Phosphoranes

Phosphoranes

Structure of a typical phosphorane

Phosphoranes are a class of organophosphorus compounds characterized by a pentavalent phosphorus atom. These compounds are notable for their trigonal bipyramidal geometry, which is a result of the phosphorus atom forming five covalent bonds. Phosphoranes are an important subject of study in chemistry due to their unique bonding and reactivity.

Structure and Bonding

Phosphoranes exhibit a trigonal bipyramidal molecular geometry, where the phosphorus atom is at the center of the structure. This geometry is a consequence of the valence shell electron pair repulsion theory (VSEPR), which predicts the arrangement of electron pairs around a central atom to minimize repulsion. In phosphoranes, the five substituents are arranged with three equatorial and two axial positions.

The bonding in phosphoranes involves the use of d-orbitals in phosphorus, allowing it to expand its valence shell beyond the typical octet. This results in the formation of five covalent bonds, which is unusual for main group elements. The axial bonds are typically longer than the equatorial bonds due to increased repulsion in the axial positions.

Synthesis

Phosphoranes can be synthesized through several methods, including the reaction of phosphines with halogens or peroxides. Another common method involves the nucleophilic substitution of phosphonium salts. The choice of method depends on the desired substituents and the specific phosphorane being synthesized.

Reactivity

Phosphoranes are known for their diverse reactivity, which includes acting as intermediates in various organic reactions. They can undergo nucleophilic attack at the phosphorus center, leading to the formation of phosphine oxides or other derivatives. Additionally, phosphoranes can participate in Wittig reactions, where they are used to convert carbonyl compounds into alkenes.

Applications

Phosphoranes have applications in organic synthesis, particularly in the formation of carbon-carbon bonds. They are also used as intermediates in the synthesis of pharmaceuticals and other complex organic molecules. The unique properties of phosphoranes make them valuable tools in the development of new synthetic methodologies.

Related Compounds

Phosphoranes are related to other organophosphorus compounds such as phosphines, phosphine oxides, and phosphonium salts. These compounds share similar phosphorus chemistry but differ in their oxidation states and reactivity.

Related Pages

• Organophosphorus chemistry
• Phosphine
• Wittig reaction
• Valence shell electron pair repulsion theory