20. Phenols

O,P-positions vs. Meta-Positions

Rank the following molecules in increasing order of acidity (least acidic to most acidic).

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Multiple Choice

Rank the following phenols in order of increasing acidity.

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(•••) Benzoic acid and phenol are insoluble in water. When sodium bicarbonate is added to the water, benzoic acid dissolves, but phenol does not. The addition of sodium hydroxide causes both to dissolve. On the basis of these observations, estimate the pKₐ of phenol. [The pKₐ of H₂CO₃ is 6.4.]

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The following compounds are only slightly soluble in water, but one of them is very soluble in a dilute aqueous solution of sodium hydroxide. The other is still only slightly soluble.

(b) Show how this difference might be exploited to separate a mixture of these two compounds using a separatory funnel.

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The following compounds are only slightly soluble in water, but one of them is very soluble in a dilute aqueous solution of sodium hydroxide. The other is still only slightly soluble.

(a) Explain the difference in solubility of these compounds in dilute sodium hydroxide.

Predict the products formed when m-cresol (m-methylphenol) reacts with (b) acetyl chloride,

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Predict the major products of the following reactions.

(b) phenol + tert-butyl chloride + AlCl3

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(••) The dye alizarin normally forms an orange solution when dissolved. However, if KOEt is added to the solution, it turns blue very rapidly. Rationalize this result. [Hint: We'll learn in Chapter 24 that hydroxyl groups bonded to a benzene ring are more acidic than normal alcohols.]

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(•••) Rationalize the fact that 1,4-dihydroxybenzene melts at a significantly higher temperature than 1,2-diydroxybenzene.

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(••) Sulpiride is an antipsychotic. Provide the reagents (a) and (b) used to complete the synthesis.

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(•••) Phenol is synthesized from cumene via the scheme shown. Suggest the mechanism for each step of the reaction shown. [The oxygens have been screened in blue and orange to assist you.]

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Though the nitro group is electron-withdrawing by resonance, when in the meta position, it doesn’t communicate by resonance with the phenoxide anion. And yet, the pKₐ value of m-nitrophenol is lowered from 10 (the pKₐ value of unsubstituted phenol) to 8.4. Explain this observation.

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e. Which has a greater Ka: cyclohexanol or phenol? f. Which is a stronger acid: cyclohexanol or phenol?

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Rank the amines in each set in order of increasing basicity.

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