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Difficulty: EasyAmines and Amides: Structure, Basicity, and Reactions

Unlike aliphatic amines, amides such as ethanamide (CH3CONH2CH_3CONH_2) do not exhibit basic properties in aqueous solution. Which of the following best explains this neutral behavior?

  1. The lone pair of electrons on the nitrogen atom is delocalized by resonance with the adjacent carbonyl group.Answer
  2. B
    The nitrogen atom in ethanamide lacks an unshared pair of electrons.
  3. C
    Ethanamide dissociates completely in water to yield hydrogen ions (H+H^+).
  4. D
    The methyl group (CH3−CH_3-) withdraws electron density from the nitrogen atom through an inductive effect.

Answer

The lone pair of electrons on the nitrogen atom is delocalized by resonance with the adjacent carbonyl group.
The neutral character of amides is due to resonance stabilization. The non-bonding lone pair of electrons on the nitrogen atom overlaps with the π\pi-system of the adjacent carbonyl group (C=OC=O). This delocalization significantly decreases the electron density on nitrogen, preventing it from accepting a proton (H+H^+) and acting as a base.

Step-by-Step Solution

1
Identify the functional group and structural elements of ethanamide (CH3CONH2CH_3CONH_2).
Ethanamide contains an amino group (−NH2-NH_2) directly attached to a carbonyl group (−C=O-C=O).
Understanding the adjacent arrangement of the carbonyl carbon and nitrogen is essential for evaluating electronic effects.
2
Analyze the availability of the nitrogen lone pair for protonation.
The nitrogen atom has a lone pair of electrons, but it interacts with the π\pi-orbital of the carbonyl group, forming a resonance structure: CH3−C(O−)=NH2+CH_3-C(O^-)=NH_2^+.
Basicity depends on the availability of a lone pair to accept a proton (H+H^+). Delocalization drastically reduces lone pair availability.
3
Conclude the acid-base nature of the molecule.
Because the lone pair is delocalized, ethanamide cannot readily act as a proton acceptor, rendering it neutral in aqueous solution.
This explains the contrast between basic amines (localized lone pair) and neutral amides (delocalized lone pair).

Key Concept

Neutrality of amides due to resonance delocalization of the nitrogen lone pair into the adjacent carbonyl group
Estimated Time:45s
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Topics

Amines and Amides: Structure, Basicity, and Reactions

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