AQA A-Level Chemistry Paper 3, June 2018: Question 24

✅ Correct Answer: C

ethylamine > ammonia > phenylamine

A stronger base accepts protons more readily from water, producing a higher concentration of hydroxide ions [OH⁻], which results in a higher pH. Therefore, the order of decreasing pH mirrors the order of decreasing base strength:

Ethylamine (strongest base, highest pH) > Ammonia > Phenylamine (weakest base, lowest pH)

💡 Key Knowledge

• Base definition: Amines act as weak Brønsted-Lowry bases by using the lone pair of electrons on the nitrogen atom to form a dative bond with a proton (H⁺).
• Ethylamine (CH₃CH₂NH₂): The ethyl group has a positive inductive (+I) electron-releasing effect. This increases the electron density on the N atom, making the lone pair more available to accept a proton than in ammonia.
• Ammonia (NH₃): Serves as the benchmark with no electron-donating or withdrawing groups.
• Phenylamine (C₆H₅NH₂): The lone pair of electrons on the nitrogen overlaps and delocalises into the benzene ring's π-electron cloud. This significantly reduces electron density on N, making the lone pair much less available to accept H⁺.

🧠 Exam Technique & Logic

• Connect pH to Base Strength: Higher base strength → greater extent of ionisation in water ( B + H₂O ⇌ BH⁺ + OH⁻ ) → higher [OH⁻] → lower [H⁺] → higher pH.
• Use the "Aliphatic vs Aromatic" rule: Primary aliphatic amine > Ammonia > Aromatic amine. Memorise this baseline hierarchy as an essential organic trend.
• Elimination method: Knowing ethylamine is stronger than ammonia immediately eliminates options A and B. Knowing phenylamine is weaker than ammonia eliminates option D.

❌ Common Errors & Misconceptions

• Inverting the pH scale: Confusing acidity and basicity—thinking that a stronger base has a lower pH (closer to 7), rather than a higher pH (closer to 14).
• Ranking phenylamine above ammonia: Forgetting that delocalisation of the nitrogen lone pair into the aromatic ring dramatically reduces basicity.
• Assuming all organic amines are stronger than ammonia: Only alkyl amines are stronger; aryl amines (like phenylamine) are considerably weaker bases than ammonia.