Which of the following resonance structures for OCN will contribute most to the correct structure of OCN?
A. O(1 lone pair)=C-N(3 lone pairs)
B. O(3 lone pairs)-C=N(1 lone pair)
C. O(2 lone pairs)=C=N(2 lone pairs)
D. They all contribute equally to the correct structure of OCN.
E. O(1 lone pair)=C(2 lone pairs)=N(1 lone pair)
The correct answer and explanation is :
The correct answer is C. O(2 lone pairs)=C=N(2 lone pairs).
Here’s why:
To determine which resonance structure of OCN⁻ (cyanate ion) contributes most to the correct structure, we must consider the formal charges on each atom and how they influence the overall structure.
Step 1: Understand the structure of OCN⁻
The cyanate ion (OCN⁻) consists of oxygen, carbon, and nitrogen atoms. Since it has a negative charge, the structure must account for this negative charge in the most stable way possible. A key factor in stability is minimizing formal charges and ensuring that atoms follow the octet rule as closely as possible.
Step 2: Analyze each resonance structure
- A. O(1 lone pair)=C-N(3 lone pairs)
- Oxygen has 1 lone pair and a double bond with carbon. The carbon has a single bond with nitrogen, and nitrogen has 3 lone pairs. This structure gives oxygen a formal charge of 0, carbon a formal charge of +1, and nitrogen a formal charge of -1. The formal charges are not balanced optimally.
- B. O(3 lone pairs)-C=N(1 lone pair)
- Oxygen has 3 lone pairs and a single bond with carbon. Carbon has a double bond with nitrogen, and nitrogen has 1 lone pair. This structure gives oxygen a formal charge of -1, carbon a formal charge of 0, and nitrogen a formal charge of 0. While this minimizes formal charges on carbon and nitrogen, it places a negative charge on oxygen, which is less stable.
- C. O(2 lone pairs)=C=N(2 lone pairs)
- Oxygen has 2 lone pairs and forms a double bond with carbon. Carbon forms a double bond with nitrogen, and nitrogen has 2 lone pairs. This structure places the negative charge on nitrogen, which is the most stable, as nitrogen is more electronegative than oxygen and can better accommodate the negative charge. Additionally, all atoms have formal charges of 0, which is ideal.
- D. They all contribute equally to the correct structure of OCN.
- This is incorrect because the resonance structures do not contribute equally. Some structures result in higher formal charges or less stability, so the resonance structures do not all contribute equally.
- E. O(1 lone pair)=C(2 lone pairs)=N(1 lone pair)
- In this structure, carbon has 2 lone pairs, which is highly unusual and leads to an unnatural bonding arrangement. This configuration is less stable and unlikely to be a major contributor.
Step 3: Conclusion
Among the given resonance structures, C is the most stable. It minimizes formal charges and places the negative charge on the most electronegative atom, nitrogen. This makes structure C the most significant resonance form for the cyanate ion (OCN⁻).