Organic Compounds Containing Oxygen: Chemistry | JEE Main
The strongest acid from the following is
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Hint 1 of 2
What primarily determines the relative acidity of substituted aromatic acids and phenols?
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Step-by-step solutionView
Correct answer
Picric acid (2,4,6-trinitrophenol) is the strongest acid among the given options due to intense resonance and inductive electron withdrawal by three nitro groups stabilizing the phenoxide conjugate base.
Option analysis
Why each option works or fails
A ·
None; this is the correct choice. 2,4,6-Trinitrophenol (picric acid) has three strongly electron-withdrawing -NO2 groups at the ortho and para positions (-M and -I effects), making its conjugate base exceptionally stable (pKa ≈ 0.38), far more acidic than typical phenols or benzoic acids.
B ·
Believing carboxylic acids are always stronger acids than phenols regardless of substituents. Check the cumulative electronic effects of substituents; multiple nitro groups stabilize a phenoxide ion to such an extent that picric acid becomes stronger than unsubstituted or weakly substituted benzoic acids.
C ·
Assuming that having two nitro groups or a single activating group imparts sufficient acidity without comparing the cumulative -M stabilization of three nitro groups. Quantify or count the conjugating nitro groups; fewer nitro groups provide significantly less resonance stabilization to the negative charge of the conjugate base.
D ·
Confusing substituent placement or mistaking mononitro or unsubstituted derivatives for the most acidic compound. Compare the number and positions (ortho/para) of electron-withdrawing nitro groups that can delocalize the conjugate base's negative charge directly via the -M effect.
Reviewed route
Solution
StepWorking
01concept
Acidic strength is directly proportional to the stability of the conjugate base formed after deprotonation. Electron-withdrawing groups (EWGs) like −NO2 exert strong −M and −I effects that delocalize and disperse the negative charge, drastically increasing acidity.
02option_verdict
Contains the strongest electron-withdrawing −NO2 substitution, which maximizes negative charge dispersal via −M and −I effects, making the conjugate base exceptionally stable and this molecule the strongest acid.
03option_verdict
Possesses weaker electron-withdrawing substituents or fewer −NO2 groups compared to Option 0, resulting in less stabilization of the phenoxide/carboxylate conjugate base.
04option_verdict
Has weaker −I/−M groups or electron-donating groups that destabilize the conjugate base relative to the nitro-substituted derivative.
05option_verdict
Lacks the strong −M withdrawing power of −NO2, providing significantly less acid-strengthening effect.
✓discriminator
Look for the molecule with the maximum number of strong −M/−I electron-withdrawing groups (specifically −NO2 at ortho/para positions) which creates the strongest acidic center.
Hints that build this answer step by step
What primarily determines the relative acidity of substituted aromatic acids and phenols?
The stability of the conjugate base formed after proton loss, enhanced by electron-withdrawing groups via resonance and inductive effects.
Why is 2,4,6-trinitrophenol (picric acid) exceptionally acidic (pKa ≈ 0.38)?
Three nitro groups at the ortho and para positions strongly stabilize the phenoxide anion through both powerful -M (resonance) and -I (inductive) effects.
Why is the -NO2 group such an effective acid-strengthener?
The -NO2 group has strong electronegativity (-I effect). At ortho and para positions, it also shows powerful resonance withdrawal (-M effect). This efficiently delocalizes the negative charge of the conjugate base.