Cyanogen azide can exist as three isomers: CNNNN, NCNNN, and NNCNN. Draw valid Lewis dot structures for each isomer. Include formal charges and any important resonance structures. Additionally, use VSEPR to predict the geometry of the underlined atom. (b) Predict which isomer is most stable? Why? Predict which isomer is least stable? Why?
The Correct Answer and Explanation is:
Cyanogen Azide Isomers: CNNNN, NCNNN, and NNCNN
(a) Lewis Structures with Formal Charges & VSEPR Geometries
Let’s draw each isomer and analyze:
1. CNNNN (Cyano-tetrazene)
Structure:
mathematicaCopyEditC≡N–N=N–N
- Carbon forms a triple bond with nitrogen.
- The first N is single-bonded to the second N (=N).
- The second and third N are double bonded.
- The last nitrogen has a lone pair.
Formal Charges:
- C (0), terminal N (0), middle N (=N, +1), next N (-1), last N (0)
Total: neutral
VSEPR Geometry (Underlined Atom = First N after CN):
- Electron groups = 2 (single bond, lone pair) → Linear
Resonance:
- Delocalization in the N=N–N region gives resonance stabilization.
2. NCNNN (Azide-cyano)
Structure:
mathematicaCopyEditN≡C–N=N–N
- Terminal N triple bonded to C.
- C single bonded to next N, followed by N=N–N.
Formal Charges:
- Terminal N (0), C (0), middle N (+1), second N (-1), third N (0)
VSEPR Geometry (Underlined Atom = Carbon):
- Electron groups = 2 → Linear
Resonance:
- Similar to CNNNN, has N=N–N delocalization.
3. NNCNN (Diazido-carbon)
Structure:
mathematicaCopyEditN=N–C≡N
- Terminal N double bonded to N.
- N single bonded to C, which is triple bonded to final N.
Formal Charges:
- Terminal N (+1), next N (-1), C (0), terminal N (0)
VSEPR Geometry (Underlined Atom = Carbon):
- Electron groups = 2 → Linear
Resonance:
- N=N–C region has fewer resonance contributors.
(b) Stability Predictions
Most Stable: CNNNN
- Best resonance delocalization in the tetrazene (N=N–N) region.
- Formal charges are well-distributed.
- Less steric hindrance and a strong CN triple bond.
Least Stable: NNCNN
- Poor resonance; N=N group not stabilized by adjacent atoms.
- More localized formal charges (+1 on N), creating instability.
- Electron-rich N adjacent to electron-rich triple bond leads to repulsion.
Conclusion:
Cyanogen azide’s most stable isomer is CNNNN, due to resonance and minimal charge separation.
Least stable is NNCNN, due to poor resonance and destabilizing formal charges.