Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

1. A Review of General Chemistry

Resonance Structures

Organic Chemistry

1. A Review of General Chemistry

Resonance Structures

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5:59 minutes

Problem 50

Textbook Question

i) Which of the following resonance structures represents the 'actual' structure of the molecule shown? (ii) Which contributes more to the resonance hybrid? (iii) Why?
(a)
(b)
(c)

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Examine the resonance structures (a), (b), and (c) to determine if they follow the rules of resonance. Ensure that all structures obey the octet rule (if applicable), conserve the total number of electrons, and maintain proper formal charges.

Identify the 'actual' structure of the molecule by understanding that the true structure is a resonance hybrid, which is a weighted average of all valid resonance structures. None of the individual resonance structures fully represents the molecule; instead, the hybrid does.

To determine which resonance structure contributes more to the resonance hybrid, evaluate the stability of each structure. Stability is influenced by factors such as: (i) minimal formal charges, (ii) negative charges on more electronegative atoms, and (iii) avoidance of charge separation unless necessary.

Compare the resonance structures (a), (b), and (c) based on the criteria above. For example, a structure with no formal charges or with charges placed on appropriate atoms (e.g., negative charge on oxygen) will contribute more to the hybrid.

Explain why the most stable resonance structure contributes more to the hybrid. The resonance hybrid is closer in character to the most stable structure because it minimizes energy and maximizes stability, which is a fundamental principle in resonance theory.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Resonance Structures

Resonance structures are different Lewis structures for the same molecule that depict the same arrangement of atoms but differ in the placement of electrons. These structures are used to represent the delocalization of electrons within a molecule, which cannot be accurately depicted by a single Lewis structure. The actual structure of the molecule is a resonance hybrid, a weighted average of all valid resonance forms.

Resonance Hybrid

The resonance hybrid is the actual structure of a molecule that is formed by combining all the resonance structures. It reflects the delocalization of electrons across the molecule, resulting in bond lengths and angles that are intermediate between those predicted by the individual resonance forms. The hybrid is more stable than any single resonance structure due to the distribution of electron density.

Contribution to Resonance Hybrid

Not all resonance structures contribute equally to the resonance hybrid; some are more stable and thus have a greater contribution. Factors influencing this include the octet rule, charge distribution, and the presence of formal charges. Structures that minimize formal charges and maintain an octet for all atoms are typically more significant contributors to the resonance hybrid.

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Related Practice

Textbook Question

Use resonance structures to identify the areas of high and low electron density in the following compounds:

Textbook Question

Use resonance structures to identify the areas of high and low electron density in the following compounds:

Textbook Question

Draw the resonance contributors of the cyclooctatrienyl dianion.

b. Which of the resonance contributors makes the smallest contribution to the hybrid?

Textbook Question

a. Draw resonance contributors for the following species, showing all the lone pairs:

b. For each species, indicate the most stable resonance contributor.

3. NO₂^-

Textbook Question

a. Draw resonance contributors for the following species. Do not include structures that are so unstable that their contributions to the resonance hybrid would be negligible. Indicate which are major contributors and which are minor contributors to the resonance hybrid.

b. Do any of the species have resonance contributors that all contribute equally to the resonance hybrid?

10.

Textbook Question

b. Do any of the species have resonance contributors that all contribute equally to the resonance hybrid?

Textbook Question

b. Do any of the species have resonance contributors that all contribute equally to the resonance hybrid?

Textbook Question

b. Do any of the species have resonance contributors that all contribute equally to the resonance hybrid?

11.

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i) Which of the following resonance structures represents the 'actual' structure of the molecule shown? (ii) Which contributes more to the resonance hybrid? (iii) Why? (a) (b) (c)

Key Concepts

Resonance Structures

Resonance Hybrid

Contribution to Resonance Hybrid

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i) Which of the following resonance structures represents the 'actual' structure of the molecule shown? (ii) Which contributes more to the resonance hybrid? (iii) Why?
(a)
(b)
(c)