Chemistry · General Principles Of Organic Chemistry · NEET
A compound is acidic if it can give away an H+ ion easily. This happens only when the ion left behind (the conjugate base) is stable. If the negative charge on that ion is spread out over many atoms, the ion is stable, so the compound is a stronger acid. Rule: the more stable the conjugate base, the stronger the acid.
When a carboxylic acid (R-COOH) loses H+, the negative charge sits on TWO oxygen atoms equally by resonance, so it is very stable. When an alcohol (R-OH) loses H+, the charge stays on ONE oxygen with no resonance, so it is much less stable. More stable ion means the carboxylic acid is the stronger acid.
The -NO2 group is electron-withdrawing. It pulls electron density away by both the -I (inductive) and -M (resonance) effects. This helps spread out the negative charge on the phenoxide ion, making it more stable. A more stable conjugate base means a stronger acid. Three -NO2 groups (picric acid) make phenol very strongly acidic.
Acetic acid has a CH3 group. Alkyl groups like CH3 are electron-DONATING (+I effect). They push electron density toward -COO-, which increases the negative charge and makes the ion LESS stable. Formic acid has no such group, so its ion is more stable and it is the stronger acid. More or bigger alkyl groups = weaker acid.
Yes. In phenol, when H+ leaves, the negative charge on oxygen is delocalised (spread) into the benzene ring by resonance, so the phenoxide ion is stable. Water and alcohols have no ring to spread the charge, so their ions are less stable. That is why phenol is more acidic than water and alcohols but still weaker than carboxylic acids.
The -NO2 group stabilises the negative charge by the -M (resonance) effect only from the ortho and para positions. From the meta position, resonance cannot reach the oxygen, so only the weaker -I effect works. So o- and p-nitrophenol have more stable ions and are more acidic than m-nitrophenol.
An sp carbon (in alkynes, 50% s-character) holds its electrons closer to the nucleus than sp2 or sp3 carbons. So the carbanion (negative carbon) left after losing H+ is more stable. That is why acidity is alkyne > alkene > alkane: HC#CH > CH2=CH2 > CH3-CH3.
Which one is the correct order of acidity?
The correct order of decreasing acidity of the following aliphatic acids is:
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.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Carboxylic acids are more acidic than phenols. In a carboxylate ion the charge is shared over two equivalent oxygen atoms, giving strong resonance stabilisation. In phenoxide the charge is spread into the ring but sits mostly on one oxygen, so it is less stabilised. Order: carboxylic acid > phenol > water > alcohol.
Compare the stability of the conjugate base (the ion left after H+ leaves). More stable ion = stronger acid. Electron-withdrawing groups (-NO2, -Cl, ring O) stabilise the ion and raise acidity. Electron-donating groups (-CH3, -OCH3, +I alkyl groups) destabilise it and lower acidity.
Picric acid is 2,4,6-trinitrophenol. It has three -NO2 groups (two ortho, one para) that all withdraw electrons by -I and -M effects. This gives maximum stabilisation of the phenoxide ion, so it is far more acidic than phenol.
Yes. The inductive (-I) effect passes through sigma bonds and weakens quickly as the group moves farther from the -COOH or -O- centre. So a withdrawing group closer to the acidic centre raises acidity more than the same group farther away.
