Major Product of Friedel-Crafts Reaction: n-Butylbenzene

Introduction:
The Friedel-Crafts reaction is a powerful method for introducing alkyl groups onto aromatic rings. In the presence of anhydrous aluminum chloride (AlCl3) as a catalyst, the reaction between n-butylbromide and benzene leads to the formation of n-butylbenzene as the major product. Let's explore the detailed reaction mechanism and rationale behind this outcome.

Reaction Mechanism:
1. Formation of the Lewis Acid-Base Complex:
- The first step involves the formation of a Lewis acid-base complex between AlCl3 and n-butylbromide.
- AlCl3 acts as a Lewis acid, accepting an electron pair from the bromine atom of n-butylbromide.
- This complexation enhances the electrophilic character of the alkyl halide, making it susceptible to attack by the aromatic ring.

2. Generation of the Electrophile:
- The Lewis acid-base complex undergoes a heterolytic cleavage of the carbon-bromine bond.
- This results in the formation of a carbocation, which is the electrophile in this reaction.
- The positive charge on the carbon atom is stabilized by resonance with the aluminum atom.

3. Electrophilic Attack on the Aromatic Ring:
- The electrophile, the carbocation, now reacts with the electron-rich benzene ring.
- The pi electrons of the benzene ring act as a nucleophile, attacking the carbocation.
- The electrophilic attack occurs on one of the ortho or para positions relative to the alkyl group.

4. Rearrangement and Product Formation:
- The initial electrophilic attack leads to the formation of a sigma complex.
- This intermediate undergoes a hydride shift or alkyl shift to facilitate the formation of a more stable carbocation.
- The rearranged carbocation is then attacked by another molecule of benzene, resulting in the formation of the final product.
- The major product obtained is n-butylbenzene, where the butyl group is attached to the benzene ring at the ortho or para position.

Rationale for the Major Product:
The major product in this Friedel-Crafts reaction is n-butylbenzene due to the following reasons:
- The reaction is regioselective, favoring the substitution at the ortho or para positions.
- The ortho and para positions are more accessible for electrophilic attack compared to the meta position.
- The carbocation intermediate formed after the electrophilic attack can undergo rearrangement to form a more stable carbocation before the final substitution takes place.
- The stability of the carbocation intermediate is enhanced by resonance with the aluminum atom, promoting the formation of the major product.

In conclusion, the Friedel-Crafts reaction between n-butylbromide and benzene in the presence of anhydrous AlCl3 catalyst leads to the major product of n-butylbenzene. The regioselectivity and stability factors play crucial roles in determining the outcome of this reaction.

N--butyl benzene is formed. Here n..butyl carbocation is formed.