Draw the Lewis structure for acetamide (CH3CONH2), an organic compound, and determine the geometry about each interior atom. Experiments show that the geometry about the nitrogen atom in acetamide is nearly planar. Which resonance structure can account for the planar geometry about the nitrogen atom?
The Correct Answer and Explanation is:
Answer:
The Lewis structure for acetamide (CH₃CONH₂) is as follows:
H
|
H₃C—C—N
|| \
O H
Lewis Structure Breakdown:
- The methyl group (CH₃) is bonded to the carbonyl carbon.
- The carbonyl group (C=O) involves a double bond between carbon and oxygen.
- The amide group (NH₂) is bonded to the carbonyl carbon and has two hydrogen atoms attached to nitrogen.
Geometry:
- Carbonyl carbon (C=O): This carbon is sp² hybridized, resulting in a trigonal planar geometry (~120° bond angles).
- Nitrogen (NH₂): Despite typically forming a pyramidal geometry (sp³ hybridization), in acetamide, nitrogen is also nearly planar, suggesting sp² hybridization.
- Oxygen (O): Double bonded to carbon with two lone pairs, it also adopts a trigonal planar arrangement (sp² hybridized).
Explanation of Planar Geometry Around Nitrogen:
Experimental evidence indicates the nitrogen atom in acetamide is nearly planar, which is not typical for nitrogen with three sigma bonds and one lone pair (normally tetrahedral). This deviation can be explained by resonance.
Key Resonance Structure:
A significant resonance form of acetamide shows delocalization of the nitrogen lone pair into the carbonyl π-system, creating a partial double bond between nitrogen and carbon:
H
|
H₃C—C=N⁺—H
| |
O⁻ H
This resonance form contributes to:
- Planar Geometry: Delocalization of the lone pair requires the nitrogen to use an unhybridized p-orbital, aligning with the p-orbitals of the C=O group, leading to a planar sp² hybridized nitrogen.
- Bond Lengths: The C–N bond is shorter than a typical C–N single bond, reflecting partial double-bond character.
- Stabilization: Resonance delocalization increases stability and explains the amide’s rigidity and reduced basicity.
In conclusion, the resonance structure with delocalized nitrogen lone pair and partial C=N and C–O⁻ character accounts for the planar geometry around nitrogen in acetamide.
