Question

Under acid catalysis, tetrahydrofurfuryl alcohol reacts to give surprisingly good yields of dihydropyran. Propose a mechanism to explain this useful synthesis.

Short answer

The compound that is made up of tetrahydrofuran ring with one alcohol group containing CH₂ group is tetrahydrofurfuryl alcohol. This compound is used as a solvent and also an intermediate for many organic syntheses.

Step-by-step solution

About tetrahydrofurfuryl alcohol

The compound that is made up of tetrahydrofuran ring with one alcohol group containing CH₂ group is tetrahydrofurfuryl alcohol. This compound is used as a solvent and also an intermediate for many organic syntheses.

About the reaction

The treatment of tetrahydrofurfuryl alcohol gives dihydropyran on acidification. The ring gets expanded and it becomes a six-membered ring. In addition to this one double bond will be formed at the first carbon from the oxygen atom.

About the mechanism

The lone pair of electrons of oxygen present in the alcohol group abstracts a proton from an acid. This results in the removal of water and the formation of a carbocation. It is subjected to an alkyl shift. On treating with water, the dihydropyran will be formed. The mechanism is shown below.