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SolutionsPreparation

1. Why not like an alcohol?

Explain why phenol cannot be obtained by heating bromobenzene with aqueous NaOH at 100 °C, whereas 1-bromobutane gives 1-butanol under those conditions.

The sp2 ring carbon cannot be attacked from the back as SN2 requires (the ring prevents it), and the aryl cation is too unstable for SN1. Moreover, the C–Br bond of bromobenzene is stronger because of conjugation with the ring. The halogen is replaced only under very harsh conditions (Dow process, via benzyne) or with nitro groups ortho/para (SNAr).

2. The cumene process

Write the three stages of the cumene process and state which by-product is obtained and why it matters.

  1. Benzene + propene (acid catalyst) → cumene.
  2. Cumene + O2 (air), radical reaction → cumene hydroperoxide.
  3. Hydroperoxide + H3O+ → phenol + acetone (Hock rearrangement).

The by-product is acetone, a widely demanded solvent and raw material; selling it adds to the profitability of the process.

3. From an amine

How would you prepare 4-methylphenol (p-cresol) from 4-methylaniline (p-toluidine)?

Diazotization with NaNO2 and aqueous H2SO4 at 0–5 °C, which gives the 4-methylbenzenediazonium salt, and heating of the aqueous solution: N2 is released and 4-methylphenol is formed.

4. Synthesis from benzene

Propose a synthesis of 3-bromophenol from benzene. Hint: OH directs ortho and para; choose a meta-directing group that can later be converted into OH.

  1. Nitration (HNO3, H2SO4) → nitrobenzene.
  2. Bromination (Br2, FeBr3): NO2 directs meta → 1-bromo-3-nitrobenzene.
  3. Reduction of NO2 (Sn or Fe, HCl) → 3-bromoaniline.
  4. Diazotization (NaNO2, H2SO4, 0–5 °C) and hydrolysis with hot water → 3-bromophenol.

5. Which reacts most easily?

Order by ease of hydrolysis with aqueous NaOH: chlorobenzene, 1-chloro-4-nitrobenzene and 1-chloro-2,4-dinitrobenzene.

Chlorobenzene < 1-chloro-4-nitrobenzene < 1-chloro-2,4-dinitrobenzene. Each ortho or para nitro group stabilizes the negative charge of the Meisenheimer complex by resonance and makes SNAr easier; chlorobenzene reacts only under the conditions of the Dow process.

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