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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5:40 minutes

Problem 8c

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.
(c) [H₂COCH₃]⁺

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Identify the structure of the compound [H2COCH3]+. This is a protonated acetaldehyde, where the positive charge is on the oxygen atom.

Draw the Lewis structure of the compound, showing all atoms, bonds, and the positive charge on the oxygen atom.

Consider the movement of electrons to form resonance structures. The positive charge on the oxygen can be delocalized by moving the lone pair of electrons from the oxygen to form a double bond with the adjacent carbon atom.

Draw the resonance structure where the oxygen forms a double bond with the carbon, and the positive charge is now on the carbon atom. This is a major contributor because it allows the positive charge to be on a less electronegative atom (carbon) compared to oxygen.

Evaluate the resonance structures: The structure with the positive charge on the carbon is a major contributor due to the stability provided by the delocalization of charge. The original structure with the positive charge on the oxygen is a minor contributor because the charge is on a more electronegative atom, which is less stable.

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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. Understanding resonance is crucial for predicting the stability and reactivity of molecules.

Major and Minor Contributors

In resonance, not all structures contribute equally to the resonance hybrid. Major contributors are the most stable structures, typically having the least charge separation and full octets on atoms. Minor contributors are less stable, often with more charge separation or incomplete octets. Identifying these helps in understanding the true electron distribution in the molecule.

Formal Charge

Formal charge is a bookkeeping tool used to determine the charge distribution within a molecule. It is calculated by assuming equal sharing of electrons in bonds. Structures with formal charges closest to zero are generally more stable. Calculating formal charges helps in identifying major and minor resonance contributors by highlighting the most stable electron arrangements.

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Related Practice

Textbook Question

Draw the resonance contributors for the following radicals:

Textbook Question

The given was missing some important resonance forms of the intermediates shown in brackets. Complete this mechanism by drawing all the resonance forms of these intermediates. Do your resonance forms help to explain why this reaction occurs under very mild conditions (water with a tiny trace of acid)?

Textbook Question

All of the rings of the four heterocyclic bases are aromatic. This is more apparent when the polar resonance forms of the amide groups are drawn, as is done for thymine here. Redraw the hydrogen-bonded guanine-cytosine and adenine-thymine pairs shown in Figure 23-24, using the polar resonance forms of the amides. Show how these forms help to explain why the hydrogen bonds involved in these pairings are particularly strong. Remember that a hydrogen bond arises between an electron-deficient hydrogen atom and an electron-rich pair of nonbonding electrons.

Textbook Question

Which of the following has delocalized electrons?

Textbook Question

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

(c)

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)

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