Distillation case study

We shall look at the case of the complete oxidation of propan-1-ol using sodium dichromate(VI) and sulfuric acid. The product we are looking for is propanoic acid, produced according to the following equation:

3C₃H₇OH + 8H₂SO₄ + 2Na₂Cr₂O₇ ⇒ 3C₂H₅CO₂H + 2Na₂SO₄ + 2Cr₂(SO₄)₃ + 11H₂O

We need to separate the propanoic acid product from any other materials present. The reaction mixture is dark due to the reduction products of the sodium dichromate(VI). Green chromium(III) sulfate will be present in the mixture. Unreacted propan-1-ol and sulfuric acid will also be present. Some propanal from partial oxidation may also be present. Water and sodium sulfate are other products of the reaction.

The reaction mixture is heated steadily in a distillation apparatus. In this way we can collect the liquid which boils very close to the target molecule. We shall boil out the low boiling point liquids and discard them. The target propanoic acid is then collected between 141-143 °C. You should see from the temperatures in the table that, unless very strong heating is applied, we should leave the inorganic chromium and sodium compounds and the sulfuric acid behind:

Material	Boiling point (ºC)
Propan-1-ol	98
Propanal	49
Propanoic acid	141
Water	100
Sulfuric acid	338
Chromium(III) sulfate*	>1000
Sodium sulfate*	>1000

*These compounds decompose before boiling.

Video - the distillation of propanoic acid

The immiscible layer seen in the video is propanal, with the water and propan-1-ol boiling over around 100 ºC. The green chromium(III) compound remains in the flask along with the sulfuric acid and sodium salt. The propanoic acid boiled over at just above 141 ºC.