Question

Oxidation of the cyanide ion produces the stable cyanate ion, \(\mathrm{OCN}^{-}\) . The fulminate ion, \(\mathrm{CNO}^{-}\), on the other hand, is very unstable. Fulminate salts explode when struck; \(\mathrm{Hg}(\mathrm{CNO})_{2}\) is used in blasting caps. Write the Lewis structures and assign formal charges for the cyanate and fulminate ions. Why is the fulminate ion so unstable? (C is the central atom in \(\mathrm{OCN}^{-}\) and \(\mathrm{N}\) is the central atom in \(\mathrm{CNO}^{-}\) )

Short answer

The Lewis structures for cyanate (OCN⁻) and fulminate (CNO⁻) ions are as follows: In OCN⁻, there is a double bond between O and C, and a triple bond between C and N; and in CNO⁻, there is a triple bond between C and N, and a single bond between N and O. The formal charges are: OCN⁻: O(-1), C(0), N(0); CNO⁻: C(-1), N(0), O(0). Fulminate ion is unstable due to the negative formal charge on the central Carbon atom, making the molecule highly reactive and explosive.

Step-by-step solution

Draw the Lewis structures

For both ions, we will first calculate the total number of valence electrons, then distribute these valence electrons in the structure, and finally optimize the structure by creating double or triple bonds if necessary. For the cyanate ion (OCN⁻): 1. Oxygen has 6 valence electrons, Carbon has 4, and Nitrogen has 5. 2. Additionally, there is 1 extra electron due to the negative charge, making a total of 16 valence electrons. 3. Distribute these electrons to create the structure: Oxygen is single bonded to Carbon, which is single bonded to Nitrogen. Oxygen has 6 nonbonding electrons (3 lone pairs), Carbon has 2 nonbonding electrons (1 lone pair), and Nitrogen has 4 nonbonding electrons (2 lone pairs). 4. Since Carbon has less than an octet, we can form a double bond with Oxygen and a triple bond with Nitrogen to complete the octet for all atoms. For the fulminate ion (CNO⁻): 1. Carbon has 4 valence electrons, Nitrogen has 5, and Oxygen has 6. 2. An extra electron due to the negative charge makes a total of 16 valence electrons. 3. Distribute these electrons to create the structure: Carbon is single bonded to Nitrogen, which is single bonded to Oxygen. Carbon has 4 nonbonding electrons (2 lone pairs), Nitrogen has 3 nonbonding electrons (1 lone pair and 1 single electron), and Oxygen has 6 nonbonding electrons (3 lone pairs). 4. To complete the octet for all atoms, we can form a triple bond between Carbon and Nitrogen, leaving a single bond between Nitrogen and Oxygen.

Assign formal charges

For each ion, we will calculate the formal charge for each atom using the formula: \[Formal\,Charge = Valence\,Electrons - Nonbonding\,Electrons - \frac{Bonding\,Electrons}{2}\] For cyanate ion (OCN⁻): 1. Oxygen: \(6 - 4 - \frac{4}{2} = -1\) 2. Carbon: \(4 - 0 - \frac{10}{2} = 0\) 3. Nitrogen: \(5 - 2 - \frac{6}{2} = 0\) For fulminate ion (CNO⁻): 1. Carbon: \(4 - 0 - \frac{10}{2} = -1\) 2. Nitrogen: \(5 - 1 - \frac{8}{2} = 0\) 3. Oxygen: \(6 - 6 - \frac{2}{2} = 0\)

Explain the instability of the fulminate ion

The instability of the fulminate ion (CNO⁻) can be attributed to the presence of a negative formal charge on the central Carbon atom. Carbon normally forms bonds with atoms to achieve a formal charge of zero, which is more stable. In the case of the fulminate ion, Carbon has three bonds, yet it still carries a negative formal charge. This property makes the molecule highly reactive and unstable, leading to its explosive nature.

Key concepts

Formal Charge

The concept of formal charge is crucial in understanding the stability of molecules. It helps us figure out the most likely arrangement of electrons in a molecule. The formal charge on an atom is calculated using the equation:

• \(Formal\,Charge = Valence\,Electrons - Nonbonding\,Electrons - \frac{Bonding\,Electrons}{2}\)

For example, in the cyanate ion (\(\mathrm{OCN}^{-}\)), the oxygen atom has a formal charge of \(-1\), while both carbon and nitrogen have a formal charge of \(0\). This balanced distribution indicates greater stability. In comparison, the fulminate ion \(\mathrm{CNO}^{-}\), assigns a formal charge of \(-1\) to the central carbon, contributing to its instability.
The need for a zero formal charge or close to zero on atoms within a molecule aligns with nature's preference for stability, hence, molecules with balanced charges tend to be more stable and less reactive than those with uneven charge distributions.

Valence Electrons

Valence electrons are the outermost electrons of an atom and they play an essential role in chemical bonding. These electrons are responsible for the formation of chemical bonds. In Lewis structures, valence electrons are depicted as dots around the elemental symbols.
For creating these structures, it's critical to first calculate the total number of valence electrons available. For the ions in our example:

• Cyanate ion \((\mathrm{OCN}^{-})\): Oxygen contributes 6, carbon contributes 4, nitrogen contributes 5, and the additional charge adds another electron, totaling 16 valence electrons.
• Fulminate ion \((\mathrm{CNO}^{-})\): Carbon contributes 4, nitrogen contributes 5, and oxygen contributes 6, plus an extra electron from the negative charge, again totaling 16 valence electrons.

Understanding the distribution of these electrons helps us draw correct Lewis structures that depict the molecule's theoretical real-world form. Ensuring all atoms meet the octet rule, where applicable, further contributes to molecule stability.

Chemical Stability

Chemical stability in a molecule is largely influenced by the arrangement of its electrons and the formal charges on its atoms. Molecules strive for a configuration where their energy is minimized, meaning they prefer to be in a state where the sum of formal charges is as close to zero as possible.
In the case of the cyanate ion \((\mathrm{OCN}^{-})\), stability is achieved through the balanced formal charges: oxygen carries a \(-1\) charge while carbon and nitrogen are neutral. The ion holds stable chemical stability enabling it to exist without high reactivity. On the other hand, the fulminate ion \((\mathrm{CNO}^{-})\) is unstable due to the unusual negative charge on the central atom, which is contrary to the typical tendencies of carbon.

• Chemically stable molecules are less reactive and don't rapidly decompose or explode, unlike unstable molecules.
• Unstable molecules like \(\mathrm{CNO}^{-}\) with a centralized negative charge are prone to decomposition or explosive reactions.

Evaluating chemical stability involves understanding both the electron arrangement and the distribution of charges across molecules.