Electrophilic Substitution Pattern of Substituted Phenol

To determine the factors influencing the halogenation reactivity of compound Z ($3\text{-}tert\text{-butylphenol}$), let us analyze the structural and electronic characteristics of the molecule and the electrophiles:

1. Structure of Compound Z: Compound Z is $3\text{-}tert\text{-butylphenol}$, where the hydroxyl group ($-\text{OH}$) is at position 1 and the bulky $tert\text{-butyl}$ group ($-\text{C(CH}_3)_3$) is at position 3.

2. Electronic Influence of the Phenolic $-\text{OH}$ Group (Option C):

   • The $-\text{OH}$ group is a strong activating group due to its positive mesomeric ($+M$) resonance effect.
   • It directs electrophilic aromatic substitution to the positions $ortho$ and $para$ relative to itself, which correspond to positions 2, 4, and 6 on the benzene ring.
   • Thus, the position of electrophilic attack is primarily governed by the electronic influence of the phenolic $-\text{OH}$ group.

3. Steric Effect of the $tert\text{-Butyl}$ Group (Option B):

   • The $tert\text{-butyl}$ group ($-\text{C(CH}_3)_3$) is extremely bulky.
   • Position 2 lies directly between the $-\text{OH}$ group and the $tert\text{-butyl}$ group, making it severely sterically hindered.
   • Position 4 is also adjacent to the $tert\text{-butyl}$ group and experiences significant steric crowding compared to position 6.
   • Thus, the steric bulk of the $tert\text{-butyl}$ group restricts access to certain positions depending on the size of the incoming halogen electrophile.

4. Steric Effect of the Halogen (Option A):

   • The atomic/ionic radii of the halogens increase down the group: $\text{Cl} < \text{Br} < \text{I}$
   • Chlorine ($\text{Cl}_2$): Chlorine is small enough to overcome the steric hindrance at all activated positions (2, 4, and 6), yielding a tri-halo substituted derivative.
   • Bromine ($\text{Br}_2$): Bromine is larger, so it cannot readily attack the most hindered position 2 between $-\text{OH}$ and the $tert\text{-butyl}$ group. Thus, substitution occurs at positions 4 and 6, giving a di-halo substituted derivative.
   • Iodine ($\text{I}_2$): Iodine is the largest among the three. Due to severe steric repulsion with the $tert\text{-butyl}$ group, substitution is restricted to the least hindered position (position 4/6), yielding a mono-halo substituted derivative.
   • Hence, the steric size of the halogen directly dictates the extent of substitution (mono, di, or tri).

5. Electronic Influence of the $tert\text{-butyl}$ Group (Option D):

   • The $tert\text{-butyl}$ group exhibits a weak $+I$ (inductive) effect and lacks $\alpha\text{-hydrogens}$ required for hyperconjugation. Its electronic contribution is negligible compared to the $+M$ effect of the $-\text{OH}$ group and does not explain the differential substitution pattern across the halogens.

Therefore, the observed pattern of electrophilic substitution is governed by:

• (A) the steric effect exerted by the halogen
• (B) the steric effect exerted by the tert-butyl group
• (C) the electronic influence of the phenolic OH group

Correct Options: A, B, C