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Q6.22(iii):
What happens when (iii) chlorobenzene is subjected to hydrolysis,

Solution :

Given Variables & Chemical Context

The reaction involves the attempted nucleophilic aromatic substitution ($S_NAr$) of chlorobenzene ($C_6H_5Cl$) using a hydroxide ion ($OH^-$), typically in the form of an aqueous base (e.g., $NaOH$ or $KOH$).

Step 1: Analyzing the C–Cl Bond Stability

In chlorobenzene, the chlorine atom is directly attached to the $sp^2$ hybridized carbon of the benzene ring. This bond exhibits partial double bond character due to resonance. The lone pair of electrons on the chlorine atom undergoes delocalization into the benzene ring [Resonance Effect].

The stabilization energy and the increased s-character of the carbon atom ($sp^2$ vs $sp^3$) result in a significantly shorter and stronger $C-Cl$ bond compared to haloalkanes. Consequently, the bond dissociation energy is higher, and the carbon center is less susceptible to nucleophilic attack.

Step 2: Thermodynamic and Kinetic Barriers

For the hydrolysis to occur under standard laboratory conditions (e.g., boiling with aqueous $NaOH$), the reaction is kinetically inert. The transition state, which would involve the formation of a negatively charged Meisenheimer complex, is highly unstable due to the electron-rich nature of the benzene ring, which repels the incoming nucleophile ($OH^-$).

Step 3: Required Reaction Conditions

To overcome the electronic and resonance barriers, extreme conditions are required to force the hydrolysis, as historically demonstrated in the Dow Process:

  • Temperature: $623\ K$
  • Pressure: $300\ atm$
  • Reagent: Aqueous $NaOH$

Step 4: Chemical Equation

The reaction proceeds via the formation of a phenoxide ion intermediate, which is subsequently protonated to yield phenol:

$C_6H_5Cl + 2NaOH \xrightarrow{623\ K, 300\ atm} C_6H_5ONa + NaCl + H_2O$

$C_6H_5ONa + H^+ \rightarrow C_6H_5OH + Na^+$

Visual Representation of the Reaction Coordinate

Potential Energy Reaction Coordinate Chlorobenzene Phenol

Analytical Summary

Under ordinary conditions, chlorobenzene does not undergo hydrolysis because the $C-Cl$ bond is strengthened by the resonance effect and the $sp^2$ hybridization of the aryl carbon. Hydrolysis is only achievable at high temperature and high pressure conditions, converting the aryl halide into sodium phenoxide, which upon acidification, yields phenol ($C_6H_5OH$).

Final Solution: Chlorobenzene is resistant to hydrolysis under normal conditions; however, under extreme conditions ($623\ K$ and $300\ atm$ pressure), it reacts with aqueous sodium hydroxide to produce sodium phenoxide, which upon treatment with an acid, forms phenol.


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