Propose mechanisms for the following reactions.
(c)

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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.

Propose mechanisms for the following reactions.(c)