Animated Solution for Chemistry - Organic Chemistry: The major product obtained in the given reaction is

Welcome to a fascinating journey into the world of intramolecular reactions! Our starting material is a beautifully complex molecule: 1-(3-chlorobutoxy)-3-methylbenzene.

Let's break it down. We have a central benzene ring adorned with two key substituents. On one side, there is a simple methyl group ($-C{H}_3$). On the other side, we have an ether linkage connected to an alkyl chain that terminates with a chlorine atom ($-O-C{H}_2-C{H}_2-CH(Cl)-C{H}_3$). This specific arrangement—an electron-rich aromatic ring tethered to a potential electrophile—is the classic hallmark of an intramolecular Friedel-Crafts alkylation waiting to happen.

Carbocation Generation

The reaction kicks off with the introduction of anhydrous aluminum chloride ($AlC{l}_3$). As a potent Lewis acid, $AlC{l}_3$ is highly electron-deficient and actively seeks out electron pairs. It finds a willing donor in the chlorine atom at the end of our alkyl chain.

The $AlC{l}_3$ coordinates with the chlorine and abstracts it as a chloride ion ($C{l}^{-}$). As the carbon-chlorine bond breaks heterolytically, the bonding electrons leave with the chlorine, leaving behind a positively charged carbon atom. This generates a secondary ($2^{\circ }$) carbocation on the alkyl chain. This carbocation is highly electrophilic and is now perfectly positioned to attack the adjacent electron-rich benzene ring.

Now comes the critical question of regioselectivity: where will the carbocation attack the benzene ring? To answer this, we must look at the directing effects of our substituents.

The ether oxygen atom possesses lone pairs of electrons that it can donate into the benzene ring via resonance (the $+M$ effect). This makes the $-OR$ group strongly activating and an ortho/para director. However, because the electrophilic carbocation is physically tethered to the oxygen atom via a relatively short carbon chain, it simply cannot reach across the molecule to attack the para position. The geometry of the tether restricts the attack exclusively to the ortho positions.

We have two ortho positions available relative to the ether oxygen: 1. The Hindered Ortho Position: This position is sandwiched directly between the bulky ether group and the methyl group. Attacking here would require overcoming significant steric repulsion. 2. The Open Ortho Position: This position is on the opposite side, far away from the methyl group, making it sterically wide open and highly accessible.

Nature always favors the path of least resistance. The carbocation swoops in and attacks the less hindered, open ortho position. As the new carbon-carbon bond forms, a six-membered ring is created, and the aromaticity of the benzene ring is temporarily broken, forming an intermediate known as a sigma complex (or Arenium ion).

Let's look closely at this specific sigma complex. When the attack occurs at this open ortho position, the positive charge delocalizes around the ring and lands directly on the carbon atom bearing the methyl group. This creates a tertiary ($3^{\circ }$) carbocation intermediate! This intermediate is exceptionally stable because it benefits from both the inductive effect ($+I$) and the hyperconjugation provided by the attached methyl group.

Finally, to regain the immense thermodynamic stability of the aromatic system, the intermediate rapidly loses a proton ($H^{+}$). The electrons from the broken carbon-hydrogen bond flow back into the ring, restoring aromaticity.

The result is our major product: a beautifully fused bicyclic compound where the newly formed six-membered oxygen-containing ring features a methyl group situated right next to the fusion point. This perfectly matches the structure shown in Option (c).