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Concept
Phosphorus tribromide
Summary
Phosphorus tribromide is a colourless liquid with the formula PBr3. The liquid fumes in moist air due to hydrolysis and has a penetrating odour. It is used in the laboratory for the conversion of alcohols to alkyl bromides.
PBr3 is prepared by treating red phosphorus with bromine. An excess of phosphorus is used in order to prevent formation of PBr5:
P4 + 6 Br2 → 4 PBr3
Because the reaction is highly exothermic, it is often conducted in the presence of a diluent such as PBr3. Phosphorus tribromide is also generated in situ from red phosphorus and bromine.
Phosphorus tribromide, like PCl3 and PF3, has both properties of a Lewis base and a Lewis acid. For example, with a Lewis acid such as boron tribromide it forms stable 1 :1 adducts such as Br3B · PBr3. At the same time PBr3 can react as an electrophile or Lewis acid in many of its reactions, for example with amines.
An important reaction of PBr3 is with alcohols, where it replaces an OH group with a bromine atom to produce an alkyl bromide. All three bromides can be transferred.
Several detailed procedures are available. In some cases, triphenylphosphine/Br2 is superior to PBr3.
The mechanism for a primary alcohol involves formation of a phosphorous ester (to form a good leaving group), followed by an SN2 substitution.
Because of the SN2 substitution step, the reaction generally works well for primary and secondary alcohols, but fails for tertiary alcohols. If the reacting carbon centre is chiral, the reaction usually occurs with inversion of configuration at the carbon alpha to the alcohol, as is usual with an SN2 reaction.
In a similar reaction, PBr3 also converts carboxylic acids to acyl bromides:
The main use for phosphorus tribromide is for conversion of primary or secondary alcohols to alkyl bromides, as described above. PBr3 usually gives higher yields than hydrobromic acid, and it avoids problems of carbocation rearrangement- for example even neopentyl bromide can be made from the alcohol in 60% yield.
Another use for PBr3 is as a catalyst for the α-bromination of carboxylic acids.
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Triphenylphosphine
Triphenylphosphine (IUPAC name: triphenylphosphane) is a common organophosphorus compound with the formula P(C6H5)3 and often abbreviated to PPh3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists as relatively air stable, colorless crystals at room temperature. It dissolves in non-polar organic solvents such as benzene and diethyl ether. Triphenylphosphine can be prepared in the laboratory by treatment of phosphorus trichloride with phenylmagnesium bromide or phenyllithium.
Phosphorus trichloride
Phosphorus trichloride is an inorganic compound with the chemical formula PCl3. A colorless liquid when pure, it is an important industrial chemical, being used for the manufacture of phosphites and other organophosphorus compounds. It is toxic and reacts readily with water to release hydrogen chloride. Phosphorus trichloride was first prepared in 1808 by the French chemists Joseph Louis Gay-Lussac and Louis Jacques Thénard by heating calomel (Hg2Cl2) with phosphorus.
Haloalkane
The haloalkanes (also known as halogenoalkanes or alkyl halides) are alkanes containing one or more halogen substituents. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely used commercially. They are used as flame retardants, fire extinguishants, refrigerants, propellants, solvents, and pharmaceuticals. Subsequent to the widespread use in commerce, many halocarbons have also been shown to be serious pollutants and toxins.