Phosphonium Salt

Definition

A phosphonium salt is a positively charged species containing a phosphorus atom with four covalently bonded substituents. These salts are important intermediates in the Wittig reaction, a powerful method for the synthesis of alkenes from carbonyl compounds and phosphorus ylides.

5 Must Know Facts For Your Next Test

1. Phosphonium salts are formed when a phosphine (PR3) reacts with an alkyl halide, resulting in a positively charged phosphorus atom with four substituents.
2. The phosphonium salt is a key intermediate in the Wittig reaction, where it reacts with a carbonyl compound to form a phosphorus ylide.
3. The phosphorus ylide then undergoes a nucleophilic addition to the carbonyl carbon, leading to the formation of an alkene product.
4. Phosphonium salts are often used as stable, isolable precursors to generate phosphorus ylides in situ for the Wittig reaction.
5. The choice of substituents on the phosphonium salt can influence the stereochemistry of the alkene product formed in the Wittig reaction.

Review Questions

• Describe the role of phosphonium salts in the Wittig reaction and explain how they are formed.
  • Phosphonium salts are key intermediates in the Wittig reaction, a powerful method for the synthesis of alkenes. They are formed when a phosphine (PR3) reacts with an alkyl halide, resulting in a positively charged phosphorus atom with four substituents. The phosphonium salt then reacts with a carbonyl compound to generate a phosphorus ylide, which undergoes a nucleophilic addition to the carbonyl carbon, leading to the formation of an alkene product. The choice of substituents on the phosphonium salt can influence the stereochemistry of the alkene product.
• Analyze the relationship between phosphonium salts, phosphorus ylides, and the mechanism of the Wittig reaction.
  • Phosphonium salts are precursors to phosphorus ylides, which are the key reactive intermediates in the Wittig reaction. The phosphonium salt is formed when a phosphine reacts with an alkyl halide, and this positively charged species then reacts with a carbonyl compound to generate the phosphorus ylide. The ylide then undergoes a nucleophilic addition to the carbonyl carbon, leading to the formation of an alkene product. The mechanism of the Wittig reaction involves the initial formation of the phosphonium salt, followed by its conversion to the phosphorus ylide, and finally the nucleophilic addition step that results in the alkene product.
• Evaluate the importance of phosphonium salts in organic synthesis, particularly in the context of the Wittig reaction and the formation of alkenes.
  • Phosphonium salts are of great importance in organic synthesis, as they serve as key intermediates in the Wittig reaction, a widely used method for the synthesis of alkenes. The Wittig reaction involves the reaction of a phosphonium salt with a carbonyl compound to generate a phosphorus ylide, which then undergoes a nucleophilic addition to the carbonyl carbon, leading to the formation of an alkene product. The versatility of the Wittig reaction, its ability to control the stereochemistry of the alkene product, and the ease of access to phosphonium salt precursors make them invaluable tools in the synthetic chemist's toolbox. Phosphonium salts are often used as stable, isolable precursors to generate phosphorus ylides in situ, allowing for the efficient and reliable synthesis of a wide range of alkene-containing compounds, which are important building blocks in the synthesis of complex organic molecules.

Related terms

Phosphorus Ylide: A phosphorus ylide is a neutral compound containing a positively charged phosphorus atom and a negatively charged carbon atom, typically used in the Wittig reaction to form alkenes.

Wittig Reaction:

The Wittig reaction is a method for the synthesis of alkenes from the reaction of a phosphorus ylide and a carbonyl compound, often facilitated by the formation of a phosphonium salt intermediate.

Nucleophilic Addition:

Nucleophilic addition is a type of reaction where a nucleophile adds to an electrophilic carbon, such as the carbonyl carbon in the Wittig reaction, to form a new product.