Animated Solution for Chemistry - Organic Chemistry: The major product of the following reaction is

A Tale of Two OH Groups

Welcome to a fascinating journey through an intramolecular reaction! In this problem, we are presented with a molecule that has a bit of a split personality: it features both a phenolic $-OH$ group attached directly to a benzene ring and an aliphatic $-OH$ group at the end of a three-carbon side chain. Our goal is to predict the major product when this molecule is treated sequentially with $HCl$ and anhydrous $AlC{l}_3$.

The Selective Attack of $HCl$

When we introduce $HCl$ to the system, it acts as a source of protons and chloride nucleophiles. But which $-OH$ group will it attack? The oxygen atom of the phenolic $-OH$ group donates its lone pair into the benzene ring through resonance. This gives the $C-O$ bond partial double bond character, making it incredibly strong and unreactive towards nucleophilic substitution.

On the other hand, the aliphatic $-OH$ on the side chain has no such resonance stabilization. It is easily protonated to form a good leaving group (water), which is then displaced by the chloride ion. This selective reaction converts our primary alcohol into a primary alkyl chloride, leaving the phenol ring untouched.

Unleashing the Lewis Acid

Next, we bring in anhydrous $AlC{l}_3$, a classic and powerful Lewis acid. Aluminum in $AlC{l}_3$ is electron-deficient and highly electrophilic. It coordinates with the chlorine atom on our newly formed side chain, weakening the $C-Cl$ bond and eventually ripping the chlorine away.

This generates a highly reactive primary carbocation at the end of the side chain. We now have a potent electrophile tethered right next to an electron-rich benzene ring. The stage is perfectly set for an intramolecular Friedel-Crafts alkylation!

The Intramolecular Attack

The carbocation is on a leash, and it wants to attack the benzene ring to regain stability. The phenolic $-OH$ is a strongly activating, ortho/para directing group. This means it increases the electron density specifically at the positions ortho and para to itself, making them the most attractive targets for our electrophile.

However, not all targets are created equal. The ortho position is located right between the bulky $-OH$ group and the tethered side chain. Attacking this position would require squeezing into a very sterically crowded space. In contrast, the para position is wide open and easily accessible.

To minimize steric hindrance, the carbocation preferentially swings around and attacks the para position. This electrophilic aromatic substitution closes the loop, forming a stable five-membered ring fused to the benzene ring. After the loss of a proton to restore aromaticity, we arrive at our final major product: a beautiful bicyclic system!