Chapter 19: Problem 39
Aldehydes can be prepared by the Wittig reaction using (methoxymethylene)triphenylphosphorane as the Wittig reagent and then hydrolyzing the product with acid. For example, (a) How would you prepare the necessary phosphorane? (b) Propose a mechanism for the hydrolysis step.
Short Answer
Expert verified
Prepare (methoxymethylene)triphenylphosphorane from triphenylphosphine and iodomethane. Hydrolyze product by acid-catalyzed attack and rearrangement.
Step by step solution
01
Identify the reactant
To prepare (methoxymethylene)triphenylphosphorane, begin with triphenylphosphine, which is a common starting material for ylide formation. It is essential because the phosphonium salt intermediate needed for the Wittig reaction can be obtained from it.
02
Form the Phosphonium Salt
React triphenylphosphine with an alkyl halide like iodomethane. This will lead to the formation of the phosphonium salt:\[\text{PPh}_3 + \text{CH}_3\text{I} \rightarrow \text{PPh}_3^+\text{CH}_3\text{I}^-\] where PPh₃ is short for triphenylphosphine.
03
Deprotonate to Form the Ylide
Treat the phosphonium salt with a strong base, such as butyllithium (BuLi), to deprotonate and form (methoxymethylene)triphenylphosphorane. The reaction is as follows:\[\text{PPh}_3^+\text{CH}_3\text{I}^- + \text{BuLi} \rightarrow \text{Ph}_3\text{P}=\text{CH}_2 + \text{BuI} + \text{H}_2\text{O}\]The methoxide that results from earlier step attacks one of the carbons in this compound, forming the ylide.
04
Mechanism of Hydrolysis - Activation
In the Wittig reaction product, typically an alkene, protonate the alkoxy group using an acid like HCl, which activates the group towards nucleophilic attack.
05
Mechanism of Hydrolysis - Nucleophilic Attack
The nucleophile, such as water, attacks the activated carbon, leading to the opening of the cyclic structure if present or rearrangement. Proton transfer occurs, stabilizing or forming a hydroxyl group, which is common in aldehyde products.
06
Mechanism of Hydrolysis - Protonation and Rearrangement
Further protonation of intermediates and rearrangements may occur, ultimately leading to the elimination of methanol and formation of the carbonyl group of the aldehyde.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Phosphorane Preparation
The preparation of phosphorane is a crucial step in the Wittig Reaction. To begin, we start with triphenylphosphine, a versatile organic compound useful for ylide formation. The initial step in creating the phosphorane involves forming a phosphonium salt. This is achieved by reacting triphenylphosphine
- with an alkyl halide, such as iodomethane
- resulting in triphenylphosphine becoming positively charged, forming a phosphonium salt
Hydrolysis Mechanism
In the Wittig reaction, hydrolysis converts an intermediate into a stable final product, often an aldehyde. The mechanism begins with the protonation of an alkoxy group found in the intermediate alkene product. Typically, an acid such as hydrochloric acid (HCl) accomplishes this.
Once protonated, this group becomes more reactive, sensitizing it to nucleophilic attack. Water, a common nucleophile, targets the reactive site, facilitating structural changes. Following this, a sequence of proton transfers occurs, which help in forming a hydroxyl group. This hydroxyl group is key as its presence suggests product stabilization. Eventually, it transitions into the more familiar form of an aldehyde by rearranging within the compound.
The process may involve the loss of side products, such as methanol, and further protonation, ensuring the aldehyde carbonyl group emerges prominently, marking the successful conclusion of hydrolysis.
Phosphonium Salt
Phosphonium salt plays a central role in ylide formation. A phosphonium salt emerges when triphenylphosphine reacts with an alkyl halide like iodomethane, creating a positively charged phosphine species. This reaction can be summarized as:\[\text{PPh}_3 + \text{CH}_3\text{I} \rightarrow \text{PPh}_3^+\text{CH}_3\text{I}^-\]The formation of phosphonium salts is significant due to their reactivity and ability to further transform into ylides. The charged state of phosphonium salts ensures they can easily participate in subsequent reactions, such as with strong bases. Thus, understanding the behavior and formation of phosphonium salts is vital for anyone wanting to harness the Wittig reaction for synthetic applications.
Ylide Formation
Ylides are among the most important intermediates in organic chemistry, particularly in the Wittig reaction. The creation of an ylide involves deprotonating a phosphonium salt. This step leverages a strong base like butyllithium (BuLi) to remove a hydrogen atom, facilitating ylide formation.
Here's how it works:
- The strong base approaches the phosphonium salt
- Deprotonation occurs as it extracts a hydrogen ion
- The remaining structure, typically a conjugate base, stabilizes as an ylide