Thiocyanate is the anion [SCN].

Thiocyanate is the anion [SCN]. For the following resonance structures: $:N \text{-} C \equiv S: \quad \longrightarrow \quad :N \equiv C \text{-} S: \quad \longrightarrow \quad :N \equiv C \text{-} S: i) Show how to convert from one structure to the other using curved arrows ii) Assign formal charges on appropriate atoms iii) Determine, then explain if one resonance structure is more likely than the other.

The Correct Answer and Explanation is:

Answer and Explanation:

i) Conversion Using Curved Arrows:

In the thiocyanate ion (SCN–), the resonance structures show the distribution of electrons around the atoms. Here’s how to convert from one resonance structure to the other using curved arrows:

1. Structure 1: :N - C ≡ S:
   • The nitrogen has a lone pair of electrons (shown as :) and is single-bonded to carbon.
   • Carbon is triple-bonded to sulfur.
2. Structure 2: :N ≡ C - S:
   • The lone pair from nitrogen forms a bond with carbon, creating a triple bond between nitrogen and carbon.
   • The bond between carbon and sulfur becomes a single bond, and sulfur gains a lone pair.
3. Curved Arrows:
   • The curved arrow starts from the lone pair on nitrogen and points toward the C-N bond to create a triple bond between nitrogen and carbon.
   • The curved arrow from the C≡S bond points to sulfur, creating a lone pair on sulfur and weakening the C≡S bond.

ii) Assign Formal Charges:

• Structure 1: :N - C ≡ S:
  • Nitrogen (N): Formal charge is 0. It has one single bond (2 electrons) and a lone pair (2 electrons).
  • Carbon (C): Formal charge is 0. It has one single bond (2 electrons) and one triple bond (6 electrons).
  • Sulfur (S): Formal charge is 0. It has three bonds (6 electrons) and one lone pair (2 electrons).
• Structure 2: :N ≡ C - S:
  • Nitrogen (N): Formal charge is 0. It has a triple bond (6 electrons) and a lone pair (2 electrons).
  • Carbon (C): Formal charge is 0. It has a triple bond with nitrogen (6 electrons) and a single bond with sulfur (2 electrons).
  • Sulfur (S): Formal charge is -1. It has one single bond (2 electrons), two lone pairs (4 electrons), and one bond to carbon.

iii) Likelihood of Resonance Structures:

While both resonance structures are plausible, Structure 1 is more likely than Structure 2. Here’s why:

• Bonding Preferences: Nitrogen prefers to form three bonds or have one lone pair, as it follows the octet rule. In Structure 1, nitrogen follows this rule with one lone pair and a single bond to carbon.
• Electronegativity: Sulfur is more electronegative than carbon, so it is better able to accommodate a negative charge, making the negative charge on sulfur in Structure 2 more reasonable.
• Charge Distribution: In Structure 1, the charges are distributed more evenly between the atoms, whereas in Structure 2, sulfur carries a negative charge, which is not optimal. Therefore, Structure 1 is favored due to better charge delocalization and bond formation.

In conclusion, the resonance structure where nitrogen is single-bonded to carbon and sulfur is triple-bonded to carbon (Structure 1) is more stable and favored.