Phosphorus Trichloride is an inorganic compound having the chemical formula PCl3. It is composed of phosphorus and chlorine, and hence, it is a binary compound consisting of two different elements.
Is PCl3 ionic or covalent? PCl3 is a covalent compound because the high ionization energy of phosphorus does not favor the formation of a P3+ ion. Hence, a covalent bond is preferred over an ionic bond to achieve a stable electronic configuration. Similar values of Pauling electronegativity (2.19 & 3.16 for P and Cl, respectively) also favor the formation of a covalent bond.
PCl3 is industrially prepared by reacting white phosphorus with chlorine.
P4 + 6Cl2 —-> 4PCl3
It can further react with Cl2 to form PCl5. Thus, it is essential to remove the PCl3 product during the manufacturing process continuously.
This is also an application of Le Chatelier’s principle, which states that product formation will be favored if the product concentration is decreased by external factors.
It exists as a colorless liquid in its standard state.
PCl3 has a melting point of -93.6 Celsius (-136.5 Fahrenheit) and a boiling point of 76.1 Celsius (169.0 Fahrenheit).
The low melting point indicates the tendency of PCl3 to exist in liquid states, while the low boiling point indicates its volatile nature.
In this article, we carry out a detailed chemical analysis to determine if PCl3 is Ionic or covalent.
Why is PCl3 Covalent?
Let us look at the electron configuration of P and Cl and determine the type of bonding.
Electron Configuration of P: [Ne] 3s2 3p3 (Number of valence electrons = 5)
Electron Configuration of Cl: [Ne] 3s2 3p5 (Number of valence electrons = 7)
Let us assume that the P3+ cation is formed. For this to be possible, we need to remove 3 electrons from the 3p orbitals.
The 3p3 configuration is highly stable due to high electron exchange stabilization energy. Hence, a lot of energy is required to remove these 3 electrons to form the P3+ cation.
This results in high ionization energy, and thus, we can rule out the possibility of ionic bonding.
However, we notice that the P atom needs 3 electrons to complete its octet and the Cl atoms need 1 electron each to complete their octet.
This is possible when electron sharing takes place between the P and Cl atoms. Thus, each bond contains one electron from the P atom and another electron from the Cl atom.
How is a Covalent bond different from an Ionic bond?
A Covalent bond is formed when two atoms share electrons to achieve a stable electronic configuration.
In the case of Ionic bonding, the two atoms in question gain and lose electrons to form a negatively charged anion and a positively charged cation, respectively.
The cation and the anion gain stability for having a complete octet, and the attraction between the cation and the anion further increases the molecule’s stability.
The difference between the two can be further explained by comparing the properties of their compounds.
|Ionic Compounds||Covalent Compounds|
|Usually crystalline solids||Usually fluids, seldomly solids|
|High Melting and Boiling Points (Very Strong Bonds)||Low Melting and Boiling Points (Weak Bonds)|
|Soluble in Polar solvents but insoluble in Non-Polar solvents||Soluble in Non-Polar solvents but insoluble in Polar solvents|
|High electrical conductivity in the dissolved or molten state||Low electrical conductivity|
Conditions for formation of Covalent Bond
1. High Ionization Potential: Both atoms should have high ionization potential as ionization potential is proportional to the energy required for ion formation.
2. High Electron Affinity: Both atoms should have high electron affinity to assist in sharing electrons.
3. Electronegativity Difference: The electronegativity difference should be as small as possible because a higher contrast in electronegativity implies more ionic character and less sharing of electrons.
Bonding in PCl3 using Lewis structure and VSEPR Theory
We employ the following procedure to determine the structure of PCl3:
Step 1: Determine the central atom. In our case, we choose P as the central atom.
Step 2: Count the total number of valence electrons in the molecule.
n1 = 5 (from P) + 3 x 7 (from 3 Cl) = 26
Step 3: Count the number of electrons needed to fulfill the octet of all atoms.
n2 = 8 x (Number of atoms) = 8 x 4 = 32
Step 4: Number of bond pairs = (n2 – n1)/2 = 6/2 = 3
Step 5: Count the number of non-bonding electrons.
n3 = n1 – (n2 – n1) = 26 – (32 – 26) = 20
Step 6: Number of lone pairs = n3/2 = 20/2 = 10
We are now ready to draw the Lewis structure of PCl3. First, we place the P atom at the center and remove three single bonds (3 bond pairs) connecting the Cl atoms.
To complete P’s octet, one of the lone pairs is placed on P, and the other 9 lone pairs are evenly distributed on the Cl atoms to complete their octet.
You can also read out the article I wrote on the lewis structure of PCl3.
After knowing the electron distribution in the molecule, VSEPR theory can be used to determine the molecule’s geometry.
The repulsion between the lone pair on the P atom and the bond pairs of the Cl atoms will force the molecule to adopt a trigonal bipyramidal geometry.
Bonding in PCl3 using Hybridization Theory
Consider the electronic configuration of P: [Ne] 3s2 3p3
The 3s and 3p orbitals are pretty close in energy and can hybridize into four equivalent sp3 orbitals.
One sp3 orbital will contain the lone pair of the P atom, and the other three sp3 orbitals will have one electron each.
The atomic orbitals of each Cl atom can now share their electrons with sp3 orbital electrons to fulfill their octet.
The expected geometry of a sp3 molecule is tetrahedral.
Still, due to the absence of a 4th bonding atom and the presence of a lone pair, the trigonal bipyramidal geometry is adopted with a Cl-P-Cl bond angle of 100 degrees (deviates from the ideal 109 degrees because of lone pair repulsion).
Properties of PCl3
1. PCl3 reacts with water violently and forms phosphorous acid (H3PO3) and hydrogen chloride (HCl)
2. PCl3 converts carboxylic acids into corresponding acid chlorides.
3. PCl3 can act as a nucleophile (the lone pair on P) and an electrophile (due to vacant d orbitals in P).
4. The P atom in PCl3 is in a +3 oxidation state. However, many higher oxidation states are possible due to the presence of d orbitals in P.
Thus, PCl3 can be easily oxidized to other Phosphorus derivatives.
5. PCl3 can undergo substitution reactions as Cl- is an excellent leaving group.
6. It is toxic and biologically corrosive.
1. PCl3 is a colorless oily liquid.
2. It is a fuming liquid because of its high volatility.
3. It has a highly pungent odor.
4. PCl3 (1.57 g/cm3) is denser than water (1.00 g/cm3)
5. PCl3 is slightly polar due to the electronegativity difference between P and Cl and hence is soluble in slightly polar solvents and organic solvents.
Uses of PCl3
1. PCl3 is the starting material in the production of compounds like PCl5, POCl3, and PSCl3. These compounds are industrially important and find applications in the production of agricultural chemicals.
2. It is used as a chlorinating agent and as a catalyst in organic synthesis.
3. It is used in the manufacture of detergents and flame retardants.
4. PCl3 can be used to make coordination compounds that find use as catalysts in organic and inorganic reactions.
PCl3 is a covalent compound as electrons are shared between the P atom and the Cl atoms. P atom is three electrons short of achieving an octet, while the Cl atoms are one electron away. The sharing of electrons results in an inert electron configuration for all the atoms.
Three bond pairs of electrons are responsible for the formation of 3 single bonds between the P and Cl atoms.
The rest of the valence electrons manifest themselves as non-bonding electrons or lone pairs on the P and Cl atoms.