Question

In the Dehydration of Cyclohexanol (used Cyclohexanol to get Cyclohexene), we had to do distillation twice. Why is that? My TA said it had something to do with Azeotropes but I can't figure out why.

Answer 1

Answer

cyclohexene is  prepared by dehydration of cyclohexanol using an acid catalyst such as phosphoric acid.

The crude product is contaminated with water, unreacted alcohol, phosphoric acid and some side products. Washing with water removes most of the impurities. Treatment with sodium carbonate solution removes traces of acid and a final wash with water removes any remaining carbonate.

The mechanism of the dehydration of cyclohexanol probably involves the formation of a carbocation.

This carbocation can react in any of the ways shown below:

1. With water to yield cyclohexanol - the starting material. (Note that all the steps in this reaction are reversible);

2. by losing a proton to yield cyclohexene;

3. with cyclohexanol to yield dicyclohexyl ether.

Dicyclohexyl ether then is a probable side product of the dehydration of cyclohexanol. It is immiscible with water is likely to co-distill and may therefore be present in the first distillate. To remove dicylcohexy ether completely a second distillation of the product is usually carried out.
In order to confirm experimentally that a mixture forms an azeotrope you need to do a compositional analysis of the vapor and liquid phases at the conditions of interest (e.g. constant temperature or pressure).