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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3:44 minutes

Problem 44c

Textbook Question

For each of these ions, draw the important resonance forms and predict which resonance form is likely to be the major contributor.
(c)

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Identify the structure of the ion: The given ion is a pyridinium ion with a positive charge on the CH2 group attached to the nitrogen-containing aromatic ring.

Determine the possible resonance forms: The positive charge on the CH2 group can be delocalized into the aromatic ring. This involves moving electrons from the double bonds in the ring to form new double bonds and shifting the positive charge around the ring.

Draw the first resonance form: Move the electrons from the double bond adjacent to the CH2 group towards the CH2 group, forming a new double bond between the CH2 and the ring, and shifting the positive charge to the nitrogen atom.

Draw additional resonance forms: Continue moving the positive charge around the ring by shifting electrons from adjacent double bonds, ensuring that each atom maintains a valid valence structure.

Predict the major resonance contributor: The major resonance form is typically the one with the least charge separation and the most stable arrangement of charges. In this case, the form where the positive charge is on the nitrogen atom is likely to be the major contributor, as nitrogen can better accommodate a positive charge due to its electronegativity.

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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 a molecule that depict the same arrangement of atoms but differ in the distribution of electrons. They are used to represent delocalized electrons within certain molecules where the bonding cannot be expressed by a single Lewis structure. The true structure is a hybrid of all possible resonance forms, with some contributing more to the hybrid than others.

Major and Minor Resonance Contributors

In resonance, not all structures contribute equally to the resonance hybrid. The major contributor is the resonance form with the lowest energy, typically having the most covalent bonds, the least charge separation, and negative charges on the most electronegative atoms. Minor contributors have higher energy due to less favorable charge distributions or incomplete octets.

Aromaticity and Stability

Aromaticity refers to the increased stability of certain cyclic, planar molecules with delocalized pi electrons, following Huckel's rule (4n+2 pi electrons). In the context of resonance, aromatic systems often have resonance forms that maintain aromaticity, contributing significantly to the stability of the molecule. The presence of an aromatic ring can influence the major resonance contributor by stabilizing positive or negative charges through delocalization.

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Related Practice

Textbook Question

For each of these ions, draw the important resonance forms and predict which resonance form is likely to be the major contributor.

(a)

Textbook Question

For each of the following compounds, draw the important resonance forms. Indicate which structures are major and minor contributors or whether they have the same energy.

(d) [H₂CNO₂]^–

Textbook Question

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

(d)

Textbook Question

For each of these ions, draw the important resonance forms and predict which resonance form is likely to be the major contributor.

(b)

Textbook Question

For each of the following compounds, draw the important resonance forms. Indicate which structures are major and minor contributors or whether they have the same energy.

(e) [CH₃C(OH)₂]⁺

(f) [CH₂CHNH]^–

Textbook Question

For each of the following compounds, draw the important resonance forms. Indicate which structures are major and minor contributors or whether they have the same energy.

(g)

(h)

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:

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For each of these ions, draw the important resonance forms and predict which resonance form is likely to be the major contributor.(c)

Key Concepts

Resonance Structures

Major and Minor Resonance Contributors

Aromaticity and Stability

Watch next

For each of these ions, draw the important resonance forms and predict which resonance form is likely to be the major contributor.
(c)