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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3:41 minutes

Problem 90

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.)

Verified step by step guidance

Step 1: Recognize that diazomethane (CH₂N₂) is a highly reactive compound due to the presence of a diazo group (-N≡N) and a negatively charged carbon atom. This makes it a good nucleophile and a source of carbene intermediates.

Step 2: Understand that the reaction is concerted, meaning all bond-making and bond-breaking events occur simultaneously in a single step. The alkene (CH₂=CH₂) acts as an electron-rich species, while diazomethane provides the carbene intermediate.

Step 3: Propose the mechanism: The diazomethane undergoes decomposition to generate a carbene (CH₂:), which is a neutral species with a divalent carbon atom and two nonbonding electrons. This carbene is highly reactive and interacts with the alkene.

Step 4: The carbene reacts with the alkene in a concerted manner. The π-electrons of the alkene attack the carbene, forming two new σ-bonds simultaneously, resulting in the formation of a cyclopropane ring.

Step 5: During the reaction, nitrogen gas (N₂) is released as a byproduct due to the decomposition of diazomethane. This release of N₂ drives the reaction forward energetically.

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Key Concepts

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

Alkenes

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond (C=C). This double bond is a site of reactivity, making alkenes important in organic synthesis. Their ability to undergo addition reactions allows them to react with various reagents, leading to the formation of more complex structures, such as cyclopropanes.

Concerted Reactions

Concerted reactions are processes in which bond-breaking and bond-forming occur simultaneously in a single step, without intermediates. This type of mechanism is characterized by a transition state where all changes happen at once. Understanding concerted mechanisms is crucial for predicting the outcome of reactions, such as the formation of cyclopropanes from alkenes and diazomethane.

Cyclopropane Formation

Cyclopropane formation involves the addition of a reagent across the double bond of an alkene, resulting in a three-membered ring structure. This reaction is typically facilitated by the presence of a reactive species, such as diazomethane, which can insert into the double bond. The unique strain and reactivity of cyclopropanes make them interesting intermediates in organic synthesis.

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Related Practice

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

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.]

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)

Textbook Question

Limonene is one of the compounds that give lemons their tangy odor. Show the structures of the products expected when limonene reacts with an excess of each of these reagents.

m. CHBr₃ and 50% aq. NaOH

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