WJEC A-Level Chemistry Unit 4, June 2025: Question 4

✅ Mark Scheme Answer

Any one of the following statements:

• 4-Bromophenol acts/reacts as an acid.
• It acts as a proton donor (donates H⁺ to OH⁻).

💡 Key Knowledge

• Phenols are weak acids (pKa ~ 10). They are acidic enough to react with strong bases like aqueous NaOH to form water-soluble phenoxide salts and H₂O .
• In contrast, standard aliphatic alcohols (e.g. ethanol, pKa ~ 16) are not acidic enough to react with aqueous NaOH .
• Phenol's acidity arises because the lone pair of electrons on oxygen delocalises into the benzene π-ring, stabilising the phenoxide anion ( C₆H₅O⁻ ).

🧠 Exam Technique

The question asks why it reacts with an alkali. Alkalis are bases/proton acceptors, so the most direct chemical reasoning is to identify the complementary role of the organic reactant: it is an acid or a proton donor.

❌ Common Errors

• Writing simply that "it has an —OH group" (aliphatic alcohols also have —OH groups, but they do not react with aqueous NaOH).
• Calling the reaction "electrophilic substitution" instead of an acid-base neutralization.

✅ Mark Scheme Answer

Award 1 mark for any one of the following points:

• Aromatic compounds / benzene rings are not susceptible to attack by nucleophiles (or repel nucleophiles due to high π-electron density).
• Aromatic compounds / halogenoarenes do not undergo nucleophilic substitution.
• The aromatic C—Br bond is too strong (stronger than in a haloalkane).

💡 Key Knowledge

In haloalkanes (aliphatic), OH⁻ easily displaces Br⁻ via nucleophilic substitution (hydrolysis). However, in bromobenzene derivatives:

• Partial double bond character: One of the p-orbital lone pairs on the bromine atom delocalises into the aromatic π-system, shortening and strengthening the C—Br bond.
• Electron repulsion: The electron-rich π-cloud of the benzene ring repels approaching nucleophiles (such as OH⁻ ).

🧠 Exam Technique

Notice what changed in the incorrect molecule: the —Br group was substituted by an —OH group. Immediately recognise this as attempted nucleophilic substitution of a halogen on an aromatic ring, which is generally not feasible under room conditions.

❌ Common Errors

• Saying "bromine is not reactive" (too vague; bromine in 1-bromobutane is readily substituted). Specify the aromatic C—Br bond.
• Stating that "NaOH is an electrophile" (hydroxide ion, OH⁻, is negatively charged and therefore a nucleophile).
• Failing to mention the word nucleophile or substitution when discussing the mechanism.