Chemistry · Alcohols, phenols and ethers · chapter 7 of 10

Class 12 Chemistry Chapter 7: Alcohols, Phenols and Ethers case study question with answers

One case-based question of the kind the board asks: a passage and a table, then 3 parts worth 4 marks in all, with the answer and the marking scheme for each. Written from the NCERT chapter.

Case study · 4 marks

A student compares the acid strengths of some phenols and of ethanol in water at 298 K, using their pKa values, listed in the table. She recalls that pKa = −log Ka, so that each unit of pKa is a factor of ten in Ka. She wants to explain the numbers by looking at what happens to the negative charge of the anion left when each compound loses its proton, and at how a group already on the ring changes that charge.

Compound	pKa
phenol	10.0
2-nitrophenol	7.2
3-nitrophenol	8.3
4-nitrophenol	7.1
4-methylphenol	10.2
ethanol	15.9

Part 1 · 1 mark

Ka of 4-nitrophenol divided by Ka of phenol is about (antilog 0.9 = 7.94)

(a) 794(b) 2.9(c) 7943(d) 0.0013

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Answer: (a) 794

Ka ratio = 10^(10.0 − 7.1) = 10².9 = 7.94 × 10² ≈ 794. 2.9 is the pKa difference itself; 7943 adds a power of ten; 0.0013 is the inverse ratio.

Part 2 · 1 mark

3-Nitrophenol has a higher pKa than 2- and 4-nitrophenol because

(a) the negative charge of the phenoxide ion is delocalised effectively onto the ortho and para carbons, where –NO₂ can withdraw it, but not onto the meta carbon(b) a meta –NO₂ group releases electrons to the ring(c) 3-nitrophenol forms intramolecular hydrogen bonds that hold the proton(d) the –NO₂ group has no effect on acidity from any position

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Answer: (a) the negative charge of the phenoxide ion is delocalised effectively onto the ortho and para carbons, where –NO₂ can withdraw it, but not onto the meta carbon

Electron withdrawing groups raise acid strength most at ortho and para, the positions that carry the phenoxide's negative charge in its resonance structures; at meta the nitro group cannot take up the charge by resonance, so its effect is smaller.

Part 3 · 2 marks

Explain why 4-methylphenol has a slightly higher pKa than phenol, and why ethanol's pKa is nearly six units higher than phenol's.

Answer and marking scheme

The methyl group releases electrons; it does not favour the phenoxide ion (it intensifies the negative charge), so 4-methylphenol ionises a little less: pKa 10.2. In the ethoxide ion the negative charge is localised on oxygen, while in the phenoxide ion it is delocalised over the oxygen and the ring, so phenoxide is far more stable and phenol ionises far more than ethanol.

–CH₃ electron releasing, does not favour the phenoxide (1)
ethoxide charge localised; phenoxide charge delocalised by resonance (1)

Describe the resonance structures of the phenoxide ion, saying where the negative charge sits in each, and use them to explain why phenoxide is more stable than phenol.

Answer and marking scheme

The phenoxide ion has five resonance structures, I to V: the negative charge is on oxygen in I and V and on the two ortho carbons and the para carbon in II, III and IV. The charge is delocalised with no charge separation, whereas phenol's own resonance structures separate charge (C=O⁺–H with negative charge on the ring), so phenol is less stable than phenoxide and ionisation is favoured.

charge on O (I, V) and on ortho, para and ortho carbons (II–IV) (1)
phenol's structures have charge separation, phenoxide's do not: phenoxide more stable (1)

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