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

Welcome to a beautiful and intricate multi-step organic synthesis problem! We are tasked with finding the major product when $\gamma$-butyrolactone reacts with $o$-cresol in the presence of aluminum chloride ($AlC{l}_3$) and heat. At first glance, this is a classic setup for a Friedel-Crafts reaction, but the presence of a cyclic ester (lactone) and a disubstituted benzene ring introduces a very elegant twist.

Friedel-Crafts Acylation

The reaction kicks off with the Lewis acid, $AlC{l}_3$, coordinating with the oxygen atom of the lactone. This coordination weakens the carbon-oxygen bond, causing the ring to open and forming a highly reactive acylium ion.

Now, this electrophile must choose where to attack the $o$-cresol ring. The benzene ring has two activating groups: a hydroxyl group ($-OH$) and a methyl group ($-C{H}_3$). The $-OH$ group is a significantly stronger activating group due to its strong $+R$ (resonance) effect. Therefore, it dictates the regiochemistry of the electrophilic aromatic substitution, directing the incoming acylium ion to its para position (which is also sterically less hindered than the ortho position between the two groups).

After the initial acylation, the oxygen-aluminum chloride complex at the end of the newly attached alkyl chain leaves, generating a primary carbocation.

Here is the catch: Primary carbocations are highly unstable. To achieve a lower energy state, the molecule undergoes a rapid $1,2$-hydride shift. A hydrogen atom from the adjacent carbon migrates over, shifting the positive charge and creating a much more stable secondary carbocation. This secondary carbocation is further stabilized by hyperconjugation from the adjacent methyl group.

Intramolecular Alkylation

Finally, this newly formed secondary carbocation acts as an internal electrophile. It swings around and attacks the benzene ring at the available ortho position (which is also para to the methyl group, making it electronically favorable).

Because the reactive center shifted one carbon closer to the benzene ring during the hydride shift, the resulting intramolecular cyclization forms a five-membered fused ring instead of a six-membered one. Furthermore, the terminal carbon is left hanging outside the new ring as a distinct methyl group. This perfectly matches the structure shown in option (c), completing our thrilling mechanistic journey!