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

A Tale of Rearrangement and Ring Closure

Imagine you are looking at a molecule that has everything it needs to react with itself. On one side, we have an electron-rich anisole derivative. On the other side, a long alkyl chain ending in a chlorine atom. When we introduce anhydrous $AlC{l}_3$, a classic Lewis acid, we set the stage for a beautiful intramolecular dance known as Friedel-Crafts alkylation.

Aluminum chloride is highly electron-deficient and has a strong affinity for halogens. It acts as a molecular thief, ripping the chlorine atom right off the end of the alkyl chain.

When the chlorine leaves with its bonding electrons, it leaves behind a positively charged carbon atom—a primary carbocation:

Now, pause and think. Is a primary carbocation stable? Not at all. It is highly unstable and desperate for electron density. But nature always seeks the lowest energy state. Look right next door to the primary carbocation! We have a tertiary carbon atom bonded to a hydrogen.

If that hydrogen shifts over with its electrons—a process known as a 1,2-hydride shift—we can form a much more stable tertiary carbocation.

This rearrangement is incredibly fast and outcompetes any immediate attack on the benzene ring.

Now we have a powerful electrophile—the tertiary carbocation—tethered directly to an electron-rich benzene ring. The methoxy ($-OC{H}_3$) group on the ring is strongly activating due to its $+R$ effect, making the ring highly nucleophilic at the ortho and para positions.

Since the para position is already occupied by the tethered chain itself, the carbocation has no choice but to swing around and attack the ortho position. This is an intramolecular electrophilic aromatic substitution.

Let's trace the newly formed ring to determine its size. The tethered chain contributes three carbons to the new ring: the two $-C{H}_2-$ groups and the tertiary carbon $C(C{H}_3{)}_2$. The benzene ring contributes two carbons: the ipso carbon (where the chain starts) and the ortho carbon (where the attack happens).

Three plus two equals five! We form a highly stable 5-membered ring fused to the benzene ring, featuring a gem-dimethyl group. This specific bicyclic framework is known as an indane derivative. The elegance of this reaction lies in how the molecule perfectly rearranges itself to find the most thermodynamically stable product.