Textbook Question
Show how you would accomplish the following syntheses. You may use whatever additional reagents you need.
(e)
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Ch. 18 - Ketones and Aldehydes
Wade9th EditionOrganic ChemistryISBN: 9780135213728
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Table of contents
- Ch.1 - Structure and Bonding107
- Ch. 2 - Acids and Bases; Functional Groups115
- Ch.3 - Structure and Stereochemistry of Alkanes100
- Ch.4 - The Study of Chemical Reactions99
- Ch.5 - Stereochemistry93
- Ch.6 - Alkyl Halides; Nucleophilic Substitution118
- Ch. 7 - Structure and Synthesis of Alkenes; Elimination158
- Ch.8 - Reactions of Alkenes171
- Ch.9 - Alkynes91
- Ch.10 - Structure and Synthesis of Alcohols143
- Ch.11 - Reactions of Alcohols104
- Ch. 12 - Infrared Spectroscopy and Mass Spectrometry22
- Ch. 13 - Nuclear Magnetic Resonance Spectroscopy45
- Ch. 14 - Ethers, Epoxides, and Thioethers72
- Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy89
- Ch. 16 - Aromatic Compounds74
- Ch. 17 - Reactions of Aromatic Compounds126
- Ch. 18 - Ketones and Aldehydes193
- Ch. 19 - Amines129
- Ch. 20 - Carboxylic Acids77
- Ch. 21 - Carboxylic Acid Derivatives141
- Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds175
- Ch. 23 - Carbohydrates and Nucleic Acids93
- Ch. 24 - Amino Acids, Peptides, and Proteins54
Chapter 18, Problem 30
Trimethylphosphine is a stronger nucleophile than triphenylphosphine, but it is rarely used to make ylides. Why is trimethylphosphine unsuitable for making most phosphorus ylides?
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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.
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Nucleophilic Addition
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.
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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
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.
(b) Show how this sequence might be used to convert cis-cyclooctene to trans-cyclooctene.
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Textbook Question
Show how you would accomplish the following syntheses. You may use whatever additional reagents you need.
(d)
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Textbook Question
Show how you would accomplish the following syntheses. You may use whatever additional reagents you need.
(f)
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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?
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Textbook Question
Write the sequence of steps required for the conversion of benzene into benzenediazonium chloride.
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