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🎯 By the end of this guide,

You will learn how to rank resonance forms by stability, identify the major resonance contributor, and apply these principle to solve resonance problems with confidence.

 

 

Challenge 1: Ranking resonance forms

 

Question:

Rank the following resonance forms, from least significant to most significant, and briefly explain the rankings.

 

 

Here’s how we’ll do it

 

Step 1: Identify atoms with incomplete octets

 

Key rule: Resonance forms with more atoms that have full octets are more significant.

 

Quick check! Which resonance form has atoms with incomplete octets?

• Structure I: All atoms have full octets (8 electrons for carbon, nitrogen, and oxygen; 2 for hydrogen). ✅

 

 

• Structure II: All atoms have full octets. ✅

 

 

• Structure III: The carbon atom with the positive charge has only three bonds, for a total of 6 electrons—it doesn’t have a full octet. 🚩

 

 

So, structure III with the incomplete octet is the least significant resonance form of the three.

 

Ranking so far:
III < ? < ?

 

Step 2: Check for formal charges

 

The next step is to count formal charges. Structures with fewer formal charges are more significant because they’re more stable.

 

Quick check! Which structure has formal charges? I or II?

 

• Structure II has no formal charges. ✅

 

 

• Structure I has two formal charges (negative on oxygen, positive on nitrogen). 🚩

 

 

So, structure II with filled octets and no formal charges is the best Lewis structure and the most significant resonance form.

 

Ranking so far:
III < I < II

 

Step 3: Consider the location of formal charge(s)

Sometimes, the tie-breaker is where those charges are placed:

 

• Positive charges are more stable on less electronegative elements.
• Negative charges are more stable on more electronegative elements.

In this case, we already ranked the forms using Steps 1 and 2, so we don’t need to apply this step here. But keep it in mind—it’ll come in handy later!

 

Final ranking:
III < I < II

 

 

Great work! By following these steps, you’ve ranked the resonance forms like a pro. 🎉

 

 

Challenge 2: Identifying the major resonance contributor

 

Question:

Which of these four resonance forms contributes the most to the given molecule?

 

 

Let’s tackle this challenge step by step, just like before.

 

Step 1: Identify any atoms that lack an octet

 

Key rule: The most significant resonance forms have the greatest number of atoms with full octets.

 

Quick check! Which resonance form has atoms with incomplete octets?

 

Structures B and C: These contain carbocations (positively charged carbons with only 6 electrons). Since they lack octets, we can eliminate these right away. 🚩

 

 

Structures A and D: Both of these have all atoms with full octets. ✅

 

 

Conclusion so far:

Eliminate B and C. Focus on A and D for the next steps.

 

Step 2: Check for formal charges

 

Key Rule: Resonance forms with fewer formal charges are more significant because they’re more stable.

 

Quick check! How many formal charges do Structures A and D have?

 

 

• Structure A: Two formal charges (a negative charge on oxygen and a positive charge on nitrogen).
• Structure D: Two formal charges (a negative charge on carbon and a positive charge on nitrogen).

 

Since both structures have the same number of formal charges, we’ll need to break the tie in the next step.

 

Step 3: Consider the location of formal charges

 

Key Rule:

 

• A negative charge is more stable on the more electronegative atom (like oxygen).
• A positive charge is more stable on the less electronegative atom (like nitrogen).

 

Quick check! Where are the formal charges in Structures A and D?

 

Structure A:

• Negative charge on oxygen (a very electronegative atom). ✅
• Positive charge on nitrogen (less electronegative, so it handles the positive charge well). ✅

 

Structure D:

 

• Negative charge on carbon (less electronegative, so it’s less stable). 🚩
• Positive charge on nitrogen (same as Structure A).

 

Structure A is more stable because its negative charge is on oxygen, a more electronegative atom, making it a greater contributor to the overall structure.

 

Final answer:

The major resonance contributor is Structure A!

 

Great work! By following these steps, you’ve determined the major resonance contributor to the given molecule like a pro. 🎉

 

Recap

 

Here’s a recap of the tools we’ve built so far to rank resonance forms and identify major (significant) resonance forms:

 

• Identify any atoms that lack an octet. The resonance form with the most atoms having filled octets is typically the most stable.
• Identify formal charges. A structure with fewer formal charges is generally lower in energy.
• Consider the location of the formal charge(s). Charges are more stable when:
  • A negative charge is on the more electronegative atom.
  • A positive charge is on the less electronegative atom.

 

 

Challenge 3: Identifying the lowest energy resonance form

 

Question:

For each pair, circle the major (lowest energy) resonance structure and provide a brief explanation for your choice.

 

Pair 1

 

 

• Structures A and B have complete octets (lone pairs on oxygen and nitrogen are not shown, but remember to draw those in for clarity).
• Both structures have one atom with a negative charge.
• Structure B with a negative charge on oxygen is more stable than structure A with a negative charge on nitrogen.

 

Answer: Structure B is the lowest energy resonance form because the negative charge is on oxygen, which is better at stabilizing it.

 

Pair 2

 

 

• In structure A, all atoms have filled octets. In structure B, the carbocation lacks an octet (it only has 6 electrons).

 

Answer: Structure A is the major (lower energy) resonance form because it has more atoms with complete octets.

 

Pair 3

 

 

• In both structures, all atoms have filled octets.
• Structure A has four atoms with formal charges while structure B has two atoms with formal charges.

 

Answer: Structure B is the major (lower energy) resonance form because it has fewer atoms with formal charges.

 

Pair 4

 

 

• Both structures have a carbocation which has 6 electrons.
• In structure A, the positive charge is located on a tertiary carbon. In structure B, the positive charge is located on a primary carbon.

 

Answer: Structure A is the major (lower energy) resonance form because the positive charge is more stable on a tertiary carbon.

 

Great job! You’ve mastered the tools to rank resonance forms, identify major contributors, and spot the lowest-energy structures. Keep practicing, and you’ll tackle resonance problems with confidence. Happy studying! 🎉