Problem 107 - Chemistry

Question

In the cyanate ion, \(\mathrm{OCN}^{-}\), carbon is the central atom. (a) Draw as many resonance structures as you can for OCN , and assign formal charges to the atoms in each. (b) Which resonance structure makes the greatest contribution to the resonance hybrid? Which makes the least contribution? Explain.

Step-by-Step Solution

Answer

Structure 2 contributes most; Structure 3 contributes least due to formal charges.

Step 1: Understanding Resonance Structures

Resonance structures occur when multiple valid Lewis structures can be drawn for a molecule or ion, differing only in the placement of electrons. It is crucial to draw these possible configurations for the cyanate ion, \(\mathrm{OCN}^-\), with carbon as the central atom.

Step 2: Drawing the First Resonance Structure

Draw the resonance structure where nitrogen (N) forms a triple bond with carbon (C), and carbon forms a single bond with oxygen (O). Assign formal charges as follows: - N: \(+1\)- C: 0- O: \(-2\)Total formal charge: \(-1\), matches the charge of the ion.

Step 3: Drawing the Second Resonance Structure

Draw the resonance structure where oxygen (O) forms a double bond with carbon (C), and carbon forms a double bond with nitrogen (N). Assign formal charges as follows:- O: 0- C: 0- N: \(-1\)Total formal charge: \(-1\), matching the charge of the ion.

Step 4: Drawing the Third Resonance Structure

Draw the resonance structure where oxygen (O) forms a triple bond with carbon (C), and carbon forms a single bond with nitrogen (N). Assign formal charges as follows:- O: 0- C: \(+1\)- N: \(-2\)Total formal charge: \(-1\), matching the charge of the ion.

Step 5: Evaluating the Contribution of Each Structure

Evaluate each structure's stability by considering formal charges. The most stable structure usually minimizes formal charges. - Structure 2 has the least separation of charges (all formal charges are close to 0). Thus, it contributes the most to the resonance hybrid. - Structure 1 and Structure 3 have higher formal charges, making them less stable. Structure 3 has a larger separation of charges, hence it makes the least contribution.

Key Concepts

Formal Charge Calculation

Understanding formal charges is essential when analyzing molecules like the cyanate ion, \(\mathrm{OCN}^{-}\). Calculating the formal charge involves counting the valence electrons an atom has in a free state, then subtracting the non-bonded electrons and half the bonded electrons in a structure.
To find the formal charge, use the formula:\[ \text{Formal Charge} = \text{Valence Electrons} - (\text{Non-Bonding Electrons} + \frac{1}{2} \text{Bonding Electrons}) \]
This calculation helps in determining which resonance structures are more stable and more likely to occur in nature. A lower formal charge typically indicates a more stable configuration.

• Assigning charges correctly ensures that the total charge of the structure equals the charge of the ion, such as \(\text{-1}\) for \(\mathrm{OCN}^{-}\).
• By calculating the formal charges for the three resonant forms, you can determine which is more favorable.

Understanding these charges aids in explaining why some structures contribute more significantly to the resonance hybrid.

Lewis Structures

Lewis structures are diagrams showing the bonding between atoms in a molecule and the lone pairs of electrons that may exist. For the cyanate ion, \(\mathrm{OCN}^{-}\), these structures highlight the possible ways that oxygen (O), carbon (C), and nitrogen (N) can be arranged with each other.

When drawing Lewis structures:

• Determine the total number of valence electrons. For \(\mathrm{OCN}^{-}\), this totals to 16 electrons (6 from O, 4 from C, 5 from N, plus 1 extra for the negative charge).
• Select the central atom, typically the least electronegative, which in this case is carbon.
• Ensure all atoms reach an octet (or duet for hydrogen), while accounting for any formal charges.

By experimenting with different configurations, we derive multiple valid resonance structures for OCN, emphasizing the flexibility and variety inherent in molecular bonding.

Resonance Hybrid

A resonance hybrid is a composite of all possible resonance structures, representing the delocalization of electrons within the molecule or ion. For the cyanate ion, \(\mathrm{OCN}^{-}\), its resonance hybrid is more stable than any of the individual resonance structures.
The importance of a resonance hybrid lies in these factors:

• The resonance hybrid embodies a more accurate depiction of electron distribution.
• It combines aspects of all dot structures to enhance stability and energy minimization.
• Contributions of each resonance structure depend on their stability; structures with minimized formal charges contribute more.

In summary, understanding resonance hybrids obliges us to look at the bigger picture in molecular chemistry, recognizing that real molecules are hybrids, not fixed in just one arrangement.