Is PBr3 a polar or nonpolar substance?

Phosphorus tribromide, or PBr3, is a white liquid with a pungent odour that generates liquid vapours in wet air due to hydrolysis. It is used in research facilities to convert alcohols to alkyl bromides.

PBr3 is made by combining red phosphorus with bromine, which is usually done in the presence of a diluent because the reaction is highly exothermic.

The following is the reaction:

2P     +     3Br2   —–>     2PBr3

Phosphorus is utilised in excess in order to prevent the production of PBr5.

Phosphorus tribromide has the ability to behave as both a Lewis base and a Lewis acid, making it useful in a variety of processes.

Is PBr3 polar or nonpolar, then? Because of its asymmetrical form, PBr3 (phosphorus tribromide) is a polar molecule. Br has an electronegativity of 2.96 while P has an electronegativity of 2.16. As a result, the electronegativity difference between the P-Br bonds is more than 0.5, making them polar. However, it is classified as a polar molecule due to its asymmetrical form.

As previously indicated, the principal function of PBr3 is to generate alkyl bromides from alcohols. It’s also utilised as a step in the Hell-Volhard-Zelinsky halogenation process.

PBr3 also functions as a catalyst for carboxylic acid alpha bromination. It’s also very soluble, so it gets hydrolyzed quickly.

It’s a potent fire suppression chemical that’s also used to make fenoprofen, alprazolam, and other medications.

PBr3 has a molar mass of 270.69 g/mol and a density of 2.852 g/cm3.

PBr3 has a boiling point of 173.2 degrees Celsius.

PBr3 has a melting point of -41.5 degrees Celsius.

Polarity as a concept

We’ll brush up on our understanding of what a polar molecule is before determining whether PBr3 is a polar or non-polar molecule.

Polar molecules are those with a difference in electronegativity between their bonds. A polar bond exists when the electronegativity difference between two elements is larger than or equal to 0.5.

This means that electrons aren’t distributed evenly among the atoms, and that electrons gravitate toward the more electronegative element.

For example, in HF, F is strongly electronegative and hence attracts electrons, giving F a partially negative charge and H a partially positive charge, resulting in a polar bond.

Non-polar molecules are described as those with an equal number of shared electrons.

There is no difference in electronegativity, hence their net dipole moment is zero. Here are a few examples:

Non-polar atoms are those that are bound to one another, such as H2, F2, and Cl2. Hydrocarbons, or molecules with a C-H bond, such as CH4, are non-polar as well. Examine the cause behind CH4’s non-polarity.

We need to apply what we’ve learned about polarity to more complex compounds now that we’ve covered the basics. That’s when net dipole moment and geometry come in useful.

When the more electronegative element draws electrons towards itself, a dipole moment is created. From the more electropositive element to the more electronegative element, an arrow is drawn.

It is said to be a non-polar molecule when the net dipole moment is zero.

The following are the steps for determining the polarity of molecules:

Check the polarity of each bond and draw the Lewis structure of the molecule according to its shape.

Determine whether or not the molecule is symmetrical or asymmetrical. If the molecule is symmetrical, the net dipole is zero, and the molecule is non-polar.

Check to verify if the dipole moment arrows cancel each other out.

You can identify the polarity of any molecule by following these easy steps.

Factors that aid in determining a molecule’s polarity

There are three basic factors that influence polarity. The following are some of them:

Electronegativity Difference– When one element is more electronegative than the other, the electrons are drawn to it, causing the element to gain a partial negative charge while the other element gains a partial positive charge.

For a bond to be considered polar, the difference must be equal to or larger than 0.5. The polarity of the link will increase as the divergence grows.

Dipole Moment– The polarity of a chemical bond is measured using the dipole moment. When the charges are separated from one another, something happens.

The charge on the molecule is multiplied by the distance between the partial negative and positive charges to get the bond dipole. It is measured in Debye, which is represented by the letter D.

Molecular Shape– The AXN formula can be used to determine the molecular shape of any molecule, where A represents the number of central atoms, X represents the number of atoms bonded to the central atom, and N represents the number of lone pairs or nonbonding electrons.

We use the VSEPR chart to determine AXN’s molecular structure after we’ve identified it. A non-polar molecule has a symmetrical form (the dipole moments cancel each other).

It is a polar molecule if the shape is asymmetrical (irregular) and the net dipole moment is never zero.

The molecular shape of PBr3 is depicted in the figure below.

Why is PBr3 classified as a polar molecule?

To answer this, we’ll examine its Lewis structure first, followed by its molecular geometry.

We start by counting the total number of valence electrons in each element to calculate the Lewis structure of PBr3.

P (group 5) is the core atom and has 5 valence electrons (since all other atoms are bonded to it).

Br (group 7) has 7 valence electrons, but because it contains 3 Br atoms, it has 7 3= 21 valence electrons.

As a result, the total number of valence electrons is 21+5=26.

Arrange all 26 valence electrons around the molecule to complete each atom’s octet.

We observe that there are two electrons left after we’ve placed all of them. On P, the final two valence electrons function as a lone pair (changing the structure and bond angle), and each atom has completed its octet. As a result, we have the Lewis structure of PBr3.

We can see from this that because Br is the more electronegative atom, electrons tend to spend more time around it, making the P- Br bond polar.

So far, just the P-Br bond has been found to be polar; now we must determine whether the entire molecule is polar. This is where molecular geometry enters the picture.

Single bond dipole moments are mostly determined by molecule shape. We can clearly determine from VSEPR theory that PBr3 has a trigonal pyramidal molecular structure with bond angles close to 109 degrees.

We can see from the structure that PBr3 has an asymmetrical structure, which means that their net dipole moment is not zero (dipole moments do not cancel out), making the molecule polar.

Conclusion

To summarise the article, we discovered that PBr3 is a polar molecule due to its asymmetrical form and the fact that the dipole moments do not cancel out, resulting in a non-zero net dipole moment.

PBr3 is used as a fire suppressant and for converting alcohols to alkyl bromides in the pharmaceutical industry. It has a molar mass of 270.69 grammes per mol.

It has a strong odour and, in some reactions, can produce poisonous chemicals such as phosphine and HBr3, thus it should be used with caution.

Read more: Is CS2 an ionic or a covalent compound?

Misha Khatri
Misha Khatri is an emeritus professor in the University of Notre Dame's Department of Chemistry and Biochemistry. He graduated from Northern Illinois University with a BSc in Chemistry and Mathematics and a PhD in Physical Analytical Chemistry from the University of Utah.

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