Nitrous oxide (N2O) has three possible Lewis structures: ∴N = N = O’↔:N ≡ N − O` :↔: N − N ≡ 0 Given the following bond lengths, N − N 167 pm N = N 120 pm N ≡ N 110 pm N = O 115 pm N − O 147 pm rationalize the observations that the N − N bond length in N₂O is 112 pm and that the N − O bond length is 119 pm. Assign formal charges to the resonance structures for N₂O. Can you eliminate any of the resonance structures on the basis of formal charges? Is this consistent with observation?
The Correct Answer and Explanation is:
The molecule nitrous oxide (N₂O) has three resonance structures:
- N = N = O (structure 1)
- N ≡ N − O (structure 2)
- N − N ≡ O (structure 3)
The bond lengths in the molecule, as given in the problem, are:
- N − N = 167 pm
- N = N = 120 pm
- N ≡ N = 110 pm
- N = O = 115 pm
- N − O = 147 pm
Analysis of Bond Lengths:
The experimentally observed N − N bond length in N₂O is 112 pm, which is between the typical N − N single bond (167 pm) and N ≡ N triple bond (110 pm). This suggests that the N − N bond in N₂O is a hybrid between a single and a triple bond, corresponding to a double bond character. The N − O bond length is observed to be 119 pm, which is shorter than a typical N − O single bond (147 pm) but longer than a N = O double bond (115 pm). This suggests a bond character between a single and double bond, likely reflecting the resonance between the structures.
Formal Charges and Resonance Structures:
- Structure 1 (N = N = O):
- The formal charges can be assigned as:
- N on the left: formal charge = 0 (neutral)
- N in the middle: formal charge = +1
- O on the right: formal charge = −1
- Structure 2 (N ≡ N − O):
- The formal charges:
- N on the left: formal charge = 0
- N in the middle: formal charge = 0
- O on the right: formal charge = −1
- Structure 3 (N − N ≡ O):
- The formal charges:
- N on the left: formal charge = +1
- N in the middle: formal charge = 0
- O on the right: formal charge = −1
Elimination of Resonance Structures Based on Formal Charges:
- Structure 1 (N = N = O) and Structure 3 (N − N ≡ O) both result in formal charges of +1 on nitrogen and −1 on oxygen, which are reasonable and consistent with formal charge rules.
- Structure 2 (N ≡ N − O) is also reasonable with a neutral formal charge on the central nitrogen atom, but it may be less favored due to its higher formal charge on oxygen in some contexts.
Given the observed bond lengths, the molecule’s actual structure is likely a mix of the resonance forms, with the N − N bond being intermediate between a single and triple bond, and the N − O bond being intermediate between a single and double bond. The formal charges help rationalize that structures with minimal formal charges (Structure 1 and 2) are more likely to be the dominant contributors to the overall resonance hybrid. This is consistent with the experimental observations of bond lengths.