Out of chlorobenzene and benzyl chloride, which one gets easily hydrolysed by aqueous NaOH and why?

When comparing chlorobenzene and benzyl chloride in terms of their susceptibility to hydrolysis by aqueous NaOH, benzyl chloride is the clear winner. The reason for this lies in the structure and reactivity of the two compounds.

Understanding the Structures

Chlorobenzene is an aromatic compound where the chlorine atom is directly bonded to a benzene ring. This bond is quite stable due to the resonance stabilization of the aromatic system. In contrast, benzyl chloride has a chlorine atom attached to a carbon that is part of a benzyl group, which is a -CH2- group connected to a benzene ring.

Reactivity of Benzyl Chloride

Benzyl chloride is more reactive towards nucleophiles like hydroxide ions (OH-) because the carbon atom bonded to chlorine is sp³ hybridized. This means it can undergo nucleophilic substitution reactions more readily. When benzyl chloride is treated with aqueous NaOH, the hydroxide ion can easily attack the carbon atom, leading to the displacement of the chlorine atom and the formation of benzyl alcohol.

Chlorobenzene's Stability

On the other hand, chlorobenzene's carbon-chlorine bond is much stronger due to the resonance stabilization of the aromatic ring. The chlorine atom is less likely to leave because it is part of a stable aromatic system. Hydrolysis of chlorobenzene would require breaking this stable bond, which is not favored under normal conditions. Therefore, chlorobenzene is significantly less reactive towards nucleophilic substitution reactions compared to benzyl chloride.

Mechanism of Hydrolysis

The hydrolysis of benzyl chloride can be summarized in a few steps:

• Nucleophilic Attack: The hydroxide ion attacks the carbon atom bonded to the chlorine.
• Leaving Group Departure: The chlorine atom leaves, resulting in the formation of a benzyl cation.
• Formation of Product: The cation is then stabilized by the surrounding benzene ring, leading to the formation of benzyl alcohol.

Comparison of Reaction Rates

In practical terms, if you were to conduct a reaction with both compounds in the presence of aqueous NaOH, you would observe that benzyl chloride reacts rapidly, while chlorobenzene remains largely unchanged. This difference in reactivity is crucial in organic synthesis and industrial applications, where the choice of reactants can significantly impact the efficiency of a reaction.

In summary, the hydrolysis of benzyl chloride occurs more readily than that of chlorobenzene due to the structural differences and the stability of the respective carbon-chlorine bonds. Understanding these concepts not only helps in predicting the behavior of these compounds but also enhances your grasp of organic chemistry principles.