what is the lewis structure for the anion -CH2CN?
The Correct Answer and Explanation is:
Lewis Structure for the Anion –CH₂CN⁻:
The molecular formula of the anion is CH₂CN⁻, which contains:
- 3 carbon atoms,
- 2 hydrogen atoms,
- 1 nitrogen atom,
- and a negative charge (total charge = –1).
Lewis Structure:
mathematicaCopyEditH H
\ /
C — C ≡ N:
|
[:]
(One lone pair on the terminal nitrogen, negative charge delocalized.)
Explanation
The anion –CH₂CN⁻, also known as the cyanomethyl anion, consists of a methyl group (CH₂) bonded to a nitrile group (C≡N). The total number of valence electrons must be calculated to draw the correct Lewis structure.
Step 1: Count valence electrons.
- Carbon (C): 4 electrons × 3 = 12
- Hydrogen (H): 1 electron × 2 = 2
- Nitrogen (N): 5 electrons
- Additional electron due to negative charge: 1
Total = 12 + 2 + 5 + 1 = 20 valence electrons
Step 2: Arrange atoms.
The basic structure is:
mathematicaCopyEditH₂C—C≡N
Here:
- The leftmost carbon is bonded to two hydrogen atoms and the middle carbon.
- The middle carbon forms a triple bond with the nitrogen atom.
Step 3: Satisfy the octet rule.
- Each hydrogen has 2 electrons (satisfied).
- The left carbon (CH₂) has four bonds: 2 to H, 1 to the middle carbon, and one lone pair (due to the negative charge).
- The central carbon forms a single bond with the left carbon and a triple bond with nitrogen (4 bonds total).
- The nitrogen forms a triple bond with carbon and holds one lone pair (to satisfy its octet).
Step 4: Formal charges.
- The negative charge primarily resides on the CH₂ carbon, making it a carbanion.
- This is supported by resonance: the lone pair on CH₂ can delocalize into the π-system of the nitrile.
The structure is stabilized by resonance and satisfies all atoms’ octet requirements, making it the most accurate Lewis representation of –CH₂CN⁻.
