How Does PBr3 Convert Alcohols to Alkyl Bromides?
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.
FAQs on PBr3 Reaction in Chemistry: Stepwise Mechanism & Key Uses
1. What is Phosphorus Tribromide (PBr₃), and what is its primary function in organic chemistry?
Phosphorus Tribromide (PBr₃) is a colourless fuming liquid that serves as a powerful brominating agent in organic synthesis. Its main purpose is to convert primary and secondary alcohols into their corresponding alkyl bromides by replacing the hydroxyl (-OH) group. It is also used to convert carboxylic acids into acyl bromides.
2. What is the step-by-step mechanism for the reaction of PBr₃ with a primary alcohol?
The reaction proceeds via a two-step mechanism:
- Step 1: Activation of the Alcohol: The oxygen atom of the alcohol's hydroxyl group acts as a nucleophile, attacking the phosphorus atom of PBr₃. This forms an intermediate, turning the hydroxyl group into a much better leaving group (a dibromophosphite).
- Step 2: Nucleophilic Substitution: A bromide ion (Br⁻), displaced in the first step, then performs a backside attack on the carbon atom bonded to the oxygen. This is a classic Sₙ2 reaction that results in the formation of the alkyl bromide and phosphorous acid (H₃PO₃) as a byproduct.
3. What are the main applications of the PBr₃ reaction?
The most important applications of Phosphorus Tribromide in chemistry include:
- Synthesis of Alkyl Bromides: It is widely used for the efficient conversion of primary and secondary alcohols into alkyl bromides.
- Preparation of Acyl Bromides: PBr₃ effectively converts carboxylic acids into acyl bromides, which are important intermediates in various organic reactions, including the Hell-Volhard-Zelinsky halogenation.
4. Why is PBr₃ often prepared 'in situ' instead of being stored and used from a bottle?
PBr₃ is often prepared 'in situ' (in the reaction mixture) by adding liquid bromine to red phosphorus. This is done primarily for safety and stability. PBr₃ is highly reactive and fumes in moist air, undergoing rapid hydrolysis to form corrosive phosphorous acid and hydrogen bromide (HBr) gas. Preparing it in situ ensures it is consumed as it forms, minimising the hazards associated with its storage and handling.
5. What is the stereochemical outcome when a chiral alcohol reacts with PBr₃?
When PBr₃ reacts with a chiral alcohol, the reaction proceeds via an Sₙ2 mechanism. A definitive feature of this mechanism is the inversion of configuration at the stereocenter. The bromide nucleophile attacks the carbon from the side opposite to the leaving group, causing the molecule's stereochemistry to flip. For example, a reaction with an (R)-alcohol will yield an (S)-alkyl bromide.
6. How does the PBr₃ reaction with a tertiary alcohol differ from its reaction with primary or secondary alcohols?
PBr₃ is ineffective for converting tertiary alcohols into alkyl bromides. The Sₙ2 mechanism required for the reaction cannot occur at a tertiary carbon due to significant steric hindrance, which blocks the backside attack by the bromide ion. Attempting this reaction typically results in an elimination (E1) reaction, where the alcohol is dehydrated to form an alkene as the major product instead of the desired alkyl bromide.
7. How does the PBr₃ reaction with a carboxylic acid differ from its reaction with an alcohol?
While both reactions involve replacing an -OH group, the site of attack and the final product are different.
- With an alcohol, the bromide ion attacks the alkyl carbon bonded to the oxygen, forming an alkyl bromide.
- With a carboxylic acid, the bromide ion attacks the more electrophilic carbonyl carbon, not the alkyl carbon. This results in the formation of an acyl bromide (R-COBr), not a bromoalkane.
