Concept: Electrophilic Aromatic Substitution (EAS) and Friedel-Crafts Alkylation

This reaction involves an aromatic compound (benzene ring) reacting with an alkyl halide in the presence of a Lewis acid catalyst (AlCl₃). This is a classic example of Friedel-Crafts alkylation, a type of electrophilic aromatic substitution.

Step-by-Step Mechanism:

Step 1: Generation of the Electrophile

The Lewis acid AlCl₃ coordinates with the chlorine atom of the alkyl chloride (CH₃-CH₂-Cl), polarizing the C-Cl bond. This leads to the formation of a carbocation electrophile.

$\mathrm{CH}{3}_{}-\mathrm{CH}{2}_{}-\mathrm{Cl}+\mathrm{AlCl}{3}_{}\to [\mathrm{CH}{3}_{}\mathrm{CH}{2}_{}{]}^{+}{\mathrm{AlCl}}^4-$

The electrophile generated is an ethyl carbocation (CH₃CH₂⁺).

Step 2: Electrophilic Attack

The strong electrophile (CH₃CH₂⁺) attacks the electron-rich benzene ring. This forms a resonance-stabilized carbocation intermediate known as a sigma complex.

Step 3: Loss of a Proton

A base (often the [AlCl₄]⁻ ion generated in step 1) deprotonates the sigma complex. This restores the aromaticity of the benzene ring and yields the final substituted product: ethylbenzene.

$\text{Benzene}+{\mathrm{CH}}^3\mathrm{CH}{2}_{}{\to }^{\mathrm{AlCl}3}C{6}_{}H{5}_{}\mathrm{CH}{2}_{}\mathrm{CH}{3}_{}+H^{+}$

Final Answer: The major product is Ethylbenzene. Among the given options, it is the structure with a benzene ring bonded to a -CH₂CH₃ group (the first option).

Related Topics & Formulae

1. Electrophilic Aromatic Substitution (EAS): A reaction where an atom (usually H) attached to an aromatic ring is replaced by an electrophile. The general mechanism involves two main steps: (a) electrophilic attack to form a sigma complex and (b) loss of a proton.

2. Friedel-Crafts Reactions: A subset of EAS reactions that use Lewis acid catalysts to create powerful electrophiles.

• Friedel-Crafts Alkylation: Introduces an alkyl group onto a benzene ring.
  General Reaction: $\mathrm{Ar-H}+\mathrm{R-X}\stackrel{\mathrm{AlX}3}{\to }\mathrm{Ar-R}+\mathrm{HX}$
• Friedel-Crafts Acylation: Introduces an acyl group (R-C=O) onto a benzene ring to form a ketone.

3. Limitations of Friedel-Crafts Alkylation:

• The ring must be more reactive than a halogenobenzene. Strongly deactivated rings (e.g., nitrobenzene) do not react.
• Carbocation rearrangements can occur if a primary carbocation is formed.
• Polyalkylation can be a problem because the alkyl group introduced is activating, making the product more reactive than the starting material.