The observed pattern of electrophilic substitution in compound Z (2-tert-butyl-4-methylphenol) can be explained by the steric effect of the tert-butyl group.

Let's analyze the structure and reactivity:

Step 1: Identify the directing effects
The phenolic -OH group is a strong ortho/para-directing group due to its electron-donating resonance effect. The tert-butyl group ((CH₃)₃C-) is also an ortho/para-directing group due to its electron-donating inductive effect. Both groups direct electrophiles to the ortho and para positions relative to themselves.

Step 2: Analyze the substitution pattern
The observed reactions show:

• Bromination occurs at position 6 (relative to OH, which is ortho to OH and para to tert-butyl).
• Chlorination occurs at position 6.
• Nitration occurs at position 6.

Position 6 is sterically less hindered compared to positions adjacent to the bulky tert-butyl group.

Step 3: Understand the steric effect
The tert-butyl group is very bulky (three methyl groups attached to a central carbon). This creates significant steric hindrance around the ortho positions to it (positions 3 and 5). Electrophiles avoid these positions due to steric repulsion, even though both electronic effects favor substitution at these positions.

Step 4: Compare with position 6
Position 6 is ortho to the OH group and para to the tert-butyl group. It is electronically activated by both groups and is sterically accessible because it is not adjacent to the bulky tert-butyl group. Thus, all electrophiles preferentially substitute at position 6.

Conclusion: The pattern is primarily governed by the steric effect of the tert-butyl group, which blocks substitution at positions 3 and 5, leaving position 6 as the most favorable site.

Related Topics

Electrophilic Aromatic Substitution: This is a fundamental reaction where an electrophile replaces a hydrogen atom on an aromatic ring. The rate and position of substitution are influenced by substituents already present on the ring.

Steric Effects: In organic chemistry, steric effects arise from the spatial arrangement of atoms. Bulky groups can hinder approach to reactive sites, overriding electronic preferences.

Directing Effects: Substituents on benzene rings influence the orientation of incoming electrophiles. Electron-donating groups (e.g., -OH, -CH₃) are ortho/para-directing, while electron-withdrawing groups (e.g., -NO₂, -COOH) are meta-directing.

Key Formulae/Concepts

No specific mathematical formulae apply here, but understanding these concepts is crucial:

• Steric Hindrance: The prevention or retardation of chemical reactions due to the spatial arrangement of large groups.
• Ortho/Para Ratio: In substituted benzenes, steric effects often reduce the amount of ortho substitution when bulky groups are present ortho to the directing group.