VI. A) (10 Pts) The Anion Below, CH2NO2, Has Three Important Resonance Contributors For The Lewis Structure Below, Supply Any Missing Formal Charges. Draw Two Additional Resonance Forms, Rank Each Of The Resonance Contributors In Order Of Importance, And Explain Your Rankings. (If Any Are Equivalent, Be Sure To Indicate So In Your Answer.)
The Correct Answer and Explanation is :
Step 1: Understanding the Anion CH2NO2
The structure of the CH2NO2 anion likely consists of a nitro group (NO2) attached to a methylene group (CH2), with a negative charge. The anion typically exists in resonance, where electrons are delocalized, resulting in multiple valid Lewis structures (resonance contributors).
Step 2: Drawing Two Additional Resonance Forms
Resonance Form 1:
- The negative charge on the oxygen can shift to the nitrogen.
- One of the nitrogen-oxygen bonds can be broken, creating a double bond between nitrogen and oxygen.
- The formal charge on the oxygen becomes -1, while nitrogen carries a positive charge.
Resonance Form 2:
- The electron pair from the carbon-oxygen bond in the CH2 group could shift to form a double bond between carbon and oxygen.
- The negative charge then resides on the oxygen, and the carbon would have no formal charge.
Step 3: Formal Charges
- In each resonance form, you must ensure that the sum of the formal charges equals the overall charge on the molecule (which in this case is -1).
- Formal charge calculation follows this formula:
Formal Charge = Valence electrons – (Non-bonding electrons + 1/2 bonding electrons) - You should fill in the formal charges based on this principle for each atom.
Step 4: Ranking the Resonance Forms
When ranking the resonance contributors, the most stable forms will have:
- A formal charge distribution that minimizes separation of charge.
- Formal charges on electronegative atoms (like oxygen) and positive charges on less electronegative atoms (like nitrogen).
Ranking Explanation:
- Form with no formal charge separation (neutral): This is typically the most stable form.
- Form with formal charge on oxygen: Oxygen is electronegative, so it stabilizes a negative charge well.
- Form with formal charge on nitrogen: Since nitrogen is less electronegative than oxygen, a positive charge on nitrogen is less stable, making this resonance form less important.
In general, resonance forms with a charge on oxygen are favored over those with a charge on nitrogen because oxygen can better stabilize negative charges due to its higher electronegativity.