Chemistry · Alcohols, Phenols And Ethers · NEET
When phenol loses H+, it becomes the phenoxide ion. The extra negative charge does not stay on one oxygen. It moves into the benzene ring and spreads over several carbons by resonance. A spread-out charge is a stable charge. Because the phenoxide ion is stable, phenol lets go of its H+ easily, so it acts as an acid. In an alcohol like ethanol, the alkoxide ion has no ring to share the charge, so the charge stays fully on oxygen and the ion is unstable. That is why alcohols are much weaker acids than phenol.
The phenoxide ion is more stable because of resonance. The negative charge is delocalised (shared) over the oxygen and the ortho and para carbons of the ring. This gives several resonance structures, so no single atom carries the full charge. The alkoxide ion (from an alcohol) has only one structure. The full negative charge sits on the oxygen, and the electron-pushing alkyl group even makes it worse. More stable conjugate base = stronger acid, so phenol wins.
Phenol has an -OH group, so it looks like an alcohol, but the -OH is attached directly to a benzene ring. This makes it behave differently. Phenol is a weak acid (pKa about 10). It can react with strong base NaOH to give sodium phenoxide. Ordinary alcohols do NOT react with NaOH because they are even weaker acids. So for NEET: phenol = weak acid that reacts with NaOH; alcohol = much weaker, does not react with NaOH.
Yes. Phenol reacts with sodium hydroxide (NaOH) to form sodium phenoxide and water, because phenol is acidic enough. Ethanol does not react with NaOH because it is too weak an acid. The difference again comes from stability of the conjugate base: phenoxide is resonance-stabilised, ethoxide is not. This simple test (reacts with NaOH or not) is a favourite NEET point to separate phenols from alcohols.
Alkyl groups (like -CH3, -C2H5) push electrons toward the oxygen through the +I (inductive) effect. This increases the electron density on the alkoxide oxygen and makes the negative charge even more concentrated and unstable. A less stable conjugate base means the alcohol holds its H+ tightly, so it is a weaker acid. In phenol there is no such electron-pushing; instead the ring pulls charge away and stabilises it.
Acidity is relative. Carboxylic acids are stronger than phenol because the carboxylate ion spreads its charge over two oxygen atoms (very effective). Phenol is weaker than carboxylic acids but stronger than water and alcohols, because phenoxide resonance is real but less powerful than carboxylate resonance. So the NEET order to remember is: carboxylic acid > phenol > water > alcohol.
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 correct option.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Carboxylic acid > phenol > water > alcohol. Carboxylate spreads charge over two oxygens (strongest), phenoxide spreads it into the ring (medium), water is neutral-ish, and alkoxide from alcohol is the least stable, so alcohol is the weakest acid.
Phenol has a pKa around 10, while ethanol has a pKa around 16. A lower pKa means a stronger acid, so phenol is far more acidic than ethanol.
Phenol gives a violet/purple colour with neutral FeCl3 solution, which confirms a phenolic -OH. Simple alcohols do not give this colour. It is an identification test, not a direct acidity test, but NEET often links it to phenols.
Electron-withdrawing groups (-NO2, -Cl, -CHO) at ortho/para pull charge away, stabilise the phenoxide, and increase acidity. Electron-donating groups (-CH3, -OCH3, -NH2) push charge in, destabilise the phenoxide, and decrease acidity. This is covered next in 'How Nitro and Other Groups Change Phenol's Acidity'.
In neutral phenol, oxygen already holds its H, so pushing its lone pair into the ring builds up positive charge on oxygen, which is unfavourable. After losing H+, the phenoxide has a negative charge to give away, so resonance into the ring is now favourable and strongly stabilising. That is why the ION is much more resonance-stabilised than the molecule.
