We use vacuum distillation for compounds with very high boiling points. Lowering the pressure with a vacuum reduces the boiling point, allowing the liquid to distill at a much lower temperature. This prevents decomposition and avoids overheating the sample.

What feature does fractional distillation have that vacuum distillation does not?

• Fractional distillation uses a fractionating column, which provides repeated condensation–evaporation cycles for better separation of liquids with similar boiling points. Vacuum distillation does not have this column because it focuses on lowering the boiling point by reducing pressure.

On a nomograph, what happens to boiling point when pressure decreases?

• Boiling point decreases.

A sample shows no optical rotation in a polarimeter. What does this tell you?

• No optical rotation means the sample is either achiral or a racemic mixture. Enantiomers in a 1:1 mixture cancel each other’s rotations, giving zero net rotation.

Which distills first in GC or distillation: high BP or low BP compound?

• Low BP