Question

One method for preparing alcohols involves hydration of alkenes with aqueous acids (such as sulfuric acid and water). In light of this fact, it seems surprising that the phosphoric acid promotes the elimination of cyclohexanol to give cyclohexene plus water at high temperatures, the very components required to make cyclohexanol. Why was your experiment successful?

Answer 1

First compare both the reaction conditons as: First reaction includes hydration of alkene where sulfuric acid is used with water (ie. dilute sulfuric acid).

Second reaction involves dehydration of cyclohexanol with phosphoric acid (concentrated) under heating yielding product cyclohexene and water as side product.

Both reaction are acid catalyse and so reversible and in either case reactants and products always will be present at equilibrium. This type of reaction are goverened by Le Chatelier's principle (which states that any change at equilibrium such as pressure, temperature, or concentration of a reactant) is applied then the equilibrium will shift so as to tend to counteract the effect of the constraint.

So, in first reaction exess of H2O is present and this will attack on cyclohexene to give cuclohexanol (with consumption of water) where as in second reaction acid is concentrated, and this will facilitate the dehydration and equilibrium favour the formation of cyclohexene from cyclohexanol with liberation of water and by this way experiment is successful.