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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4:08 minutes

Problem 30

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?

Verified step by step guidance

Understand the structure of trimethylphosphine (P(CH₃)₃) and triphenylphosphine (P(C₆H₅)₃). Trimethylphosphine has three methyl groups attached to the phosphorus atom, while triphenylphosphine has three phenyl groups attached to the phosphorus atom.

Recall that phosphorus ylides are compounds with a phosphorus atom bonded to a carbon atom that carries a negative charge (a carbanion). These ylides are typically formed by deprotonating a phosphonium salt, which is created by the reaction of a phosphine with an alkyl halide.

Consider the steric and electronic properties of trimethylphosphine. The methyl groups are small and do not provide significant steric hindrance, making trimethylphosphine a stronger nucleophile compared to triphenylphosphine. However, this also means that the resulting phosphonium salt is less stable due to the lack of resonance stabilization.

Analyze the stability of ylides formed from trimethylphosphine. The methyl groups on trimethylphosphine do not have the ability to delocalize the negative charge on the carbanion through resonance. In contrast, the phenyl groups in triphenylphosphine can stabilize the ylide through resonance, making the ylide more stable and easier to handle.

Conclude that trimethylphosphine is unsuitable for making most phosphorus ylides because the ylides formed are highly unstable due to the lack of resonance stabilization. This instability makes them difficult to isolate and use in reactions, limiting their practical applications.

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Key Concepts

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

Nucleophilicity

Nucleophilicity refers to the ability of a species to donate an electron pair to an electrophile, forming a chemical bond. In organic chemistry, stronger nucleophiles are more reactive and can participate in various reactions, such as forming ylides. Trimethylphosphine is a stronger nucleophile than triphenylphosphine due to its less sterically hindered structure, allowing it to more easily approach electrophiles.

Steric Hindrance

Steric hindrance occurs when the spatial arrangement of atoms within a molecule prevents other molecules from approaching closely. In the case of triphenylphosphine, the bulky phenyl groups create significant steric hindrance, making it less reactive as a nucleophile. Conversely, while trimethylphosphine is less hindered and more nucleophilic, its own steric properties and electronic effects can make it less suitable for forming stable ylides.

Phosphorus Ylides

Phosphorus ylides are compounds characterized by a phosphorus atom bonded to a carbon atom that carries a negative charge. They are typically formed through the reaction of a phosphine with an alkyl halide. The stability and reactivity of ylides depend on the nature of the phosphine used; trimethylphosphine, despite being a strong nucleophile, may lead to unstable ylides due to its electronic properties and the resulting instability of the ylide formed.

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Related Practice

Textbook Question

What two sets of reagents (each consisting of a carbonyl compound and phosphonium ylide) can be used for the synthesis of the following alkene?

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

Propose mechanisms for the following reactions.

(c)

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.

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