Q.20

The IR stretching frequency of the carbonyl (C=O) group of a typical saturated ketone

is 1715 cm-1 . The IR stretching frequencies for the carbonyl groups present in three

different acetophenone (methyl phenyl ketone) derivatives are given. Match the

molecules in Group I with their corresponding frequencies in Group II

(A) P-1, Q-2, R-3

(B) P-2, Q-3, R-1

(C) P-3, Q-1, R-2

(D) P-3, Q-2, R-1

IR Stretching Frequency in Acetophenone Derivatives: Matching Guide with Correct Answer

Acetophenone derivatives show varied carbonyl IR stretching frequencies due to substituent effects on the C=O bond. The correct matching for p-amino, p-nitro, and p-methoxy acetophenones aligns with electron-donating and withdrawing influences compared to the 1715 cm⁻¹ baseline of saturated ketones.

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Group I and II Details

Group I lists para-substituted acetophenones:

• P: p-amino acetophenone (–NH₂ group)

• Q: p-nitro acetophenone (–NO₂ group)

• R: p-methoxy acetophenone (–OCH₃ group)

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Group II provides frequencies:

• 1: 1677 cm⁻¹

• 2: 1700 cm⁻¹

• 3: 1684 cm⁻¹

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All frequencies are lower than 1715 cm⁻¹ due to conjugation with the phenyl ring, which weakens the C=O bond through resonance.

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Substituent Effects Explained

Electron-donating groups (EDG) like –NH₂ and –OCH₃ enhance resonance donation to the carbonyl, further lowering frequency by increasing C=O bond length. Electron-withdrawing groups (EWG) like –NO₂ reduce this resonance, strengthening the bond and raising frequency closer to the aliphatic value.

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• p-Amino (–NH₂, strong EDG via lone pair resonance): Lowest frequency at 1677 cm⁻¹ (1).

• p-Methoxy (–OCH₃, moderate EDG via inductive/resonance): 1684 cm⁻¹ (3).

• p-Nitro (–NO₂, strong EWG): Highest at 1700 cm⁻¹ (2).

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Options Analysis

Correct Answer

Option (A) P-1, Q-2, R-3 is correct, as confirmed by standard IR spectroscopy principles for conjugated ketones.