Find the compound which undergoes nucleophilic substitution reaction exclusively by an SN1 mechanism

The correct option is Benzyl chloride.

To understand why benzyl chloride undergoes nucleophilic substitution reactions exclusively (or highly preferentially) via an S_N1 mechanism among the given options, let's break down the mechanism and analyze the structures of the carbocations formed by each compound.

1. Understanding the S_N1 Mechanism:
The S_N1 (Substitution Nucleophilic Unimolecular) mechanism is a two-step process:
• Step 1 (Rate-determining step): The leaving group (chloride ion, Cl^-) departs, leaving behind a carbocation intermediate.
• Step 2: The nucleophile attacks the carbocation to form the final substituted product.
Because the first step involves the formation of a carbocation, the rate of an S_N1 reaction is directly proportional to the stability of the resulting carbocation intermediate. The more stable the carbocation, the lower the activation energy for its formation, and the more favored the S_N1 pathway becomes.

2. Analyzing the Carbocation Stability of the Options:

• Benzyl chloride (C₆H₅CH₂Cl):
Upon ionization, benzyl chloride loses a chloride ion to form the benzyl carbocation:
$C_6{H}_5-C{H}_2-\mathrm{Cl}\to {[C_6{H}_5-C{H}_2]}^{+}+{\mathrm{Cl}}^{-}$
The positive charge on the benzylic carbon is directly adjacent to the π-electron system of the benzene ring. This allows the positive charge to be delocalized throughout the aromatic ring via resonance. Because the positive charge is spread over multiple carbon atoms (ortho and para positions), the benzyl carbocation is exceptionally stable, which drives the reaction to proceed rapidly and exclusively through the S_N1 pathway.

• Chlorobenzene (C₆H₅Cl):
In chlorobenzene, the chlorine atom is directly attached to an sp² hybridized carbon of the aromatic ring. The lone pairs on chlorine undergo resonance with the benzene ring, giving the C-Cl bond double-bond character. This makes the bond extremely strong and difficult to break. Furthermore, the resulting phenyl carbocation (if formed) would be highly unstable because the positive charge is on an sp² hybridized carbon and cannot be stabilized by resonance. Thus, chlorobenzene does not undergo S_N1 (or even S_N2) reactions under normal conditions.

• Ethyl chloride (CH₃CH₂Cl):
Ionization of ethyl chloride yields the ethyl carbocation (a primary carbocation, CH₃CH₂⁺). Primary carbocations are highly unstable and lack resonance stabilization. Consequently, ethyl chloride undergoes substitution reactions almost exclusively via the S_N2 mechanism, which does not require a carbocation intermediate.

• Isopropyl chloride ((CH₃)₂CHCl):
Isopropyl chloride forms the isopropyl carbocation (a secondary carbocation, (CH₃)₂CH⁺) upon ionization. While more stable than a primary carbocation due to inductive effects and hyperconjugation from the two methyl groups, it is significantly less stable than the resonance-stabilized benzyl carbocation. Secondary halides typically undergo substitution via a mixture of S_N1 and S_N2 mechanisms depending heavily on reaction conditions (solvent, nucleophile strength, temperature).

Conclusion:
Among the given choices, Benzyl chloride is unique in its ability to form an exceptionally stable, resonance-stabilized benzylic carbocation intermediate. This makes it undergo nucleophilic substitution exclusively via the S_N1 mechanism.