A-Level Chemistry October/November 2024 Q7(i): Compare the basicities of C2H5NHCOCH3, C2H5NHC2H5 and NH3. Explain your answer. most ba…

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  Order of Basicity

  Most basic: C₂H₅NHC₂H₅

  Least basic: C₂H₅NHCOCH₃

  Overall order: C₂H₅NHC₂H₅ > NH₃ > C₂H₅NHCOCH₃

  Explanation

  Basicity in these compounds depends on the availability of the lone pair of electrons on the nitrogen atom to accept a proton (H⁺).

  1. C₂H₅NHC₂H₅ (diethylamine) vs NH₃ (ammonia): Diethylamine is a stronger base than ammonia. The two ethyl (C₂H₅) groups are alkyl groups which are electron-donating. They exert a positive inductive effect (+I effect), pushing electron density onto the nitrogen atom. This increases the electron density on the nitrogen, making its lone pair more available to be donated to a proton compared to the nitrogen in ammonia.

  2. C₂H₅NHCOCH₃ (N-ethylacetamide) vs NH₃ (ammonia): N-ethylacetamide is a much weaker base than ammonia. The lone pair of electrons on the nitrogen atom is adjacent to the pi-system of the carbonyl (C=O) group. This lone pair is delocalised by overlapping with the p-orbitals of the C=O group. This delocalisation spreads the electron charge and makes the lone pair significantly less available to accept a proton. Therefore, it is the least basic of the three compounds.

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  How the marks are awarded

  • M1 — Stating the correct order of basicity: C₂H₅NHC₂H₅ as most basic, followed by NH₃, and C₂H₅NHCOCH₃ as least basic.
  • M2 — Explaining that basicity is due to the availability of the lone pair on the nitrogen atom and its ability to accept a proton (H⁺).
  • M3 — Correctly explaining why the amine is more basic: the alkyl/ethyl groups have a positive inductive effect, which is electron-donating, increasing the availability of the N lone pair.
  • M4 — Correctly explaining why the amide is less basic: the lone pair on the nitrogen is delocalised into the adjacent C=O group, making it less available.

  Common mistakes

  • Getting the order of basicity incorrect, often placing ammonia as more basic than the amine.
  • Failing to explain the effect of the alkyl groups in the amine, simply stating 'it has two ethyl groups' without mentioning the positive inductive effect or electron-donating nature.
  • Incorrectly describing the effect in the amide, for example, only mentioning the electronegativity of the oxygen atom (inductive effect) instead of the crucial delocalisation of the lone pair (resonance effect).
  • Giving a vague definition of basicity without mentioning the key role of the nitrogen's lone pair of electrons.

  Examiner tip: Always relate the basicity of nitrogen compounds to the availability of the nitrogen's lone pair, considering both inductive and delocalisation (resonance) effects from adjacent groups.

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