Question

(5) The boiling point of limonene is 176 ºC. You purified it by steam distillation, which allowed isolation at lower temperature. Why was this successful (hint - look up the term "azeotrope")?

(6) Another possible method of purification would be column chromatography. Compare the structure of limonene to that of biphenyl (experiment 3). What solvent mixture would you use to purify limonene by column chromatography?

Answer 1

(5) During steam distillation of limonene, it forms a azeotropic mixture with water. Azeotrope is a constant boiling point mixture of two liquids which boil at a constant temperature and the properties remain fixed during distillation.

The azeotropic mixtutre of limonene and water ( they have a particular ratio), boils at lower temperature than the boiling point of limonene. Actually the azeotropic mixture boils in between the boiling point of the component liquids. The boiling point of water is 100 ⁰C. So, the distillation occurs at a boiling point in between 100 and 176 ⁰C.

(6) Both limonene and biphenyl are hydrocarbon and non-polar in nature. You have to choose a non-polar solvent to separate. Petroleum ether ( which is a mixture of hydrocarbons) is the good solvent system for the purification of the limonene in column chromatography.