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Phosphorus Tribromide (PBr3): Formation and Uses

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Formation of Phosphorus Tribromide

Red phosphorus is treated with bromine to create PBr3. It is necessary to employ an abundance of phosphorus to prevent the development of PBr5.


$2P + 3B{r_2}\xrightarrow{{}}2PB{r_3}$


The reaction is frequently carried out with a diluent like PBr3 because of how strongly exothermic it is.


Potassium tribromide has the chemical formula PBr3. It's a colourless liquid with a distinct, piercing order. It is employed in chemical analysis, as a catalyst, and in the synthesis of other chemicals. In the laboratory, it is used to convert alcohols to alkyl bromides.


Reactions of Phosphorus Tribromide

Like PCl3 and PF3, phosphorus tribromide possesses both Lewis base and acid characteristics. For instance, it produces stable 1:1 adducts such as Br3B PBr3 with a Lewis acid-like boron tribromide.


Additionally, in many of its reactions, such as those with amines, PBr3 can behave as an electrophile or Lewis acid. To convert carboxylic acids into the appropriate acyl bromide, phosphorus tribromide (PBr3) is frequently utilised.


The most significant reaction of PBr3 is with alcohols, where it converts an OH group into an alkyl bromide by substituting a bromine atom. Each of the three bromides is transferable.


$PB{r_3} + 3ROH\xrightarrow{{}}3RBr + HP(O){(OH)_2}$


If the carbon centre that is reacting is chiral, the reaction typically takes place with an SN2 reaction-typical configuration inversion at the alcohol alpha carbon. PBr3 also changes carboxylic acids into acyl bromides in a similar process.


$PB{r_3} + 3RCOOH\xrightarrow{{}}3RCOBr + HP(O){(OH)_2}$


To be persistent and hazardous per se, the majority of reagents are too reactive to hydrolyze. There aren't any significant problems with low levels of bromide anions after neutralisation, but high local acidity could be a problem. Organic components connected to the reagent will be reflected in longer-term environmental consequences. There should be caution while handling aqueous waste streams since higher MW organic cations (such as quats and ionic liquids) might be harmful or inhibitive to some aquatic life forms.


Ph3P and Ph3PO should not be released into aqueous waste streams because they provide a special environmental risk. There may be local restrictions on the amount of phosphate that can be discharged because inorganic P-based chemicals produce phosphate on hydrolysis, and there are worries about eutrophication. Polybromo Organics have the potential to bio-absorb and persist.


Application of Phosphorus Tribromide

As a catalyst for the -bromination of carboxylic acids, PBr3 has another utility. In contrast to acyl chlorides, acyl bromides are less often produced, but they are nonetheless employed as intermediates in the Hell-Volhard-Zelinsky halogenation. The carboxylic acid and PBr3 initially react to create the more bromination-reactive acyl bromide.


Precautions for Phosphorus Tribromide

Toxic HBr, which PBr3 produces, react strongly with water and alcohol.


$PB{r_3} + 3{H_2}O\xrightarrow{{}}{H_3}P{O_3} + 3HBr$


Being aware that phosphorus acid can break down beyond roughly 160 °C to give phosphine, which can cause explosions when in contact with air, is important when working up by distillation in reactions that produce phosphorous acid as a by-product.


Uses of Phosphorus Tribromide

As mentioned above, the fundamental application of phosphorus tribromide is the transformation of primary or secondary alcohols into alkyl bromides. With PBr3, yields are typically higher than those of hydrobromic acid, and carbocation rearrangement issues are avoided. For instance, 60% of the alcohol can be converted to neopentyl bromide.


The main benefit of PBr3 is that it enables the conversion of chiral alcohols to bromides while maintaining configuration. They also demonstrate the reaction's mechanism, which involves the intermediate alkyl phosphites.


Phosphorus tribromide (PBr3) is frequently employed in the Hell-Volhard-Zelinsky halogenation process for the -bromination of carboxylic acids to produce the appropriate acyl bromide.


Important Questions

1. Consider the following reaction:

$Ethanol \xrightarrow[]{PBr_3} X \xrightarrow[]{Alc. KOH}Y\xrightarrow[H_2O, \ Heat]{H_2SO_4, \ Room \ temperature}Z$

What is product Z?


Answer: $C{H_3}C{H_2}OH$


2. Who is known by the name of mercaptans?

Answer: Thio-alcohols


Conclusion

Since phosphorus tribromide is crucial to the bromination of acids and the conversion of alcohols, we must learn everything there is to know about this molecule.


It has a variety of uses. It can be used to put out fires, make medications, function as catalyst, and analyse sugar, among other things. But in addition to irritating the respiratory system and other internal organs of the human body, it can seriously harm the skin.


Additionally, it is extremely poisonous and potentially explosive. Three Br atoms each have an electronegativity value of 2.96 in PBr3, while one P atom has an electronegativity of 2.19. With such a large difference, P and Br have polar bonds where each has a + partial charge near P, and a - partial charge near Br.

Last updated date: 28th Sep 2023
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FAQs on Phosphorus Tribromide (PBr3): Formation and Uses

1. What causes in situ PBr3 preparation?

When phosphonate comes into touch with air, it can explode because HBr is corrosive by nature. Therefore, PBr3 and PI3 are prepared in situ due to their high reactivity and the generation of hazardous and corrosive compounds on hydrolysis. The fundamental advantage of PBr3 is that it facilitates the conversion of chiral alcohols to bromides while retaining configuration, as demonstrated by the two articles mentioned above. They also show the actual reaction process by passing through the intermediate alkyl phosphites.

2. What is PBr3 typically employed for?

For brominating carboxylic acids to create the appropriate acyl bromide in the Hell Volhard Zelinsky halogenation, phosphorus tribromide (PBr3) is frequently utilised. Alkyl bromides can also be made from primary and secondary alcohols using this method. The fundamental advantage of PBr3 is that it facilitates the conversion of chiral alcohols to bromides while retaining configuration, as demonstrated by the two articles above. They also demonstrate the reaction process by passing through the intermediate alkyl phosphites.

3. PBr3: Is it an electrophile?

In many of its reactions, such as those with amines, PBr3 can behave as an electrophile or Lewis acid. The most significant reaction of PBr3 is with alcohols, where it converts an OH group into an alkyl bromide by substituting a bromine atom. Each of the three bromides is transferable. The alcohol is transformed into a good leaving group during the "activation" stage by creating an O-P bond and displacing Br from P (notice that this is effectively nucleophilic substitution at phosphorus).