Acid Catalyzed Reaction of Dihydropyran with Primary Alcohol

To determine the nature of the major product in the given reaction:

1. Reaction Mechanism:
   The starting material is 3,4-dihydro-2$H$-pyran (DHP), which contains an enol ether functional group.

   • Step 1: Protonation
     In the presence of an acid catalyst ($\text{H}^+$, anhydrous), the double bond is protonated at the C-3 position ($\beta$-carbon relative to the ring oxygen). This forms a highly resonance-stabilized oxocarbenium ion (carbocation at the C-2 position, adjacent to the oxygen atom):
     $\text{DHP} + \text{H}^+ \longrightarrow \text{[Oxocarbenium intermediate]}^+$

   • Step 2: Nucleophilic Attack
     The alcohol ($\text{RCH}_2\text{OH}$) acts as a nucleophile and attacks the electrophilic C-2 carbon of the oxocarbenium ion:
     $\text{[Oxocarbenium intermediate]}^+ + \text{RCH}_2\text{OH} \longrightarrow \text{[Protonated Tetrahydropyranyl ether]}^+$

   • Step 3: Deprotonation
     Loss of a proton regenerates the acid catalyst ($\text{H}^+$) and yields 2-alkoxytetrahydropyran (a THP ether).

2. Functional Group Analysis of the Product:
   At the C-2 carbon of the tetrahydropyran ring:

   • It is bonded to the ring oxygen atom ($-\text{O}-$).
   • It is bonded to the exocyclic alkoxy group ($-\text{OCH}_2\text{R}$).
   • It is bonded to one hydrogen atom ($-\text{H}$) and one carbon atom from the ring ($-\text{CH}_2-$).

   A carbon atom attached to two alkoxy/aryloxy groups ($-\text{OR}$ and $-\text{OR}'$), a hydrogen atom, and an alkyl chain constitutes an acetal (cyclic acetal).

Thus, the major product formed is an acetal.

Correct Option: (B) an acetal