Why PCl5 is Polar or Nonpolar?
The chemical compound PCl5 is also known as Phosphorus Pentachloride. It is one of the most essential phosphorus chlorides found among others consisting of POCl3 and PCl3. PCl5 is a water-sensitive, colorless, and moisture-sensitive solid compound. Its commercial samples can sometimes appear to be greenish-yellow in color when contaminated with hydrogen chloride.
It is a crystalline solid compound with an irritating odor and is highly toxic when a hydrolysis reaction takes place. PCl5 is highly reactive to water by nature and forms hydrochloric acid and, phosphoric acid as an outcome of an exothermic reaction producing heat. This heat can be sufficient enough to ignite any combustible material such as plastic. Phosphorous Pentachloride is strongly corrosive to metals and tissues.
This PCl5 compound can be easily obtained by chlorinating Phosphorus Trichloride with an elemental chlorine. By adding excess of chlorine to white phosphorus dissolved in chloroform, a very small amount of PCl5 can be produced. This extraction procedure can be accomplished using the garage chemist’s procedure only.
Why PCl5 is Polar or Nonpolar?
The molecular structure of PCl5 doesn’t carry any unbalanced localized charge or rather any dipoles in this chemical compound. Plus, Cl is more electronegative than P in PCl5. Hence, the bonds formed in this molecule are Polar Bonds. But the molecule PCl5 overall is a nonpolar chemical compound due to its Cl atoms forming a Trigonal Bipyramidal structure. Here, all these Cl atoms share an equivalent charge. Hence, it is a Nonpolar substance with Polar Bonds.
PCl5 is a nonpolar molecule, but how?
The electron pull balances out electronegativity, in both the vertical and horizontal axis of the Phosphorous Pentachloride molecule. One of the axis of this PCl5 falls to be linear while the other one forms a triangular shape which results in a no net dipole moment. Even though the molecule carries no dipole charge, a large amount of electrons still leads the molecule to form strong temporary forces allowing it, a melting point of 161˚C as an outcome.
This charge is also responsible for its boiling point which is 167˚C. As a result of this charge, this molecule is found to be solid at room temperature and pressure. The molecular geometry of Phosphorous Pentachloride (PCl5 ) is trigonal bipyramidal. PCl5 has symmetric charge distribution of Chlorine atoms around the central atom Phosphorous. As charge distribution is equal and there is no net dipole moment therefore, PCl5 molecule is nonpolar.
What is the hybridization of phosphorus pentachloride?
If we see the lewis structure of this molecule!
In PCl5, Phosphorus atom contains a total of 5 valence electrons. One electron is shared by each chlorine atom. The elements present in the third period comprise of d orbitals along with s and p orbitals in the periodic table.
The energy of these 3d orbitals is very close to the energy of 3s and 3p orbitals. The energy of these 3d orbitals is also equivalent to 4p and 4s orbitals. As a result, the hybridization including 3s, 3p, and 3d or 3d, 4s, and 4p are all feasible. Due to a difference in energies of 4s and 3p orbitals, no hybridization including 3p, 3d, and 4s orbitals is possible.
The five orbitals i.e. 1s, 3p, and 1d orbitals are only available free for hybridization. Therefore, it can obtain a set of 5sp3d hybrid orbitals (according to VSEPR theory).
It is prominent that all bond angles of 90 degrees and 120 degrees formed in trigonal bipyramidal geometry are not identical. So, the 5sp3d orbitals of phosphorus overlap with the p orbitals of chlorine atoms. These p orbitals are singularly occupied and together they form all the 5 P–Cl sigma bonds in PCl5. Hence, the PCl5 molecule has a sp3d hybridization and a trigonal bipyramidal geometry shape.
What are the points to determine the polarity of pcl5?
Generally, a nonpolar molecule has the bonds sharing equal electronegativity in participating atoms. Though if atoms are arranged such that polar bonds vector sum cancels out itself then also a molecule becomes nonpolar.
Here, in PCl5 the bonds are symmetric but due to the electronegativity of chlorine being higher, the molecule has polar bonds canceling their vector sum. This cancellation is responsible for nonpolar nature as there is no dipole moment and lone pair of electrons. Due to this PCl5 forms a trigonal bipyramidal geometry consisting of a pair of an equatorial and axial bond. As these horizontal and vertical bonds cancel out their charges, the molecule has no net charge. Due to no net charge, the whole molecule has a nonpolar nature forming polar ionic bonds.
Lewis Structure of Phosphorous Pentachloride
PCl5 Ball and Stick Model
Why PCl5 is nonpolar even with an odd number of vectors around the central atom?
This is because of the 3-dimensional arrangement (i.e. Ball and stick diagram) of PCl5.
We know that Cl is more electronegative than P, therefore the bonds formed in PCl5 are polar, but the molecule as overall is nonpolar. Why?
Due to the arrangement of Cl atoms in PCl5 such that it forms a trigonal bipyramidal molecular geometry where electrons are shared equally. Thus, no dipole charge is produced and hence the Compound PCl5 is nonpolar even with an odd number of vectors.
What is the dipole moment of Phosphorus Pentachloride (PCl5)?
The polarity of the P-Cl polar bonds cancel out each other due to the trigonal bipyramidal geometry of the molecule. Since the molecule Phosphorus Pentachloride (PCl5) is symmetrically arranged around the central (P) atom, there is no charge distributed unevenly. So, PCl5 has no net (i.e. zero) dipole moment.
Why PCl5 is unstable?
In PCl5, phosphorus forms 5 polar bonds with cl atoms out of which the two axial bonds have more Bond length than the other three existing equatorial Bond lengths. This results in repulsion and hence leads to the weakening of axial bonds. Thus, PCl5 becomes unstable due to weak axial bonds.
Why does PCl5 exist as a cation and anion in the solid-state ( i.e. in the form of PCl4^+ and PCl6^-)?
It takes Lattice energy to form ions from PCl5.
PCl5+PCl5+energy→PCl+4+PCl-6
Increased lattice energy and better packing efficiency!
Lattice energy is the largest factor in determining the stability of an ionic solid.
The extra energy compensates and helps in the transfer of a chloride ion from one molecule to another in PCl5. Thus, PCl5 exists as an ionic solid. This situation is also a result of weak axial bonds in PCl5.
As trigonal bipyramidal structure of PCl5 molecule persists in nonpolar solvents!
In solutions of polar solvents:
PCl5 undergoes self-ionization.
Dilute solutions dissociate as:
PCl5 ⇌ PCl+4 + Cl-6
At higher concentrations, the second equilibrium becomes more prevalent:
2 PCl5 ⇌ PCl+4 + PCl−6
2 PCl5 ⇌ PCl+4 + PCl−6
(PCl+4)=Cation is Tetrahedral
(PCl−6)=Anion is Octahedral
In conclusion:
Phosphorus Pentachloride is used as a chlorinating agent. In the pharmaceutical industry, it is used to manufacture penicillin and cephalosporin. It also acts as a catalyst for cyclization and condensation reactions and hence, is very useful. But this chemical is extremely dangerous when comes in contact with water. It is toxic to touch and inhale. PCl5 has zero dipole moment. It is an ionic solid compound exhibiting as a cation or anion in a solid-state. It is a completely nonpolar compound forming polar ionic bonds. It is unstable due to weak axial bonds.
