Properties Ammonia is a colourless gas with a pungent odour. Its freezing and boiling points are 198.4 and 239.7 K respectively. In the solid and liquid states, it is associated through hydrogen bonds as in the case of water and that accounts for its higher melting and boiling points than expected on the basis of its molecular mass. The ammonia molecule is trigonal pyramidal with the nitrogen atom at the apex. It has three bond pairs and one lone pair of electrons as shown in the structure. Ammonia gas is highly soluble in water. Its aqueous solution is weakly basic due to the formation of OH– ions. NH3(g) + H2O(l) ⇌ NH4+ (aq) + OH– (aq) … On a large scale, ammonia is manufactured by Haber’s process. N2(g) + 3H2(g) ⇌ 2NH3(g); ∆fH0 = – 46.1 kJ mol–1 In accordance with Le Chatelier’s principle, high pressure would favour the formation of ammonia. The optimum conditions for the production of ammonia are a pressure of 200 × 10^5 Pa (about 200 atm), a temperature of ~ 700 K and the use of a catalyst such as iron oxide with small amounts of K2O and Al2O3 to increase the rate of attainment of equilibrium. The flow chart for the production of ammonia is shown in Fig. 7.1. Earlier, iron was used as a catalyst with molybdenum as a promoter. … The presence of a lone pair of electrons on the nitrogen atom of the ammonia molecule makes it a Lewis base. It donates the electron pair and forms linkage with metal ions and the formation of such complex compounds finds applications in detection of metal ions such as Cu2+, Ag+: Cu2+ (aq) + 4 NH3(aq) ⇌ [Cu(NH3)4]2+(aq)
-- NCERT Class 12 Chemistry (pre-2023 edition), Chapter 7, p. 176Phenol Acidic Nature
Phenol Acidic Nature, explained for NEET
Why phenol is acidic — and why NEET keeps testing the reasoning behind it.
A common confusion on exam day: students know phenol is acidic but pick the wrong explanation for why, or they misjudge acidity order when substituents enter the picture.
The core fact. Phenol (C₆H₅OH) has a pKa of approximately 10, making it roughly 10⁶ times more acidic than a typical aliphatic alcohol (pKa 16–18). It is, however, a weaker acid than carboxylic acids (pKa 4–5). Phenol turns blue litmus red slowly and reacts with NaOH but does NOT react with NaHCO₃ — that bicarbonate test distinguishes phenols from carboxylic acids. (NCERT Class 12 Chemistry Chapter 7, Part 2, page 206.)
Why the acidity gap? When phenol loses its O–H proton, the resulting phenoxide ion (C₆H₅O⁻) is stabilised by resonance: the negative charge delocalises into the benzene ring across five contributing structures. In an aliphatic alkoxide (like CH₃CH₂O⁻), no such delocalisation is available — the charge sits localised on oxygen. Greater stabilisation of the conjugate base means greater acidity of the parent compound.
Substituent effects — the NEET trap zone. Electron-withdrawing groups (–NO₂, –Cl, –CN) on the ring further stabilise phenoxide → increase acidity (lower pKa). Electron-donating groups (–CH₃, –OCH₃, –NH₂) destabilise phenoxide → decrease acidity (higher pKa). Position matters: para-nitrophenol is more acidic than ortho-nitrophenol (intramolecular hydrogen bonding in the ortho isomer partially stabilises the undissociated form). The pattern NEET pattern: phenol electrophilic substitution has appeared in four NEET papers (2021–2025), often asking you to rank substituted phenols by acid strength.
Watch-out. Don't confuse resonance stabilisation of phenoxide (which explains acidity) with the activating effect of –OH on electrophilic substitution (which explains ortho-para directing). They share the same resonance arrows but answer different questions.
Can you answer these Phenol Acidic Nature MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Phenol reacts with aqueous NaOH but does NOT react with aqueous NaHCO₃. What does this indicate about phenol's acid strength?
Show answer and why every option is right or wrong
Answer: B. B is correct. Phenol (pKa ≈ 10) reacts with NaOH (strong base) but not NaHCO₃, indicating it is weaker than carbonic acid (pKa₁ ≈ 6.35) but stronger than water (pKa ≈ 15.7). NCERT Class 12 Chemistry Chapter 7, Part 2, page 206.
Why A is wrong: A is wrong because phenol does NOT react with NaHCO₃. If it were stronger than carbonic acid, it would displace CO₂ from bicarbonate, which it does not.
Why C is wrong: C is wrong because phenol IS acidic — it reacts with NaOH and has pKa ≈ 10, well below water's pKa of 15.7. It is distinctly more acidic than water.
Why D is wrong: D is wrong because phenol (pKa ≈ 10) and carbonic acid (pKa₁ ≈ 6.35) differ by nearly 4 pKa units — they are not equal in strength.
The pKa of phenol is approximately:
Show answer and why every option is right or wrong
Answer: C. C is correct. Phenol has pKa ≈ 10, as stated in NCERT Class 12 Chemistry Chapter 7, Part 2, page 206. This places it between carboxylic acids (pKa 4–5) and aliphatic alcohols (pKa 16–18).
Why A is wrong: A is wrong because pKa 4–5 corresponds to carboxylic acids (e.g. acetic acid, pKa 4.76), not phenol.
Why B is wrong: B is wrong because pKa 16–18 corresponds to aliphatic alcohols (e.g. ethanol). Phenol is roughly 10⁶ times more acidic than these.
Why D is wrong: D is wrong because pKa 25 corresponds to very weak carbon acids (e.g. acetylene). Phenol is far more acidic.
Which of the following does NOT react with aqueous NaHCO₃ to liberate CO₂?
Show answer and why every option is right or wrong
Answer: B. B is correct. Phenol (pKa ≈ 10) is too weak an acid to decompose NaHCO₃. Carboxylic acids (options A, C, D; pKa 4–5) are strong enough to displace carbonic acid and release CO₂.
Why A is wrong: A is wrong because acetic acid (pKa 4.76) is a stronger acid than carbonic acid and readily reacts with NaHCO₃ to give CO₂.
Why C is wrong: C is wrong because benzoic acid (pKa 4.20) is a carboxylic acid and reacts with NaHCO₃ to release CO₂.
Why D is wrong: D is wrong because formic acid (pKa 3.75) is a carboxylic acid and readily liberates CO₂ from NaHCO₃.
Phenol is more acidic than ethanol. The primary reason is:
Show answer and why every option is right or wrong
Answer: C. C is correct. Phenoxide ion stability arises from resonance: the lone pair on oxygen delocalises into the ring over five contributing structures. This lowers the energy of the conjugate base, making phenol a stronger acid. NCERT Class 12 Chemistry Chapter 7, Part 2, page 206.
Why A is wrong: A is wrong because bond length difference is not the primary factor. Acidity depends on conjugate base stability, not O–H bond length alone.
Why B is wrong: B is wrong because the phenyl group is electron-withdrawing by resonance (not electron-donating). Its –I effect is weak; the dominant effect is resonance stabilisation of the phenoxide anion.
Why D is wrong: D is wrong because molecular weight has no bearing on acid strength. Acidity depends on the stability of the conjugate base.
Arrange the following in order of increasing acid strength: ethanol, phenol, p-nitrophenol.
Show answer and why every option is right or wrong
Answer: A. A is correct. Ethanol (pKa ≈ 16) < phenol (pKa ≈ 10) < p-nitrophenol (pKa ≈ 7.15). The –NO₂ group at the para position is a strong electron-withdrawing group that further stabilises the phenoxide ion through extended resonance and –I effect. NCERT Class 12 Chemistry Chapter 7, Part 2, page 206.
Why B is wrong: B is wrong because it reverses the entire order. Ethanol is the weakest acid (highest pKa), not the strongest. The electron-withdrawing –NO₂ group increases acidity, not decreases it.
Why C is wrong: C is wrong because phenol (pKa ≈ 10) is more acidic than ethanol (pKa ≈ 16), not less. Resonance stabilisation of phenoxide gives phenol substantially greater acidity.
Why D is wrong: D is wrong because p-nitrophenol is MORE acidic than phenol, not less. The –NO₂ group withdraws electron density from the ring, stabilising the conjugate base further. (Trap: ignoring resonance-vs-induction balance for substituent effects.)
Among the following substituted phenols, which is the LEAST acidic?
Show answer and why every option is right or wrong
Answer: A. A is correct. The –OCH₃ group is an electron-donating group (+M effect dominates). It increases electron density on the ring, destabilises the phenoxide ion, and thus decreases acidity. p-Methoxyphenol (pKa ≈ 10.2) is less acidic than even unsubstituted phenol (pKa ≈ 10.0). NCERT Class 12 Chemistry Chapter 7, Part 2, page 206.
Why B is wrong: B is wrong because –Cl is an electron-withdrawing group (–I effect dominates over weak +M), stabilising phenoxide and making p-chlorophenol (pKa ≈ 9.4) more acidic than phenol itself.
Why C is wrong: C is wrong because –NO₂ is a strong electron-withdrawing group that greatly stabilises phenoxide, making p-nitrophenol (pKa ≈ 7.15) one of the most acidic substituted phenols in this list.
Why D is wrong: D is wrong because unsubstituted phenol (pKa ≈ 10.0) is more acidic than p-methoxyphenol. The electron-donating –OCH₃ group destabilises the phenoxide ion, making the methoxy derivative the least acidic.
A student argues that phenol is acidic because the –OH group directly donates its lone pair to the ring, weakening the O–H bond. Which statement best corrects this reasoning?
Show answer and why every option is right or wrong
Answer: D. D is correct. Acid strength is determined by the thermodynamic stability of the conjugate base relative to the acid. Phenoxide is stabilised by charge delocalisation across the ring. While lone-pair donation from oxygen into the ring does occur in phenol itself, acidity is properly explained by comparing phenoxide stability to alkoxide stability, not by invoking O–H weakening. NCERT Class 12 Chemistry Chapter 7, Part 2.
Why A is wrong: A is wrong because although lone-pair donation does occur, acidity is not explained by O–H bond weakening. The correct explanation centres on conjugate base (phenoxide) stabilisation through resonance.
Why B is wrong: B is wrong because the –OH group absolutely does participate in resonance with the ring — its lone pairs conjugate with the π system. This resonance is precisely what stabilises phenoxide and explains acidity.
Why C is wrong: C is wrong because the benzene ring in phenol is actually electron-rich (activated by the –OH group). The ring does not pull off the proton. Acidity arises from resonance stabilisation of the phenoxide anion after deprotonation.
Consider the following compounds:
(i) 2,4-Dinitrophenol
(ii) p-Cresol (4-methylphenol)
(iii) Phenol
(iv) p-Nitrophenol
The correct order of increasing acid strength is:
Show answer and why every option is right or wrong
Answer: B. B is correct. p-Cresol (–CH₃ is electron-donating, destabilises phenoxide, pKa ≈ 10.3) < phenol (pKa ≈ 10.0) < p-nitrophenol (one –NO₂, pKa ≈ 7.15) < 2,4-dinitrophenol (two –NO₂ groups, pKa ≈ 4.09). Each additional electron-withdrawing group stabilises the phenoxide ion further, lowering pKa. The multi-step reasoning: classify each substituent's electronic effect, then rank by cumulative stabilisation of conjugate base.
Why A is wrong: A is wrong because it places phenol as less acidic than p-cresol. The –CH₃ group is electron-donating (+I), making p-cresol LESS acidic than unsubstituted phenol, not more.
Why C is wrong: C is wrong because it completely reverses the order. 2,4-Dinitrophenol is the MOST acidic in this set (two strong EWGs), not the least. Electron-withdrawing groups increase acidity by stabilising the phenoxide ion.
Why D is wrong: D is wrong because it places p-nitrophenol as less acidic than phenol. The –NO₂ group is strongly electron-withdrawing and significantly increases acidity (pKa drops from 10 to ~7.15).
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Phenol Acidic Nature: quick recall before you leave
How do you solve a Phenol Acidic Nature question? A worked example
- 1
Given
Four phenol derivatives with different para-substituents: –NO₂ (strong EWG, –I and –M), –Cl (EWG, –I dominates), –H (no substituent), –CH₃ (EDG, +I).
- 2
Required
Decreasing order of acid strength (lowest pKa first).
- 3
Concept
Acid strength of substituted phenols depends on the stability of the corresponding phenoxide ion. Electron-withdrawing groups (EWGs) stabilise phenoxide by dispersing the negative charge → increase acidity. Electron-donating groups (EDGs) destabilise phenoxide → decrease acidity. (NCERT Class 12 Chemistry Chapter 7, Part 2, page 206.)
- 4
Formula
No arithmetic formula required. Apply the substituent-effect rule: EWG → lower pKa; EDG → higher pKa. Reference pKa values: phenol ≈ 10.0.
- 5
Substitution / classification
• –NO₂: strong EWG (–I and –M) → strongly stabilises phenoxide → most acidic• –Cl: moderate EWG (–I dominates over weak +M) → moderately stabilises phenoxide• –H: reference compound• –CH₃: weak EDG (+I, hyperconjugation) → slightly destabilises phenoxide → least acidic
- 6
Ranking
Decreasing acid strength: p-nitrophenol > p-chlorophenol > phenol > p-methylphenol.
Approximate pKa values: 7.15 < 9.4 < 10.0 < 10.3. - 7
Final answer
(a) > (d) > (b) > (c) — p-nitrophenol > p-chlorophenol > phenol > p-methylphenol.
- 8
Common trap
Confusing the +M (resonance-donating) effect of –Cl with its –I (inductive-withdrawing) effect. In the para position, the –I effect of chlorine dominates over the weak +M, making p-chlorophenol MORE acidic than phenol. Students who focus only on resonance donation may incorrectly place p-chlorophenol as less acidic than phenol.
- 9
Similar NEET-style question
Arrange in order of increasing acid strength: 2,4,6-trinitrophenol, m-nitrophenol, o-cresol, phenol.
(Answer: o-cresol < phenol < m-nitrophenol < 2,4,6-trinitrophenol.)
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What to remember before solving Phenol Acidic Nature questions
Which Phenol Acidic Nature formulas do you need for NEET?
pKa of phenol vs aliphatic alcohol
Phenols ~10⁶× more acidic than aliphatic alcohols due to resonance stabilisation of phenoxide ion. Electron-withdrawing substituents lower pKa further.
| Symbol | Quantity | SI Unit |
|---|---|---|
| pKa | -log Ka | - |
Valid when
- Aqueous solution
- Substituent effects shift values
More in Alcohols, Phenols, Ethers and Carbonyl Compounds: 3 exam traps and mistakes · 1 formula · 2 question patterns from its other lessons.
Phenol Acidic Nature questions from past NEET papers
1 question from NEET 2022. Answers verified against NTA official keys.
All 23 past-paper questions from Alcohols, Phenols, Ethers and Carbonyl Compounds →
How does NEET ask about Phenol Acidic Nature?
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
Predict acidity ordering of substituted phenols, or product of nitration/halogenation.
Common distractors
ignores resonance vs induction balance
Uses inductive only without considering resonance
Sources
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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