Nitrogen-Containing Organic Compounds — A/L Chemistry

How do I practise Nitrogen-Containing Organic Compounds?

1 question with a worked solution

Check before you answer: Basicity Trends: Explaining aqueous phase basicity orders (Secondary aliphatic amine > Primary > Tertiary > Ammonia > Aniline) balancing inductive electron release against steric hindrance and hydration solvation energies.

1. Arrange the following nitrogen-containing bases in order of decreasing basicity in dilute aqueous solution: diethylamine ((C₂H₅)₂NH), ethylamine (C₂H₅NH₂), ammonia (NH₃), and phenylamine (aniline, C₆H₅NH₂).
   1. A) Ethylamine > Diethylamine > Ammonia > Phenylamine
   2. B) Phenylamine > Ammonia > Ethylamine > Diethylamine
   3. C) Diethylamine > Ammonia > Ethylamine > Phenylamine
   4. D) Diethylamine > Ethylamine > Ammonia > Phenylamine
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   Correct answer: D) Diethylamine > Ethylamine > Ammonia > Phenylamine

   In aqueous solution, aliphatic secondary amines are more basic than primary amines because the +I inductive effect of two alkyl groups increases electron density on nitrogen, while steric hindrance and hydration energies remain favorable: (C₂H₅)₂NH > C₂H₅NH₂ > NH₃. In phenylamine (C₆H₅NH₂), the lone pair on nitrogen is delocalized into the aromatic benzene ring through resonance, making it drastically less available for protonation, so it is the weakest base.

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   Step 1: Evaluate alkyl inductive effects: Ethyl groups exert a +I inductive electron-donating effect toward nitrogen, increasing electron pair availability relative to ammonia: C₂H₅NH₂ > NH₃ and (C₂H₅)₂NH > C₂H₅NH₂. Step 2: Balance steric hindrance and aqueous hydration: For ethylamines in water, the order is secondary > primary > tertiary > ammonia, making (C₂H₅)₂NH more basic than C₂H₅NH₂. Step 3: Evaluate resonance delocalization in phenylamine: The lone pair on -NH₂ conjugates with the π-system of the benzene ring (delocalization), stabilizing the neutral base and lowering pKb. Step 4: Final decreasing order of basicity: (C₂H₅)₂NH > C₂H₅NH₂ > NH₃ > C₆H₅NH₂. Answer: Diethylamine > Ethylamine > Ammonia > Phenylamine.
   Modelled on: the A/L Chemistry examination pattern

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Grade 12 · Term 3