Ch. 16 - Reactions of Aldehydes and Ketones • More Reactions of Carboxylic Acid Derivatives

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 16 - Reactions of Aldehydes and Ketones • More Reactions of Carboxylic Acid Derivatives

Problem 82

Chapter 17, Problem 82

Unlike a phosphonium ylide that reacts with an aldehyde or a ketone to form an alkene, a sulfonium ylide reacts with an aldehyde or a ketone to form an epoxide. Explain why one ylide forms an alkene, whereas the other forms an epoxide.

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Understand the structure of the ylides: A phosphonium ylide contains a positively charged phosphorus atom bonded to a carbon atom with a lone pair of electrons, while a sulfonium ylide contains a positively charged sulfur atom bonded to a carbon atom with a lone pair of electrons.

Analyze the reactivity of the ylides: Phosphonium ylides are more nucleophilic and less electrophilic compared to sulfonium ylides due to the difference in electronegativity between phosphorus and sulfur. Sulfur is more electronegative, making sulfonium ylides more electrophilic and less nucleophilic.

Consider the reaction mechanism for phosphonium ylides: Phosphonium ylides react with aldehydes or ketones via the Wittig reaction. The nucleophilic carbon of the ylide attacks the carbonyl carbon, forming a betaine intermediate, which rearranges to produce an alkene and a triphenylphosphine oxide byproduct.

Consider the reaction mechanism for sulfonium ylides: Sulfonium ylides react with aldehydes or ketones via a different pathway. The electrophilic sulfur atom stabilizes the intermediate, leading to a concerted cyclization reaction that forms a three-membered epoxide ring.

Summarize the key difference: The difference in product formation arises from the nature of the ylide and the reaction mechanism. Phosphonium ylides favor the formation of alkenes due to their nucleophilicity and the Wittig reaction pathway, while sulfonium ylides favor the formation of epoxides due to their electrophilicity and the stabilization of the cyclization intermediate.

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Key Concepts

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

Ylide Chemistry

Ylides are neutral molecules containing a positively charged atom adjacent to a negatively charged atom, typically involving phosphorus or sulfur. In organic reactions, ylides can act as nucleophiles, attacking electrophiles such as carbonyl compounds. The nature of the ylide, whether phosphonium or sulfonium, influences the type of reaction it undergoes with aldehydes or ketones.

Mechanism of Alkene Formation

Phosphonium ylides react with aldehydes or ketones through a mechanism known as the Wittig reaction, which involves the formation of an intermediate that ultimately leads to the elimination of a phosphine oxide and the formation of an alkene. This reaction is characterized by the formation of a carbon-carbon double bond, which is a key feature of the Wittig reaction, making it a powerful tool in organic synthesis.

Epoxide Formation

Sulfoniun ylides react with carbonyl compounds to form epoxides through a different mechanism that involves a cyclic transition state. This reaction typically proceeds via a nucleophilic attack on the carbonyl carbon, followed by intramolecular closure to form a three-membered epoxide ring. The unique electronic properties of the sulfonium ylide facilitate this pathway, distinguishing it from the alkene-forming reaction of phosphonium ylides.