VIII. The Cyanate Ion, OCN, Has Three Resonance Structures Shown Below. For The Lewis Structure For OCN You Should Take Formal Charges Into Account To Find The Best Lewis Structure For The Molecule. Assign Formal Charges Above Each Atom On Each Structure And Select The Best Lewis Structure. (12) Formal Charges Formal Charges Formal Charges :0-CEN : Toc=N:
The Correct Answer and Explanation is:
To determine the best Lewis structure for the cyanate ion (OCN⁻), we must consider all resonance structures and formal charges to select the most stable form.
Resonance Structures of OCN⁻
There are three primary resonance structures:
Structure 1:
:Ö–C≡N:
- Oxygen has 3 lone pairs, single bonded to carbon.
- Carbon triple bonded to nitrogen.
Formal Charges:
- Oxygen: 6 – (6 nonbonding + 1 bonding) = -1
- Carbon: 4 – (4 bonding) = 0
- Nitrogen: 5 – (6 bonding + 2 nonbonding) = 0
Structure 2:
:Ö≡C–N:
- Oxygen triple bonded to carbon.
- Carbon single bonded to nitrogen (with nitrogen having 3 lone pairs).
Formal Charges:
- Oxygen: 6 – (2 bonding + 2 lone pairs) = +1
- Carbon: 4 – (4 bonding) = 0
- Nitrogen: 5 – (6 nonbonding + 1 bonding) = -2
Structure 3:
:O=C=N:
- Oxygen double bonded to carbon.
- Carbon double bonded to nitrogen.
Formal Charges:
- Oxygen: 6 – (4 nonbonding + 2 bonding) = 0
- Carbon: 4 – (4 bonding) = 0
- Nitrogen: 5 – (4 bonding + 2 nonbonding) = 0
Analysis of Formal Charges and Stability
- Structure 1 has only one formal charge (-1 on oxygen), and the negative charge is on the most electronegative atom (oxygen) – this is chemically favorable.
- Structure 2 has a +1 formal charge on oxygen and -2 on nitrogen, which is less favorable due to the +1 on electronegative oxygen and large -2 on nitrogen.
- Structure 3 has all atoms with 0 formal charge, which might seem best, but O=C=N has less realistic bonding (O=C=N is neutral, but doesn’t match OCN⁻ overall charge of -1). So, it’s not valid for the anion.
✅ Best Lewis Structure: Structure 1
:Ö–C≡N:
- Minimizes formal charges.
- Negative charge is on the most electronegative atom (oxygen).
- Matches the -1 overall charge of the ion.
When evaluating resonance structures for the cyanate ion (OCN⁻), we focus on assigning formal charges and checking for stability based on these charges. Three resonance forms are possible, each with different bonding between oxygen, carbon, and nitrogen. Formal charge is calculated using the formula:
Formal Charge = Valence electrons – (nonbonding electrons + ½ bonding electrons).
In Structure 1, the oxygen is singly bonded to carbon and holds a negative formal charge, carbon is neutral, and nitrogen is triple bonded and also neutral. This is favorable because the negative charge is on oxygen, which is highly electronegative and can stabilize the charge well.
In Structure 2, the oxygen has a triple bond to carbon, giving it a +1 formal charge, and nitrogen has a -2 charge. This is less stable because oxygen carrying a positive charge is unfavorable, and a -2 charge on nitrogen is too excessive.
In Structure 3, all atoms have formal charges of zero, which may initially appear ideal. However, this structure does not account for the overall negative charge of the cyanate ion (OCN⁻), making it inconsistent with the ion’s actual charge.
Therefore, the best Lewis structure for cyanate ion is Structure 1 (O⁻–C≡N), as it reflects proper charge distribution, resonance stability, and aligns with the known -1 charge of the anion.
