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: Describe the mechanism for the synthesis of dihydropyran from tetrahydrofurfuryl alcohol and arenesulfonic acid. Answer: The mechanism for the synthesis of dihydropyran involves four main steps: 1) Protonation of the alcohol using arenesulfonic acid, forming a tetrahydrofurfuryl oxonium ion, 2) Nucleophilic attack by a second molecule of tetrahydrofurfuryl alcohol, forming a cyclic intermediate, 3) Deprotonation of the cyclic intermediate by arenesulfonic acid, neutralizing the positive charge, and 4) Formation of the final dihydropyran product.

Step-by-step solution

Protonation of the alcohol

The first step in this reaction involves the protonation of the hydroxyl group in tetrahydrofurfuryl alcohol. The lone pair on the oxygen atom of the hydroxyl group interacts with the hydrogen from ArSO3H, resulting in the formation of tetrahydrofurfuryl oxonium ion and ArSO3-. This makes the oxygen more electrophilic and activates the molecule for the subsequent steps.

Formation of the oxonium ion

Now that we have a positively charged oxygen in the tetrahydrofurfuryl oxonium ion, the molecule is more susceptible to nucleophilic attack. The lone pair of electrons on the oxygen atom in another molecule of tetrahydrofurfuryl alcohol act as a nucleophile and attacks the electrophilic carbon adjacent to the oxonium ion. This forms a new carbon-oxygen bond and leads to the formation of a cyclic intermediate.

Removal of a proton

In this step, we have a positively charged cyclic intermediate. To neutralize the positive charge and complete the formation of the dihydropyran product, the oxygen atom which initially acted as a nucleophile now abstracts a proton from a nearby molecule of ArSO3H. This forms a new hydroxyl group and regenerates the ArSO3- anion.

Formation of dihydropyran

Now that the cyclic intermediate has been neutralized, we arrive at our final product, dihydropyran. The desired dihydropyran molecule is formed with a new carbon-oxygen bond connecting the two reacting tetrahydrofurfuryl alcohol molecules. In conclusion, the mechanism for the synthesis of dihydropyran from tetrahydrofurfuryl alcohol and arenesulfonic acid involves protonation of the alcohol, formation of an oxonium ion, nucleophilic attack to form a cyclic intermediate, and deprotonation to obtain the neutral product.