Problem 83 - Chemistry and Chemical Reactivity

Question

The cyanate ion, NCO", has the least electronegative atom, \(\mathbf{C},\) in the center. The very unstable fulminate ion, CNO \(^{-}\), has the same formula, but the \(\mathrm{N}\) atom is in the center. (a) Draw the three possible resonance structures of \(\mathrm{CNO}^{-}\) (b) On the basis of formal charges, decide on the resonance structure with the most reasonable distribution of charge. (c) Mercury fulminate is so unstable it is used in blasting caps. Can you offer an explanation for this instability? (Hint: Are the formal charges in any resonance structure reasonable in view of the relative electron negativities of the atoms?)

Step-by-Step Solution

Answer

The second resonance structure is the most stable, and the instability of mercury fulminate arises from poorly located formal charges.

Step 1: Draw Resonance Structures

First, depict the three possible resonance structures for \( \mathrm{CNO}^{-} \). Start by arranging the atoms \( C, N, \) and \( O \) in the sequence \( C \text{-} N \text{-} O \) with a total of 16 valence electrons \((4 \text{ from } C, 5 \text{ from } N, 6 \text{ from } O, \text{ and } 1 \text{ for the charge})\). The first structure involves a triple bond between C and N, and a single bond between N and O with a negative charge on O. The second structure involves a double bond between C and N and N and O with the negative charge on the N atom. The third structure involves a single bond between C and N, and a triple bond between N and O with the negative charge on C.

Step 2: Calculate Formal Charges

For each resonance structure, calculate the formal charge on each atom using the formula: \( \text{Formal Charge} = \text{Valence Electrons} - (\text{Non-bonding Electrons} + \frac{1}{2}\text{Bonding Electrons}) \). - First Structure: C = 0, N = +1, O = -1.- Second Structure: C = 0, N = -1, O = 0.- Third Structure: C = -1, N = +1, O = 0.

Step 3: Evaluate Stability Based on Formal Charges

The most stable resonance structure is often the one where formal charges are minimized and where negative charges reside on the most electronegative atoms. In this case, the second structure with the charge distribution (C = 0, N = -1, O = 0) is more stable. This is because the negative charge is on the nitrogen atom, which is more electronegative than carbon but less than oxygen, but maintains a more reasonable formal charge distribution for a linear structure.

Step 4: Explain Instability

The instability of mercury fulminate can be explained by the presence of unreasonable formal charges and the arrangement of electronegative atoms in its structures. Especially, when formal charges are poorly located as in the case of the first and third resonance structures where electron density is not optimally distributed. This leads to strain and potential reactive instability.

Key Concepts

Formal Charges

Formal charges help us understand how charges are distributed across different atoms in a molecule.
They are calculated using the formula: \[\text{Formal Charge} = \text{Valence Electrons} - (\text{Non-bonding Electrons} + \frac{1}{2}\text{Bonding Electrons})\] The goal is to find the structure where these charges are minimized, as this often indicates a more stable structure.
For example, consider the resonance structures of the fulminate ion \( \mathrm{CNO}^{-} \).

• The first structure might have formal charges like C = 0, N = +1, and O = -1.
• The second could be C = 0, N = -1, O = 0.
• And the third one C = -1, N = +1, O = 0.

The most stable structure is typically the one with the lowest formal charges and/or where negative charges are on the more electronegative atoms. This brings us to the concept of electronegativity.

Electronegativity

Electronegativity reflects how strongly an atom attracts electrons in a bond. Nitrogen, for example, is more electronegative than carbon, which means it attracts electrons more strongly.
In resonance structures, placing a negative charge on more electronegative atoms can lead to a more stable arrangement.

• For \( \mathrm{CNO}^{-} \), a structure with a negative charge located on nitrogen or oxygen may be more reasonable, given their high electronegativity.
• This principle helps in choosing the best resonance structure.

Whenever possible, align the charges with electronegative atoms to increase molecular stability. This leads us to the next concept: ion stability.

Ion Stability

Ion stability depends largely on the location of charges, the electronegativity of atoms, and the strain within the molecule. Stable ions usually have minimized formal charges and balanced electron distribution.
In the \( \mathrm{CNO}^{-} \) ion, resonance structures where formal charges are unreasonable (e.g., negative charges on less electronegative atoms) can make the ion unstable.
The instability of compounds like mercury fulminate is because of such poorly placed charges and unbalanced structure.

• Good resonance structures distribute negative charges on electronegative atoms.
• When charges are mismatched, it can cause reactivity and instability.

Understanding these principles helps explain why certain ions are more reactive or even explosive, as seen in mercury fulminate.