Phosphonium Ylides

Definition

Phosphonium ylides are a class of organic compounds containing a positively charged phosphorus atom bonded to a negatively charged carbon atom. They are important intermediates in organic synthesis, particularly in the Wittig reaction, which allows for the formation of carbon-carbon double bonds.

5 Must Know Facts For Your Next Test

1. Phosphonium ylides have a formal positive charge on the phosphorus atom and a formal negative charge on the adjacent carbon atom.
2. The formal charges in phosphonium ylides are stabilized through resonance, with the negative charge delocalized across the phosphorus-carbon bond.
3. Phosphonium ylides are commonly used in the Wittig reaction to synthesize alkenes from the reaction with carbonyl compounds.
4. The reactivity of phosphonium ylides is influenced by the substituents attached to the phosphorus and carbon atoms, which can modulate the stability of the formal charges.
5. Phosphonium ylides are typically generated in situ from the reaction of a phosphonium salt and a strong base, such as an alkoxide or amide.

Review Questions

• Explain how the formal charges in phosphonium ylides are stabilized through resonance.
  • The formal positive charge on the phosphorus atom and the formal negative charge on the adjacent carbon atom in phosphonium ylides are stabilized through resonance. The negative charge can be delocalized across the phosphorus-carbon bond, with the carbon atom sharing its lone pair of electrons with the positively charged phosphorus. This resonance stabilization helps to minimize the overall energy of the system and contributes to the stability and reactivity of phosphonium ylides.
• Describe the role of phosphonium ylides in the Wittig reaction and how the reaction mechanism involves the formal charges.
  • Phosphonium ylides are key intermediates in the Wittig reaction, which is used to form carbon-carbon double bonds. In the reaction, the phosphonium ylide reacts with a carbonyl compound (aldehyde or ketone), with the negatively charged carbon of the ylide attacking the carbonyl carbon. This forms a cyclic intermediate called an oxaphosphetane, where the formal positive charge on the phosphorus is stabilized. The oxaphosphetane then undergoes a retro-[2+2] cycloaddition to eliminate triphenylphosphine oxide and generate the desired alkene product, with the formal charges playing a crucial role in the mechanism.
• Analyze how the substituents on the phosphorus and carbon atoms of a phosphonium ylide can influence its stability and reactivity.
  • The substituents attached to the phosphorus and carbon atoms of a phosphonium ylide can significantly impact its stability and reactivity. Electron-withdrawing groups on the phosphorus atom can stabilize the formal positive charge, while electron-donating groups can destabilize it. Similarly, electron-withdrawing groups on the carbon atom can stabilize the formal negative charge, while electron-donating groups can destabilize it. The balance of these electronic effects determines the overall stability of the phosphonium ylide and its reactivity in processes like the Wittig reaction. Careful selection of the substituents can be used to tune the reactivity of phosphonium ylides to achieve desired outcomes in organic synthesis.

Related terms

Wittig Reaction: A chemical reaction in organic chemistry that allows for the formation of carbon-carbon double bonds from the reaction of a phosphonium ylide and a carbonyl compound.

Formal Charge: The charge assigned to an atom in a molecule, calculated by subtracting the number of unshared electrons and half the number of shared electrons from the atom's valence electron count.

Resonance Structures: Multiple Lewis structures that can be drawn for a molecule, where the atoms are arranged differently but the overall connectivity and formal charges are the same.