Question

Dihydropyran is synthesized by treating tetrahydrofurfuryl alcohol with an arenesulfonic acid, \(\mathrm{ArSO}_{3} \mathrm{H}\). Propose a mechanism for this conversion.

Short answer

Question: Propose a mechanism for the synthesis of dihydropyran from tetrahydrofurfuryl alcohol and an arenesulfonic acid, including the formation of a cyclic intermediate and the final bond formation. Answer: The mechanism involves the protonation of the oxygen atom in the hydroxyl group of tetrahydrofurfuryl alcohol by arenesulfonic acid, forming a tetrahydrofurfuryl oxonium ion. Next, an intramolecular nucleophilic attack occurs, with the adjacent carbon's hydrogen attacking the positively charged oxygen, creating a 6-membered cyclic intermediate. Finally, the cyclic intermediate loses a proton, leading to the formation of a double bond between the oxygen and the adjacent carbon, resulting in the synthesis of dihydropyran.

Step-by-step solution

Identify the reactants

Firstly, let's get familiar with the reactants - tetrahydrofurfuryl alcohol, which is a 5-membered ring containing an oxygen atom and a hydroxyl (OH) group attached to one of the carbons. The other reactant is arenesulfonic acid (ArSO3H), where Ar represents an aromatic ring and SO3H is a sulfonic acid group.

Protonation of oxygen in tetrahydrofurfuryl alcohol

The mechanism starts with the protonation of the oxygen atom in the hydroxyl group of tetrahydrofurfuryl alcohol by arenesulfonic acid. This makes the hydroxyl group a better leaving group, forming tetrahydrofurfuryl oxonium ion (a charged intermediate with a positively charged oxygen) and arenesulfonate ion. \[\mathrm{ArSO}_{3}\mathrm{H} + \mathrm{R}_{\mathrm{OH}} \rightarrow \mathrm{ArSO}_{3}^{-} + \mathrm{R}_{\mathrm{O}^{+}\mathrm{H}_{2}}\]

Intramolecular nucleophilic attack on the oxonium ion

In the next step, the adjacent carbon's hydrogen starts to act as a nucleophile (due to the presence of electron cloud). The hydrogen nucleophile attacks the positively charged oxygen in the tetrahydrofurfuryl oxonium ion, forming a 6-membered cyclic intermediate. \[\mathrm{R}_{\mathrm{O}^{+}\mathrm{H}_{2}} \rightarrow \mathrm{Cyclic\ Intermediate}\]

Deprotonation and bond formation

Finally, the cyclic intermediate loses a proton from the sulfonate ion to form a double bond between the oxygen and the adjacent carbon, leading to the formation of dihydropyran. \[\mathrm{Cyclic\ Intermediate} + \mathrm{ArSO}_{3}^{-} \rightarrow \mathrm{Dihydropyran} + \mathrm{ArSO}_{3}\mathrm{H}\] The proposed mechanism for the conversion of tetrahydrofurfuryl alcohol and arenesulfonic acid to dihydropyran involves protonation of the oxygen atom in the hydroxyl group, intramolecular nucleophilic attack, and deprotonation for bond formation.