What is triphenylphosphine compound?

by BY P GHOSH on November 14, 2021 in ORGANIC

What is triphenylphosphine compound?

Triphenylphosphine is a common organophosphorus as well as tertiary phosphine compound with molecular formula C₁₈H₁₅P. It is also expressed as P(C₆H₅)₃ or Ph₃P. The molecular weight of triphenylphosphine is 262.29 g / mol.

The IUPAC name of triphenylphosphine is triphenylphosphane. It is a white crystalline solid and its density 1.1 g / cc. Triphenylphosphine exists as relatively air stable, colorless at room temperature.

Triphenylphosphine is a less polar compound. The dipole moment of triphenylphosphine is about 1.42D.

Triphenylphosphine is insoluble in water but it dissolved in non polar organic solvents such as benzene and diethyl ether.

The melting point of triphenylphosphine is 350K and its boiling point is 650K. Triphenylphosphine is a good reducing agent and act as a neutral ligand. That is why it is used in the synthesis of organic and organometallic compounds.

Preparation and molecular structure of triphenylphosphine

Triphenylphosphine can be prepared in the laboratory by the reaction of PCl₃ with phenyl magnesium bromide or phenyl lithium.

PCl₃ + 3C₆H₅MgBr → P(C₆H₅)₃ + 3MgClBr

PCl₃ + 3C₆H₅Li → P(C₆H₅)₃+ 3LiCl

Triphenylphosphine is also synthesized in industry by the reaction of phosphorous trichloride, bromo benzene and metallic sodium.

PCl₃ + 3C₆H₅Br + 6Na → P(C6H5)₃+ 3NaCl + 3NaBr

The central phosphorous atom in triphenylphosphine is sp3 hybridized. The phosphorous atom is directly attached with three phenyl group through P – C sigma bond and one sp3 hybridized orbital contains one lone pair of electron.

Hence triphenylphosphine attain pyramidal structure with propeller-like arrangement of the three phenyl groups.

Why triphenylphosphine is a poor base or good nucleophile?

The phosphorous atom in triphenylphosphine is sp3 hybridized. The sp3 hybridized orbital of phosphorous atom occupied on lone pair of electron which phosphorous atom can donate to the electron deficient species.

In spite of triphenylphosphine is a poor base. Because of the lone pair on ‘P’ atom participate in resonance with the π electron of three phenyl ring.

Hence the lone pair on phosphorous atom is less available for donation to the electron deficient species.

Consequently triphenylphosphine acts as a weak base. The pKa value of triphenylphosphine is 7.64. Since it is a poor base and hence acts as a good nucleophile.

In co-ordination chemistry, triphenylphosphine is a neutral ligand but it is considered as a strong field ligand due to its high splitting efficiency.

Application and reactions of triphenylphosphine

Triphenylphosphine is commonly used in the synthesis of many organic and organometallic compounds.

Some reactions of Triphenylphosphine:

Triphenylphosphine is a reducing agent. It undergoes slow oxidation by air to give triphenylphosphine oxide. 2Ph₃P + O₂→ 2Ph₃P=O

Triphenylphosphine is also oxidizes by halogen elements to give triphenyl-phosphine halides. For example, Cl₂ adds to triphenylphosphine to give triphenylphosphine dichloride [Ph₃PCl]Cl.

Triphenylphosphine is a weak base. It forms triphenylphosphonium salts, when combines with strong acid [HBr] or alkyl halide.

Ph₃P + HBr → Ph₃PH⁺ Br^–

Ph3P + PhBr → [Ph4P⁺Br^–] [tetraphenylphosphonium salt]

Application of triphenylphosphine

Organic reactions:

Triphenylphosphine is broadly used in organic synthesis of different organic compounds. It undergoes organic reactions due to its nucleophilicity and its reducing character.

(I)Alkenes from epoxides: Epoxides are readily reduced to the corresponding alkenes by triphenylphosphine, for example,

Again, triphenylphosphine also reduces amine oxides to amines. The major driving force in these reactions is the unusual high strength of P = O bond.

(II) Azoxy benzene from nitroso benzene:

Since triphenylphosphine is a reducing agent, it reduces nitroso benzene to give azoxy benzene.

C₆H₅NO + Ph₃P → C₆H₅N = N⁺(O^–)–C₆H₅ [azoxy benzene]

(III) What is Wittig reaction in organic chemistry?

Alkene from carbonyl compounds:

The most important application of triphenylphosphine is in Wittig reaction. In Wittig reaction, different alkenes are prepared from aldehyde and ketone by using triphenylphosphine.

At first, triphenylphosphine is reacts with alkyl halide [CH3Br] and strong base phenyl lithium resulting in the formation of methylene triphenyl phosphorane compound.

Now, the resulting compound is reacts with carbonyl compounds and forms an intermediate compound which instantly dissociate to the expected alkene along with triphenylphosphine oxide.

This reaction is called Wittig reaction. For example, acetaldehyde gives prop-1-ene and cyclohexanone gives methylenecyclohexane.