Phenol — practice questions

1. Test with Sodium Hydrogencarbonate (NaHCO₃): Add NaHCO₃ solution to a sample of each.
   • Ethanoic acid: Effervescence (fizzing) is observed as CO₂ gas is produced. (CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂)
   • Phenol & Ethanol: No observable reaction.
2. Test with Sodium Hydroxide (NaOH): To the remaining two unidentified samples (phenol and ethanol), add aqueous NaOH.
   • Phenol: The phenol, which may be partially insoluble, will dissolve to form a colourless solution of sodium phenoxide. (C₆H₅OH + NaOH → C₆H₅ONa + H₂O)
   • Ethanol: No reaction. Ethanol is already miscible with water, so no change is observed.

Alternative/Confirmatory Test: Add neutral iron(III) chloride (FeCl₃) solution. Phenol gives a characteristic violet/purple colouration, while ethanol and ethanoic acid do not.

1. Step 1: Electrophile Generation. The electron-rich phenol ring polarises the Br-Br molecule, creating a $\delta^+$ and $\delta^-$ end. Br$^{\delta+}$ acts as the electrophile.
2. Step 2: Electrophilic Attack. A pair of π-electrons from the benzene ring attacks the Br$^{\delta+}$ atom. The attack occurs at position 4 (para) as directed by the -OH group. This breaks the aromatic system and forms a positively charged carbocation intermediate (an arenium ion), where the charge is delocalised across the ring and the oxygen atom.
3. Step 3: Regeneration of the Aromatic Ring. The C-H bond at position 4 breaks. The electron pair from this bond moves back into the ring to restore the stable aromatic π-system. A proton (H⁺) is lost. The H⁺ reacts with the Br⁻ formed in step 2 to make HBr.

(Note: A diagram would show the curly arrows from the ring to Br-Br, the structure of the carbocation intermediate with the positive charge delocalised, and the curly arrow from the C-H bond back into the ring to expel H⁺).