3 free questions from this chapter (the full set has 30). Marking: Multi-correct: +4 all correct, +1 per correct option if none wrong, −2 if any wrong · Numerical: +4 / 0 · Matching: +3 / −1.
Question 1Level 2 · ExamSingle correct
Arrange benzene, toluene (methylbenzene), anisole (methoxybenzene) and phenol in increasing order of their normal boiling points. Exactly one option is correct.
Abenzene < anisole < toluene < phenol
Bbenzene < toluene < anisole < phenol
Ctoluene < benzene < phenol < anisole
Dbenzene < toluene < phenol < anisole
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Answer: B
Phenol (M = 94) forms intermolecular O–H⋯O hydrogen bonds; this gives it by far the highest boiling point (≈ 182 °C).
Anisole (M = 108) has no O–H, so it cannot hydrogen bond with itself, but it is polar (C–O–C) and heavier than toluene: bp ≈ 154 °C.
Toluene (M = 92), almost non-polar, relies on dispersion forces: bp ≈ 111 °C. Benzene (M = 78), lightest and non-polar: bp ≈ 80 °C.
Order: benzene < toluene < anisole < phenol.
Note the comparison toluene/phenol: almost equal molar masses, yet a difference of about 70 °C, which is due to hydrogen bonding.
Answer: (B).
Difficulty: HardQuestion 2Level 3 · AdvancedSingle correct
Consider the following statements.
I. Phenol boils at a much higher temperature than toluene, although the two have nearly the same molar mass.
II. The C–O bond in phenol is shorter than the C–O bond in methanol.
III. Phenol is a weaker acid than ethanol.
IV. When CO2 is bubbled through an aqueous solution of sodium phenoxide, phenol separates out.
Which of the statements are correct? Exactly one option is correct.
AI and II only
BII, III and IV only
CI, II and IV only
DI and IV only
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Answer: C
I. Correct. Phenol (M = 94, bp ≈ 182 °C) is associated by intermolecular hydrogen bonds; toluene (M = 92, bp ≈ 111 °C) is not.
II. Correct. In phenol the C–O bond (≈ 136 pm) is shorter than in methanol (≈ 142 pm) because (i) the carbon is sp2 hybridised and (ii) a lone pair on O is delocalised into the ring, giving the C–O bond partial double-bond character.
III. Incorrect. Phenol (pKa ≈ 10) is a much stronger acid than ethanol (pKa ≈ 16): phenoxide is resonance stabilised, ethoxide is not (and is destabilised by +I of C2H5).
IV. Correct. Carbonic acid (pKa1 ≈ 6.4) is a stronger acid than phenol, so C6H5O− + CO2 + H2O → C6H5OH + HCO3−; phenol, sparingly soluble, separates as an oily layer/turbidity.
Correct: I, II and IV only.
Answer: (C).
Difficulty: Very hardQuestion 3Level 3 · AdvancedSingle correct
Arrange the following in decreasing order of acid strength (in water, 298 K):
I. 4-methylphenol II. phenol III. 3-nitrophenol IV. 4-nitrophenol V. 2,4,6-trinitrophenol
Exactly one option is correct.
AV > III > IV > II > I
BV > IV > III > I > II
CIV > V > III > II > I
DV > IV > III > II > I
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Answer: D
Acid strength is decided by the stability of the phenoxide ion.
V: three NO2 groups at the 2-, 4- and 6-positions withdraw charge by both –R and –I; the negative charge is delocalised onto three nitro groups. pKa ≈ 0.4, comparable to a strong carboxylic acid.
IV: NO2 para to O− delocalises the charge onto its oxygens (–R) as well as by –I. pKa ≈ 7.15.
III: from the meta position NO2 cannot accept the charge by resonance (the negative charge sits only at ortho/para carbons); only –I operates. pKa ≈ 8.4.
II: phenol, pKa ≈ 10.0.
I: CH3 (+I and hyperconjugation) destabilises the phenoxide: pKa ≈ 10.3.
Order: V > IV > III > II > I.
Answer: (D).
Difficulty: Very hard