Phosphorus trichloride is a chemical substance known by its chemical formula PCl3. It exists as a colorless to yellow fuming liquid and considered to be toxic in nature. Many of you may have a question regarding whether PCl3 is a polar substance or not. In this article, I will answer this and will cover its properties and applications.
So, Is PCl3 Polar or Nonpolar? PCl3 is a polar molecule because of its tetrahedral geometrical shape having a lone pair on Phosphorus atom and the difference between the electronegativity of Chlorine(3.16) and Phosphorus(2.19) atoms resulting in unequal sharing of electrons and develop positive and negative poles across the molecule making it a polar molecule.
Phosphorus trichloride has its systematic IUPAC name as Trichlorophosphane. Its other names are
Phosphorus(III) chloride and Phosphorous chloride.
At room temperature, it exists in the liquid state and is colorless to yellowish in appearance. It is also considered a toxic substance for living beings.
It smells unpleasant similar to HCL. It is volatile in nature and reacts vigorously with water to produce HCl gas.
The molecular mass of the PCl3 molecule is 137.33 g/mol. It is calculated as below
Mol mass of PCl3 = 1 * 30.9 (Mol mass of P) + 3 * 35.4 (Mol mass of Cl) = 137.33 g/mol.
If we talk about the chemical composition of Phosphorus trichloride, It consists of 1 Phosphorus atom and 3 Chlorine atoms.
All three chlorine atoms are connected to Phosphorus atom via a single covalent bond and complete its octet leaving behind a lone pair on the Phosphorus atom.
Due to lone pair on Phosphorus atom, the repulsion between lone pair and bond pairs, the shape of the molecule becomes tetrahedral with all 3 chlorine atoms at three vertices of pyramid and Phosphorus leading as the central atom.
In the PCl3 molecule, the phosphorus atom is considered to have an oxidation state of +3 and chlorine has an −1 oxidation state.
In addition to this, the electronegativity of Phosphorus is 2.19 and that of Chlorine is 3.19.
Being higher electronegative, chlorine atom attracts the bonded electron pair slightly towards its side and gains partial negative charge and phosphorus atom gains partial positive charge.
Polar v/s Nonpolar Molecules
The molecules are held different types of bonds including ionic, covalent, metallic, hydrogen bonding. The covalent bonds can be polar and nonpolar depending upon various factors discussed in the below topics.
Let us check what differences do polar and nonpolar molecules make between them
Polar Molecules: The molecules that have unequal distribution of charge on its atoms are polar molecules.
The electronegativity of atoms in these molecules differs from each other and have an unequal share of bonded electrons.
It is generally seen that the geometrical shape of polar molecules is asymmetric(distorted) due to which the charge disperses unevenly on the molecule.
The dipole moment of polar molecules is always equaled to non zero. Few examples of polar molecules are OF2, CH2O, etc.
You can check out the reason for the polarity of CH2O.
Nonpolar Molecule: These are the molecules that have an equal distribution of charge on its atoms.
The electronegativity of atoms in these molecules is equal in most cases. And the geometrical shape of nonpolar molecules is always symmetric due to which the charge distribution on the atoms of these molecules is equally dispersed.
The dipole moment of nonpolar molecules is always equaled to zero.
Few examples of nonpolar molecules are CS2, O2, etc. You can check out the reason for the non-polarity of CS2.
Why is PCl3 a Polar Molecule?
Phosphorus trichloride molecule is made up of 3 chlorine and 1 phosphorus atom. As discussed above the difference between the electronegativity of Chlorine and Phosphorus atom develops the polarity across the P-Cl bond.
All the 3 P-Cl bonds are polar having a partial negative charge on chlorine atom and the partial positive charge on Phosphorus atom.
Phosphorus atom also contains a lone pair on it and as per VSEPR theory there occurs repulsion between lone pair and bond pairs due to which the 3 P-Cl bonds face a force in downward direction resulting in the distorted shape.
In addition to this, the geometrical shape of PCl3 molecule is tetrahedral due to which all the three chlorine atoms are at one side (at three vertices of the pyramid) and Phoshprus in the center (at the top corner of the pyramid).
As a result, the dipole of the molecules turns out to be non zero originating in the downward direction of chlorine atoms.
Therefore, the PCl3 molecule is polar.
Important Points to determine Polarity
If you want to determine the polarity of a molecule, then there are few points that must be kept in mind in order to find out whether a given molecule is polar or not.
Electronegativity: The electronegativity of an atom is its strength with which it attracts the bonded pair to its side. Greater the value of electronegativity of an atom more is its polarity.
If there is a difference between the electronegativity of two atoms forming a covalent bond with each other, the bond formed will be polar in nature and vice versa.
The polarity of a covalent bond is directly proportional to the difference between the electronegativity of the atoms.
Here, Cl is higher electronegative than P, therefore P-Cl bond is polar in nature.
Dipole Moment: The dipole of a molecule is the measure of its polarity. It is measured as the product of the charge on atoms and the distance between the centers of positive and negative charge.
D = Q * R
It is denoted by D and has an SI unit as Debye. The dipole moment of a polar molecule is always equaled to non zero and nonpolar molecules always have zero dipole moment.
PCl3 is a polar molecule therefore its dipole moment is 0.97 D.
Geometrical shape: The geometrical shape of the molecule is an important and physical parameter that helps to determine the polarity of a molecule.
It is generally seen that the polar molecules have asymmetric or distorted geometrical shapes whereas the nonpolar molecules have symmetrical geometrical shapes.
It is also possible to have a polar bond within a nonpolar molecule but its polarity is canceled out by each other due to symmetric shape.
Below is the image of the 3D geometrical shape of PCl3 molecule
In the industries, it is synthesized by the reaction of chlorine with a refluxing solution of white phosphorus in the presence of phosphorus trichloride with the continuous removal of PCl3 to avid the formation of PCl5.
P4 (Phosphorus) + 6 Cl2 (Chlorine) ————-> 4PCl3 (Phosphorus trichloride)
Properties of PCl3
- The molecular mass of phosphorus trichloride is 137.33 g/mol.
- At room temperature, it exists as a liquid with colorless to yellowish texture.
- It has an unpleasant odor similar to HCl.
- It reacts vigorously with water and forms HCl gas.
- The density of this substance is 1.574 g/cm3.
- The melting point of this substance is −93.6 °C or −136.5 °F and its boiling point is 76.1 °C or 169.0 °F.
- The vapor pressure of this compound is 13.3 kPa.
- The dipole moment of PCl3 is 0.97 D.
Uses of PCl3
- This is widely used for the manufacturing of phosphites.
- It is also used to produce various organophosphorus compounds.
- It is widely used for the manufacturing of insecticides and pesticides.
- On oxidation, PCl3 gives POCl3 which has great use as plasticizers for PVC and flame retardants.
- PCl3 can also be directly used as a reagent in organic synthesis.
Phosphorus trichloride is a chemical compound having 3 chlorine atoms and 1 Phosphorus atom with one lone pair on Phosphorus atom.
As per VSEPR theory, the shape of PCl3 is tetrahedral and chlorine being higher electronegative than Phosphorus attracts the bonded electron pair slightly towards its side and gains partial negative charge leaving behind positive charge on Phosphorus atom.
Due to asymmetric shape and difference between the electronegativity, the PCl3 molecule is polar in nature.
Friends, if you have any queries regarding the polarity of PCl3, feel free to ask in below comment section. I will reach out to you quickly.