Question

What is the role of the phosphoric acid in a dehydration reaction, explain your answer with the required equations

Answer 1

An alcohol can be dehydrated to form an alkene in the presence of a strong acid. The acid used for dehydration can be either H₃PO₄ or H₂SO₄which acts as a catalyst and increases the rate of reaction. Example: Dehydration of cyclohexanol. 1 mole of cyclohexanol produces 1 mole of cyclohexene by stoichiometry.

Steps of mechanism:

Step 1: Protonation of the alcohol group by the acid making the OH- a better leaving group. This step is slightly exothermic.

Step 2: Loss of water group from alcohol to form carbocation. This step is highly endothermic.

Step 3: Removal of a beta hydrogen by base (water) to form the alkene. This step is exothermic. The overall reaction is slightly endothermic (favorable at high temperatures).

Note: Concentrated H₂SO₄ could produce many side products as it is a strong oxidizing agent. It oxidizes some of the alcohol to CO₂and reduces itself to SO₂. Both of these gases must be separated from the alkene.