Understanding the Reaction: Friedel-Crafts Alkylation

The given reaction is Friedel-Crafts alkylation of benzene with 1-chloro-2-methylpropane (isobutyl chloride) in the presence of anhydrous AlCl₃ catalyst.

The general reaction is: $C H_{6} + R-Cl \overset{Anhyd. AlCl₃}{\to} C H_{5} R + HCl$

Step 1: Identify the Carbocation Intermediate

The Lewis acid catalyst (AlCl₃) generates a carbocation from the alkyl halide.

1-chloro-2-methylpropane undergoes ionization:

(C H_{3})_{2} C H C H 2 Cl \overset{AlCl₃}{\to} (C H_{3})_{2} C^{+} H C H_{2} + {[Al {Cl}_{4}]}^{-}

The primary carbocation formed is unstable and immediately rearranges via a hydride shift to form a more stable tertiary carbocation.

Step 2: Carbocation Rearrangement (Hydride Shift)

A hydride ion (H⁻) shifts from the carbon adjacent to the carbocation center to stabilize it.

(C H_{3})_{2} C^{+} - C H_{3} \to (C H_{3})_{2} C H - C^{+} (C H_{3})_{2}

The final, stable carbocation is the tert-butyl carbocation, (CH₃)₃C⁺.

Step 3: Electrophilic Aromatic Substitution

The stable tertiary carbocation acts as an electrophile and attacks the electron-rich benzene ring.

Benzene + {(C H_{3})_{3} C}^{+} \to tert-Butylbenzene + H^{+}

Therefore, the major product of this reaction is tert-butylbenzene.

Step 4: Analyze the Given Options

We must find which product is not possible. The actual product is tert-butylbenzene. Any product that is not tert-butylbenzene or its isomer is not formed.

Let's evaluate the structures (from top to bottom in the question):

Option 1: Isobutylbenzene (C₆H₅-CH₂-CH(CH₃)₂). This is the direct, unrearranged product.
Option 2: sec-Butylbenzene (C₆H₅-CH(CH₃)-CH₂-CH₃). This is an isomer but not formed.
Option 3: tert-Butylbenzene (C₆H₅-C(CH₃)₃). This is the actual product.
Option 4: n-Butylbenzene (C₆H₅-CH₂-CH₂-CH₂-CH₃). This is an isomer but not formed.

Since the reaction proceeds via carbocation rearrangement, the unrearranged product (Option 1: Isobutylbenzene) is not possible.

Final Answer

The product that is not possible is Isobutylbenzene (the first option).

Related Topics & Formulae

Key Theory: Carbocation Rearrangement

In Friedel-Crafts alkylation, if the initially formed carbocation is not stable (e.g., primary), it can rearrange to a more stable carbocation (secondary or tertiary) via a hydride shift or alkyl shift.

Stability order: Tertiary > Secondary > Primary > Methyl

Important Formulae

General Friedel-Crafts Alkylation:

Ar-H + R-X \overset{Lewis Acid}{\to} Ar-R + HX

(Where Ar = Aryl group, R = Alkyl group, X = Cl, Br)

Limitations of Friedel-Crafts Alkylation

Carbocation rearrangements can occur.
Polyalkylation is common as the product is more reactive than benzene.
Does not work with deactivated benzene rings (e.g., nitrobenzene).