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SolutionsReactivity

1. Products

Write the main product of each reaction:

  1. Phenol + aqueous NaOH
  2. Phenol + NaOH, then CH3CH2Br
  3. Phenol + acetic anhydride in pyridine
  4. Phenol + excess bromine water
  5. Phenol + dilute HNO3 at 20 °C
  6. Hydroquinone + Na2Cr2O7, H2SO4
  1. Sodium phenoxide (C6H5O− Na+).
  2. Ethoxybenzene (phenetole), by Williamson synthesis.
  3. Phenyl acetate.
  4. 2,4,6-Tribromophenol (white precipitate).
  5. Mixture of 2-nitrophenol and 4-nitrophenol.
  6. p-Benzoquinone.

2. Cleaving an ether

Which products are obtained when anisole (methoxybenzene) is heated with excess HI? Why is iodobenzene not formed?

Phenol and methyl iodide. I− attacks the methyl carbon of the protonated ether by SN2; the sp2 ring carbon allows neither SN2 nor SN1, and the phenol formed does not react with HI.

3. More reactive than benzene

Explain why phenol is brominated without a catalyst whereas benzene needs FeBr3. Why does bromine enter ortho and para?

An oxygen lone pair is conjugated with the ring and increases its electron density: the ring is much more nucleophilic and, in addition, the intermediate cation (σ complex) is stabilized by a structure in which oxygen carries the positive charge and every atom has an octet. That structure only exists when the electrophile enters ortho or para.

4. Separating the nitrophenols

Nitration of phenol gives a mixture of 2-nitrophenol and 4-nitrophenol. Which one distils in steam and why?

2-Nitrophenol: it forms an intramolecular hydrogen bond between OH and NO2, does not associate with other molecules or with water and is more volatile. 4-Nitrophenol forms intermolecular hydrogen bonds and stays in the flask.

5. From phenol to aspirin

Give the sequence leading from phenol to aspirin and the name of each reaction.

  1. Phenol + NaOH → sodium phenoxide (acid-base reaction).
  2. Phenoxide + CO2, 125 °C, 100 atm → sodium salicylate; H3O+ → salicylic acid (Kolbe-Schmitt reaction).
  3. Salicylic acid + acetic anhydride (cat. H2SO4) → acetylsalicylic acid (esterification, acetylation of the phenolic OH).

6. Antioxidants

Why is BHT (2,6-di-tert-butyl-4-methylphenol) a good antioxidant?

It gives the OH hydrogen to a radical (for example, a peroxyl radical) and forms a phenoxyl radical stabilized by resonance in the ring and sterically protected by the two tert-butyl groups; this radical is unreactive and does not propagate the oxidation chain.

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