Acidity as an Equilibrium
Both alcohols and phenols are Brönsted acids: each can donate a proton from its hydroxyl group to a base. NCERT classes them together because both react with active metals such as sodium, potassium and aluminium, liberating hydrogen and forming alkoxides or phenoxides respectively. The decisive difference appears only with a milder base: phenol dissolves in aqueous NaOH to give sodium phenoxide, whereas an alcohol does not react.
For phenol the ionisation equilibrium is written as
$$\ce{C6H5OH + H2O <=> C6H5O^- + H3O^+}$$
The position of this equilibrium is captured by the acid dissociation constant $K_\text{a}$ and, more conveniently, by $\text{p}K_\text{a} = -\log K_\text{a}$. A central rule governs every comparison in this topic:
The lower the pKa value, the stronger the acid. A higher pKa means a weaker acid.
Acidity is therefore not about how easily the O–H bond breaks in isolation, but about how stable the resulting anion is. An acid is strong when its conjugate base (here, the phenoxide or alkoxide ion) is stabilised, because stabilisation pulls the equilibrium toward dissociation. This single idea — stability of the anion drives acidity — is the lens through which every part of this topic is read.
Why Phenol Is Acidic
In phenol the –OH group is bonded directly to an sp² hybridised carbon of the benzene ring. Two structural consequences follow. First, the sp² carbon is more electronegative than an sp³ carbon, so it withdraws electron density from oxygen. This increases the polarity of the O–H bond and makes the proton easier to release. Second, and far more important, the lone pairs on oxygen are conjugated with the aromatic ring even in the neutral molecule, giving the C–O bond partial double-bond character (its length, 136 pm, is shorter than in methanol).
When phenol loses its proton, the negative charge that appears on oxygen is not trapped there. It is delocalised into the ring. This delocalisation stabilises the phenoxide ion, lowers the energy of the products and so favours ionisation. NCERT states the principle plainly: "the delocalisation of negative charge makes phenoxide ion more stable and favours the ionisation of phenol."
Acidity is decided by the anion, not the neutral molecule
Phenol itself also has resonance structures, but in the neutral molecule those structures carry a positive charge on oxygen and a negative charge in the ring — a charge-separated, higher-energy situation. In the phenoxide ion there is no such charge separation, so the resonance is far more effective. The driving force for acidity is the extra stabilisation of the anion relative to the neutral acid.
Always compare conjugate bases when ranking acids — never compare the neutral acids directly.
Resonance in the Phenoxide Ion
The phenoxide ion is described by five resonance structures. In two of them the negative charge sits on the oxygen atom; in the remaining three it is carried by the carbon atoms of the ring — specifically the two ortho positions and the one para position. The charge never lands on a meta carbon. This pattern is the key to understanding substituent effects later.
The negative charge of phenoxide is shared over oxygen and the two ortho and one para ring carbons. Spreading the charge over four atoms lowers the ion's energy and is the reason phenol ionises far more than an alcohol.
Why Alcohols Are Not Acidic
In an alcohol the –OH is bonded to an sp³ carbon of an alkyl group. When the proton is removed, the alkoxide ion is formed with the negative charge localised entirely on a single oxygen atom. There is no ring, no conjugation and therefore no way to delocalise that charge. A concentrated negative charge is high in energy and unstable, so the equilibrium lies far to the left.
Worse, alkyl groups are electron-releasing. An electron-releasing group such as −CH3 or −C2H5 pushes electron density toward oxygen, intensifying the already-localised charge and decreasing the polarity of the O–H bond. This both weakens the parent acid and destabilises the alkoxide. The result is that acid strength among alcohols falls as alkyl substitution rises. NCERT gives the order in which the bulkier, more substituted alcohols are the weakest acids — water remains a stronger acid than every common alcohol.
| Feature | Phenol → phenoxide | Alcohol → alkoxide |
|---|---|---|
| Carbon bearing –OH | sp² (aromatic, electron-withdrawing) | sp³ (alkyl) |
| Charge in the anion | Delocalised over O + ortho/para C | Localised on a single O |
| Effect of attached group | Ring withdraws / disperses charge | Alkyl group releases, intensifies charge |
| Anion stability | High (resonance stabilised) | Low |
| Result | Reacts with NaOH; acidic | No reaction with NaOH; very weakly acidic |
A second NCERT illustration confirms the ranking: when an alkoxide is added to water, the equilibrium
$$\ce{C2H5O^- + H2O <=> C2H5OH + OH^-}$$
lies to the right, showing water to be the better proton donor (stronger acid) and the alkoxide a stronger base than hydroxide. Sodium ethoxide is therefore a stronger base than sodium hydroxide.
Phenol vs Alcohol vs Water vs Carboxylic Acid
Placing the four families on one pKa scale is a frequent NEET requirement. The ordering, from weakest to strongest acid, is alcohol < water < phenol < carboxylic acid. The driving factor in each case is the degree to which the conjugate base disperses its negative charge.
| Compound | Formula | pKa (approx.) | Why |
|---|---|---|---|
| Ethanol | $\ce{C2H5OH}$ | 15.9 | Localised charge; alkyl +I destabilises alkoxide |
| Water | $\ce{H2O}$ | 15.7 | Localised charge, but no destabilising alkyl group |
| Phenol | $\ce{C6H5OH}$ | 10.0 | Phenoxide resonance over ring |
| Acetic acid | $\ce{CH3COOH}$ | 4.76 | Carboxylate charge shared equally over two O atoms |
Two comparisons deserve emphasis. Phenol is roughly a million times more acidic than ethanol — the gap from pKa 15.9 to 10.0 spans nearly six powers of ten — purely because of phenoxide delocalisation. Yet phenol is still much weaker than a carboxylic acid. In the carboxylate ion the charge is shared equally over two electronegative oxygen atoms by two equivalent resonance structures, whereas in phenoxide most of the resonance burden falls on less electronegative ring carbons. Equal sharing over oxygens beats partial sharing onto carbon, so the carboxylic acid wins.
The acidity of phenol is the gateway to its chemistry — phenoxide is the reactive species in Kolbe's and the Williamson ether synthesis. See Reactions of Phenols.
Effect of Substituents
Once the resonance picture is fixed, the influence of any substituent on the ring follows from a single principle: whatever stabilises the phenoxide ion increases acidity; whatever destabilises it decreases acidity. Substituents act through two electronic effects — the inductive effect (–I/+I, transmitted through sigma bonds) and the resonance/mesomeric effect (–R/+R, transmitted through the conjugated system).
Electron-withdrawing groups increase acidity
Groups such as −NO2, the halogens (−X) and −CHO pull electron density away from the ring. In the phenoxide ion this disperses the negative charge further and stabilises the anion, so ionisation is more favourable and acidity rises. The nitro group is the textbook example: NCERT notes that "the presence of electron withdrawing groups such as nitro group enhances the acidic strength of phenol," and that the effect is most pronounced at the ortho and para positions.
Electron-donating groups decrease acidity
Groups such as −CH3 (alkyl) and −OCH3 push electron density into the ring. This intensifies the negative charge on the phenoxide ion and destabilises it, opposing ionisation. NCERT states that "electron releasing groups, such as alkyl groups, in general, do not favour the formation of phenoxide ion resulting in decrease in acid strength. Cresols, for example, are less acidic than phenol." The three cresols sit at pKa 10.1–10.2, just above phenol's 10.0.
Acid strength rises (pKa falls) as electron-withdrawing power increases: an electron-donating methyl group (p-cresol) leaves phenol slightly weaker, while one, then three nitro groups make the molecule progressively far stronger. Picric acid (2,4,6-trinitrophenol) is a strong acid.
Ortho, Meta and Para Effects
The position of an electron-withdrawing group decides how it can act. Recall that in the phenoxide ion the negative charge resides on oxygen and on the ortho and para carbons. A group placed at ortho or para is directly conjugated with a charge-bearing carbon, so its –R (resonance) effect can pull that charge onto itself — for nitro, onto the nitro oxygens. A group at the meta position sits on a carbon that never bears the charge, so it cannot use resonance; only the weaker, distance-attenuated –I effect operates.
This is why the nitrophenols rank as they do: p-nitrophenol (pKa 7.1) and o-nitrophenol (pKa 7.2) are far stronger acids than m-nitrophenol (pKa 8.3), which is in turn stronger than phenol (10.0). The 2022 NEET statement question turns precisely on this: the three mononitrophenols do not have equal acidity.
In p-nitrophenoxide the negative charge that reaches the para carbon is delocalised further onto the oxygens of the nitro group. This additional resonance — available only at ortho and para — is why those isomers are markedly more acidic than the meta isomer.
"All three nitrophenols are equally acidic" — false
A common distractor claims the three mononitrophenols have the same acidity because each has one –NO2 group. They do not. The ortho and para isomers benefit from the –R (resonance) effect of the nitro group; the meta isomer can only exert the –I effect. Acidity therefore runs para ≈ ortho > meta, with all three more acidic than phenol.
Position matters: –R works only from ortho/para; from meta a group is reduced to its inductive effect alone.
Acidity-Ordering Worked Examples
Arrange in increasing order of acid strength: propan-1-ol, 2,4,6-trinitrophenol, 3-nitrophenol, 3,5-dinitrophenol, phenol, 4-methylphenol. (NCERT Example 7.4)
Approach. Rank by how strongly the conjugate base is stabilised. An alcohol has no ring resonance, so it is weakest. A methyl group donates electrons, so 4-methylphenol is weaker than phenol. Each nitro group withdraws electrons and strengthens acidity; more nitro groups (and ortho/para placement) mean greater strength.
Answer. propan-1-ol < 4-methylphenol < phenol < 3-nitrophenol < 3,5-dinitrophenol < 2,4,6-trinitrophenol.
Why is ortho-nitrophenol more acidic than ortho-methoxyphenol? (NCERT Exercise 7.15)
Answer. The nitro group is electron-withdrawing (–I and –R), so it disperses and stabilises the negative charge of the phenoxide ion, increasing acidity. The methoxy group is electron-releasing (its lone pair feeds the ring by +R), so it intensifies the charge on the anion and destabilises it, decreasing acidity. Hence o-nitrophenol is the stronger acid.
Among phenol, o-nitrophenol, m-nitrophenol and p-nitrophenol, which is the weakest acid, and why?
Answer. Phenol (pKa 10.0) is the weakest, as it carries no electron-withdrawing substituent. Of the three nitrophenols the meta isomer (pKa 8.3) is weakest because the nitro group there acts by the –I effect only; the ortho (7.2) and para (7.1) isomers gain extra resonance stabilisation.
Acidity of Phenols in one screen
- Acidity is set by anion stability: a more stabilised conjugate base means a stronger acid and a lower pKa.
- Phenol is acidic because the phenoxide ion delocalises its charge over oxygen and the ortho/para ring carbons (5 resonance structures); alkoxide charge stays localised, so alcohols are barely acidic.
- Acidity order: alcohol (≈15.9) < water (15.7) < phenol (10.0) < carboxylic acid (≈4.76).
- Electron-withdrawing groups (–NO2, –X, –CHO) raise acidity; electron-donating groups (–CH3, –OCH3) lower it.
- –NO2 at ortho/para uses both –I and –R; at meta only –I. So nitrophenol acidity: para ≈ ortho > meta > phenol.
- Picric acid (2,4,6-trinitrophenol) is a strong acid — three nitro groups stabilise the picrate ion strongly.