Textbook Question
Show how Wittig reactions might be used to synthesize the following compounds. In each case, start with an alkyl halide and a ketone or an aldehyde.
(a) Ph–CH=C(CH3)2
(b) Ph–C(CH3)=CH2
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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 31b
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.
Verified step by step guidance1
Step 1: Understand the reaction mechanism. Triphenylphosphine (PPh₃) acts as a strong nucleophile and attacks the epoxide ring, leading to the formation of a betaine intermediate. This intermediate then cyclizes to form an oxaphosphetane.
Step 2: Recognize the stereochemical implications. The epoxide opening by triphenylphosphine occurs with inversion of configuration at the carbon atom being attacked. This inversion is key to converting cis-cyclooctene to trans-cyclooctene.
Step 3: Apply the reaction sequence. First, react cis-cyclooctene with a peracid (e.g., mCPBA) to form the corresponding epoxide. Ensure the stereochemistry of the epoxide matches the cis configuration of the starting alkene.
Step 4: Introduce triphenylphosphine to the epoxide. The nucleophilic attack by PPh₃ opens the epoxide ring, forming the betaine intermediate, which cyclizes to the oxaphosphetane. This intermediate collapses to yield the trans-cyclooctene and triphenylphosphine oxide.
Step 5: Confirm the stereochemical outcome. The reaction sequence results in the inversion of configuration, converting the cis-cyclooctene to trans-cyclooctene. Verify the stereochemistry of the product using spectroscopic methods or other analytical techniques.
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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 nucleophile to donate an electron pair to an electrophile, forming a chemical bond. Strong nucleophiles, like triphenylphosphine, are highly reactive and can effectively attack electrophilic centers, such as the carbon atoms in epoxides. Understanding nucleophilicity is crucial for predicting reaction pathways and mechanisms in organic chemistry.
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Nucleophilic Addition
Epoxide Ring Opening
Epoxide ring opening is a reaction where a nucleophile attacks one of the carbon atoms in an epoxide, leading to the cleavage of the three-membered ring. This reaction typically results in the formation of a more stable product, such as a betaine intermediate. The regioselectivity and stereochemistry of the attack are influenced by the nature of the nucleophile and the substituents on the epoxide.
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Cyclization and Rearrangement
Cyclization and rearrangement involve the transformation of intermediates into cyclic structures or the reorganization of atoms within a molecule. In this context, the betaine formed from the epoxide opening can cyclize to form an oxaphosphetane, which is a cyclic compound containing phosphorus. This intermediate can then collapse to yield an alkene and triphenylphosphine oxide, illustrating the dynamic nature of organic reactions.
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Related Practice
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?
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Textbook Question
(a) Outline the syntheses indicated in Solved Problem 18-2, beginning with aldehydes and alkyl halides.
(b) Both of these syntheses of 1-phenylbuta-1,3-diene form the central double bond. Show how you would synthesize this target molecule by forming the terminal double bond.
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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
Show how Wittig reactions might be used to synthesize the following compounds. In each case, start with an alkyl halide and a ketone or an aldehyde.
(c) Ph–CH=CH–CH=CH–Ph
(d)
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Textbook Question
Write the sequence of steps required for the conversion of benzene into benzenediazonium chloride.
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