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

We are given a reactant that features an alkene chain bridging two distinct benzene rings: one is unsubstituted, and the other carries an electron-donating methoxy (${\text{-OCH}}_3$) group at the para position. The reaction proceeds in two distinct phases, governed by the reagents ${\text{Cl}}_2/{\text{CCl}}_4$ and anhydrous ${\text{AlCl}}_3$.

Electrophilic Addition The first reagent, ${\text{Cl}}_2$ in a non-polar solvent like ${\text{CCl}}_4$, is a classic recipe for the electrophilic addition across a carbon-carbon double bond. The alkene $\pi$-electrons attack the chlorine molecule, forming a cyclic chloronium ion intermediate, which is subsequently opened by a chloride ion. This yields a vicinal dichloride:

Carbocation Generation Enter anhydrous ${\text{AlCl}}_3$, a powerful Lewis acid. Its primary role in such environments is to abstract a halide ion to generate a carbocation. But we have two chlorine atoms! Which one leaves? Nature always favors the path of lowest energy, meaning the most stable carbocation will dictate the reaction.

If the chlorine adjacent to the methoxy-phenyl group leaves, it forms a benzylic carbocation: ${\text{CH}}_3{\text{O-C}}_6{\text{H}}_4{\text{-CH}}^{+}{\text{-CH(Cl)-CH}}_2{\text{-C}}_6{\text{H}}_5$. This carbocation is exceptionally stable because the positive charge is delocalized not just into the benzene ring, but is further stabilized by the strong $+M$ (resonance) effect of the para-methoxy group. The alternative secondary carbocation lacks this profound stabilization.

Intramolecular Friedel-Crafts Alkylation Now for the real magic. We have a highly electrophilic carbocation and a nucleophilic unsubstituted benzene ring tethered just a few carbons away. This sets the stage for an intramolecular Friedel-Crafts alkylation.

The unsubstituted benzene ring will attack the carbocation from its ortho position. To predict the structure of the resulting fused ring, we simply count the atoms in the newly formed cycle: 1. The ortho carbon of the attacking ring. 2. The ipso carbon of the attacking ring. 3. The ${\text{-CH}}_2\text{-}$ group. 4. The $\text{-CH(Cl)-}$ group. 5. The carbocation center ${\text{-CH}}^{+}\text{-}$.

This perfectly closes into a stable $5$-membered ring, creating an indane derivative. Because the carbocation carbon was originally attached to the methoxy-phenyl group, this bulky group will end up on the carbon directly adjacent to the new fusion bond. This precise regiochemistry perfectly matches the structure shown in option (c).