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

10. Addition Reactions

Carbene

Organic Chemistry

10. Addition Reactions

Carbene

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Problem 59

Textbook Question

Nitrogen-containing heterocycles form particularly stable carbenes and are commonly used as ligands in organometallic chemistry. (a) Why is the carbene shown particularly stable? [It may be helpful to draw the molecular orbital picture.]

Verified step by step guidance

Examine the structure of the carbene provided in the image. The carbene is part of a nitrogen-containing heterocycle, specifically an imidazole ring. This structure is characterized by two nitrogen atoms and a conjugated π-system.

Understand the electronic stabilization of the carbene. The lone pair of electrons on the carbene carbon is stabilized by resonance with the adjacent nitrogen atoms. This delocalization of electrons reduces the electron density on the carbene carbon, making it less reactive and more stable.

Consider the role of the nitrogen atoms in the heterocycle. The nitrogen atoms are electron-rich and can donate electron density into the π-system of the ring. This further stabilizes the carbene through resonance and inductive effects.

Analyze the molecular orbital picture. The carbene carbon has an sp2 hybridization, with one empty p orbital and one lone pair of electrons. The lone pair can overlap with the π-system of the ring, forming a delocalized molecular orbital that contributes to the stability of the carbene.

Recognize the importance of aromaticity. The imidazole ring is aromatic, meaning it follows Huckel's rule (4n+2 π electrons). Aromaticity provides additional stabilization to the carbene, as the conjugated π-system is energetically favorable and resists disruption.

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Key Concepts

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

Carbenes

Carbenes are reactive intermediates containing a carbon atom with only six valence electrons, making them electron-deficient. They can exist in two forms: singlet, where the two non-bonding electrons are paired, and triplet, where they are unpaired. The stability of a carbene can be influenced by the surrounding atoms or groups, particularly through resonance or hyperconjugation, which can help stabilize the electron-deficient carbon.

Molecular Orbitals

Molecular orbital (MO) theory describes the behavior of electrons in molecules by combining atomic orbitals to form molecular orbitals. These MOs can be bonding, non-bonding, or antibonding, and their occupancy determines the stability and reactivity of the molecule. A molecular orbital diagram can illustrate how the electrons are distributed in a molecule, providing insight into the stability of carbenes and their interactions with other species.

Ligands in Organometallic Chemistry

Ligands are ions or molecules that can donate a pair of electrons to a metal center, forming coordination complexes. In organometallic chemistry, nitrogen-containing heterocycles often act as bidentate or multidentate ligands, stabilizing metal centers through chelation. The stability of these complexes is crucial for catalysis and other chemical processes, as the nature of the ligand can significantly influence the reactivity and selectivity of the metal.

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Related Practice

Textbook Question

Predict the product of the following reactions.

(a)

Textbook Question

Cyclopropanation using any of the reagents discussed here is stereospecific.

(b) Draw a reaction coordinate diagram for cyclopropanation.

Textbook Question

In addition to radicals, anions, and cations, a fourth class of reactive intermediates is carbenes. A neutral species, the simplest carbene has a molecular formula of CH₂.

(h) Of the reactions studied here in Chapter 8, cyclopropanation is most similar to which?

Textbook Question

To determine the stereochemistry of curacin A by synthesis, it would have been necessary to prepare all stereoisomers of the C₁₇―C₂₀ cyclopropane fragment. How would the reaction in Figure 16.30 be modified to produce the other stereoisomers shown here?

Textbook Question

The reaction of an alkene with diazomethane forms a cyclopropane ring. Propose a mechanism for the reaction. (Hint: It is a concerted reaction.)

Textbook Question

Show how you would accomplish each of the following synthetic conversions.

a. trans-but-2-ene → trans-1,2-dimethylcyclopropane

Textbook Question

Show how you would synthesize each compound using methylenecyclohexane as your starting material.

(g)

Textbook Question

Propose mechanisms consistent with the following reactions.

(d)

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