Major Products of Nitration for Substituted Aromatic Compounds P Q and S

To determine the major products formed upon nitration of the compounds $\text{P}$, $\text{Q}$, and $\text{S}$, we analyze the directing effects and activation levels of the substituents on the aromatic rings in Electrophilic Aromatic Substitution ($\text{EAS}$).

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General Principle of Regioselectivity in EAS:

1. Activating vs. Deactivating Groups: If an aromatic ring has multiple substituents directing to different positions, the regiochemistry is governed primarily by the strongest activating group (highest electron-donating ability).
2. Substituent Strength Order:
   $\text{Strong }+M\text{ (e.g., }-\text{OH}, -\text{OCH}_3\text{)} > \text{Weak } +H/+I\text{ (e.g., }-\text{CH}_3\text{)} > \text{Deactivating } -M/-I\text{ (e.g., }-\text{COOH}, -\text{COR}\text{)}$
3. Steric Effects: For $o/p$-directing groups, the para position is typically favored over the ortho position due to steric hindrance, provided the para position is vacant.

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1. Compound P (4-hydroxybenzoic acid):

• Substituents present:

  • $-\text{OH}$ group at position 1 (Strong $+M$ group, highly activating, $o/p$-directing).
  • $-\text{COOH}$ group at position 4 (Strong $-M$ group, deactivating, $m$-directing).

• Directing Effect:

  • Since $-\text{OH}$ is strongly activating and $-\text{COOH}$ is deactivating, the orientation of the incoming nitronium ion ($\text{NO}_2^+$) is strictly controlled by the $-\text{OH}$ group.
  • The position para to $-\text{OH}$ is occupied by the $-\text{COOH}$ group.
  • Therefore, substitution takes place at the position ortho to the $-\text{OH}$ group (position 2 or 6).

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2. Compound Q (1-methoxy-4-methylbenzene / $p$-methylanisole):

• Substituents present:

  • $-\text{OCH}_3$ group at position 1 (Strong $+M$ group, strongly activating, $o/p$-directing).
  • $-\text{CH}_3$ group at position 4 (Hyperconjugative $+H$ and inductive $+I$ group, moderately activating, $o/p$-directing).

• Directing Effect:

  • Resonance activation ($+M$) by the lone pairs on the oxygen atom of $-\text{OCH}_3$ is significantly stronger than the hyperconjugative activation ($+H$) of the $-\text{CH}_3$ group.
  • Thus, the reaction is directed by the $-\text{OCH}_3$ group.
  • The position para to $-\text{OCH}_3$ is occupied by the $-\text{CH}_3$ group.
  • Therefore, nitration occurs at the position ortho to the $-\text{OCH}_3$ group.

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3. Compound S (Phenyl benzoate):

• Structure: $\text{C}_6\text{H}_5-\text{C}(=\text{O})-\text{O}-\text{C}_6\text{H}_5$

• Ring Activation Analysis:

  • Benzoyl Ring ($\text{C}_6\text{H}_5-\text{CO}-$): The ring is directly attached to a carbonyl carbon, which pulls electron density away via $-M$ and $-I$ effects. This ring is deactivated.
  • Phenoxy Ring ($-\text{O}-\text{C}_6\text{H}_5$): The ring is directly attached to an oxygen atom whose lone pairs donate electron density into the ring via $+M$ effect. This ring is strongly activated.

• Directing Effect:

  • Electrophilic attack occurs exclusively on the more electron-rich, activated phenoxy ring.
  • The ester oxygen atom acts as an $o/p$-director for the phenoxy ring.
  • Due to lower steric hindrance, attack at the para position relative to the ester oxygen atom yields the major product.

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Conclusion:

• Compound P: $\text{NO}_2$ enters ortho to $-\text{OH}$.
• Compound Q: $\text{NO}_2$ enters ortho to $-\text{OCH}_3$.
• Compound S: $\text{NO}_2$ enters para to $-\text{O}-$ on the phenoxy ring.

This set of major products corresponds to Option C.