Question

E1 eliminations of alkyl halides are rarely useful for synthetic purposes because they give mixtures of substitution and elimination
products. Explain why the sulfuric acid-catalyzed dehydration of cyclohexanol gives a good yield of cyclohexene even
though the reaction goes by an E1 mechanism. (Hint: What are the nucleophiles in the reaction mixture? What products are
formed if these nucleophiles attack the carbocation? What further reactions can these substitution products undergo?)

Answer 1

Answer – We know E¹ reaction is two step reaction and the formation of carbocation is rate determining step. So the formation of cyclohexene from the cyclohexanol is done by E¹ reaction, but it is the very slow reaction and also not gives a good yield, since there is no any other force to eliminate the –OH from cyclohexanol and forming cyclohexene.

We know –OH group is not good leaving group and due to that with just E¹ reaction without acid-catalyzed. So when we added sulfuric acid then there is the rate of reaction increased and first step to protonate the –OH molecule to –OH₂ with a positive charge. Now the positive charged –OH₂ is a very good leaving group and formed carbocation very fast. As the carbocation formed then there is HSO₄^- abstract the protons and form the double bond very quickly. So we will get a very good yield of the product by eliminating water molecule.