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MCQEasyJEE 2023Electronic Effects (Inductive, Resonance, Hyperconjugation)

JEE Chemistry 2023 Question with Solution

The strongest acid from the following is:

  • A
    Option 1 shows phenol: a benzene ring bearing an OH group at the top with no other substituent.
  • B
    Option 2 shows chlorophenol: a benzene ring bearing an OH group at the top and a Cl substituent on the ring.
  • C
    Option 3 shows methylphenol: a benzene ring bearing an OH group at the top and a CH3 substituent on the ring.
  • D
    Option 4 shows nitrophenol: a benzene ring bearing an OH group at the top and an NO2 substituent on the ring.

Answer

Correct answer:D

Step-by-step solution

Standard Method

Given: The options are substituted phenols.

Find: Which compound is the strongest acid.

The strength of an acid is determined by the stability of its conjugate base.

Electron-withdrawing groups stabilize the conjugate base after loss of H+\text{H}^+, while electron-donating groups destabilize it.

From the given options:

  • A: phenol, no substituent
  • B: -Cl\text{-Cl}, electron-withdrawing through −I-I effect
  • C: -CH3\text{-CH}_3, electron-donating through +I+I effect
  • D: -NO2\text{-NO}_2, strong electron-withdrawing through −I-I and −M-M effects

Since -NO2\text{-NO}_2 stabilizes the phenoxide ion most effectively, the nitro-substituted phenol is the strongest acid among the given compounds.

Therefore, the correct option is D.

Substituted Phenols Analysis

Comparison image showing four phenolic compounds: phenol, chlorophenol, methylphenol, and nitrophenol, indicating inductive effects of substituents on acidity.

The acid strength of a phenolic compound depends on the electron-withdrawing or electron-donating groups attached to the benzene ring. The electron-withdrawing group increases the acidity of the compound.

Among the options, the nitro group (-NO2)\text{(-NO}_2\text{)} is a strong electron-withdrawing group through the inductive effect (−I)(-I), making the compound with -NO2\text{-NO}_2 attached to the ring the most acidic.

Hence, the strongest acid is nitrophenol and the correct option is D.

Common mistakes

  • Assuming that any substituted phenol is automatically more acidic than phenol is incorrect. The effect depends on whether the substituent withdraws or donates electron density. Always compare conjugate base stability.

  • Treating -CH3\text{-CH}_3 and -NO2\text{-NO}_2 as having similar effects is wrong. -CH3\text{-CH}_3 donates electron density and reduces acidity, whereas -NO2\text{-NO}_2 strongly withdraws electron density and increases acidity.

  • Looking only at the presence of a substituent and ignoring −I-I and −M-M effects leads to an incorrect choice. The strongest acid is identified by the substituent that best stabilizes the phenoxide ion.

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