Chemistry · Alcohols, Phenols And Ethers · NEET
When phenol loses its H+, it becomes the phenoxide ion, which has a negative charge on the oxygen. A -NO2 group is electron-withdrawing: it pulls electron density away from that oxygen by both the -I effect (through the bonds) and the -R effect (through resonance). This spreads out the negative charge, so the phenoxide ion is more stable. A more stable ion means H+ leaves more easily, so acidity goes up. That is why nitrophenol is a stronger acid than plain phenol.
A -CH3 group is electron-donating (+I effect). It pushes electron density toward the ring and the oxygen. This adds MORE negative charge onto an already negative phenoxide oxygen, so the ion becomes less stable. A less stable ion means H+ leaves less easily. So cresol (methylphenol) is a weaker acid than phenol. Rule: donating groups lower acidity, withdrawing groups raise it.
The correct order is para > ortho > meta nitrophenol. The -R (resonance) withdrawal of -NO2 works strongly only from the ortho and para positions, not from meta, so meta is the weakest of the three. Between ortho and para, ortho is slightly lowered because it forms intramolecular hydrogen bonding (the -NO2 and -OH are next to each other and grab their own H). So the NEET answer is p-nitrophenol > o-nitrophenol > m-nitrophenol.
Picric acid has THREE -NO2 groups, placed at the 2, 4, and 6 positions (two ortho, one para). All three are electron-withdrawing and all sit where -R resonance can act. Together they pull a large amount of negative charge off the phenoxide oxygen, making the ion extremely stable. More withdrawing groups at ortho/para = far stronger acid. This is why picric acid is more acidic than mono-nitrophenol, phenol, or cresol.
Yes, position is very important for -R (resonance) effects. Resonance from an electron-withdrawing group only stabilises the phenoxide charge when the group is at the ortho or para position. From the meta position, resonance cannot reach the negative oxygen, so a meta group affects acidity mainly through the weaker -I effect. That is why p-nitrophenol is much stronger than m-nitrophenol even though both have one -NO2.
Step 1: Count and identify the groups. More electron-withdrawing groups (-NO2, -Cl, -CHO, -COOH) means more acidic. Step 2: Check their position; ortho/para withdrawing groups help much more than meta. Step 3: Electron-donating groups (-CH3, -OCH3, -NH2, -OH) make it less acidic. So a phenol with three ortho/para -NO2 groups (picric acid) beats one with a single -NO2, which beats plain phenol, which beats cresol (has -CH3).
Which one is the most acidic compound?
Statement I: The acidic strength of monosubstituted nitrophenol is higher than phenol because of the electron withdrawing nitro group. Statement II: o-nitrophenol, m-nitrophenol and p-nitrophenol will have the same acidic strength as they have one nitro group attached to the phenolic ring. Choose the most appropriate answer.
In the reaction C6H5OH (conc. HNO3 / conc. H2SO4) -> X + Y, where X and Y are o-nitrophenol and p-nitrophenol, the suitable method to separate products X and Y is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Electron-withdrawing groups (like -NO2, -Cl, -CN, -CHO) increase phenol acidity by stabilising the phenoxide ion. Electron-donating groups (like -CH3, -OCH3, -NH2) decrease acidity by destabilising the ion. This matters for NEET because 'which is most/least acidic' is a very common question type.
In o-nitrophenol the -OH and -NO2 groups are next to each other, so they form intramolecular hydrogen bonding (they bond to themselves). This locks up the -OH hydrogen and slightly lowers acidity. In p-nitrophenol the groups are far apart, so no such lock happens, and it stays the strongest of the two.
Yes, picric acid (2,4,6-trinitrophenol) is unusually strong for a phenol; its three -NO2 groups make it comparable to, and in fact stronger than, many carboxylic acids. For NEET, just remember it is the most acidic of the common phenol options.
Halogens like -Cl are electron-withdrawing by the -I effect, so they slightly increase acidity. For example, p-chlorophenol is more acidic than phenol but much less acidic than p-nitrophenol, because -Cl withdraws less strongly than -NO2.
Usually as 'arrange in order of acidity' or 'which is the most/least acidic', and sometimes as Statement I / Statement II reasoning. The safe strategy is: count withdrawing vs donating groups, then check ortho/para vs meta positions.
