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

21. Aldehydes and Ketones: Nucleophilic Addition

Wittig Reaction

Organic Chemistry

21. Aldehydes and Ketones: Nucleophilic Addition

Wittig Reaction

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6:13 minutes

Problem 50c

Textbook Question

Propose mechanisms for the following reactions.
(c)

Verified step by step guidance

Step 1: Recognize the reaction type. This is a Wittig reaction, which is used to convert a carbonyl group (C=O) into an alkene (C=C) using a phosphonium ylide reagent.

Step 2: Identify the reactants. The starting material is cyclohexanone, which contains a ketone functional group. The reagent is triphenylphosphonium methylide (Ph₃P=CH₂), which acts as the ylide in the Wittig reaction.

Step 3: Describe the mechanism initiation. The phosphonium ylide attacks the electrophilic carbon of the carbonyl group in cyclohexanone, forming a betaine intermediate. This step involves nucleophilic addition.

Step 4: Explain the intermediate rearrangement. The betaine intermediate undergoes a [2+2] cycloaddition to form an oxaphosphetane intermediate, which is a four-membered ring containing oxygen and phosphorus.

Step 5: Detail the final step. The oxaphosphetane intermediate breaks down via elimination to produce the desired alkene (cyclohexyl methylene) and triphenylphosphine oxide (Ph₃P=O) as a byproduct.

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Key Concepts

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

Reaction Mechanisms

A reaction mechanism is a step-by-step description of how a chemical reaction occurs at the molecular level. It outlines the sequence of elementary steps, including bond breaking and formation, and the intermediates formed during the reaction. Understanding mechanisms is crucial for predicting the products and understanding the kinetics and thermodynamics of the reaction.

Types of Organic Reactions

Organic reactions can be classified into several types, including substitution, addition, elimination, and rearrangement reactions. Each type has distinct characteristics and mechanisms. Recognizing the type of reaction helps in predicting the behavior of reactants and the nature of the products formed, which is essential for proposing accurate mechanisms.

Curved Arrow Notation

Curved arrow notation is a visual representation used in organic chemistry to illustrate the movement of electrons during a reaction. Arrows indicate the direction of electron flow, showing how bonds are broken and formed. Mastery of this notation is vital for accurately depicting reaction mechanisms and understanding the underlying processes in organic reactions.

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Related Practice

Textbook Question

Suggest the appropriate carbonyl and Wittig reagent to make the following alkenes.

a. (E)-7-methylnon-4-en-3-one

Multiple Choice

Determine the carbonyl and ylide that formed the following product.

Multiple Choice

Predict the major, organic product for the following reaction.

Textbook Question

Trimethylphosphine is a stronger nucleophile than triphenylphosphine, but it is rarely used to make ylides. Why is trimethylphosphine unsuitable for making most phosphorus ylides?

Textbook Question

The Wittig reaction is useful for placing double bonds in less stable positions. For example, the following transformation is easily accomplished using a Wittig reaction.

(a) Show how you would use a Wittig reaction to do this.

(b) Show how you might do this without using a Wittig reaction, and then explain why the Wittig reaction is a much better synthesis.

Textbook Question

Like other strong nucleophiles, triphenylphosphine attacks and opens epoxides. The initial product (a betaine) quickly cyclizes to an oxaphosphetane that collapses to an alkene and triphenylphosphine oxide.

(a) Show each step in the reaction of trans-2,3-epoxybutane with triphenylphosphine to give but-2-ene. What is the stereochemistry of the double bond in the product?

Textbook Question

(b) Show how this sequence might be used to convert cis-cyclooctene to trans-cyclooctene.

Textbook Question

Show how you would accomplish the following syntheses efficiently and in good yield. You may use any necessary reagents.

(e)

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