Phosgene (COCl2) Lewis dot structure, molecular geometry or
shape, electron geometry, bond angle, polar or nonpolar,
hybridization
The IUPAC name for the COCl2 molecule is carbonyl dichloride. It is a toxic, colorless gas that emits a suffocating odor. It is more commonly known as phosgene.
Phosgene is particularly important as a reagent in the polymer and plastic manufacturing industry.
In this article, you will find interesting information such as how to draw the Lewis structure of phosgene (COCl2), what is its molecular geometry or shape, electron geometry, bond angle, hybridization, formal charges, etc.
The Lewis structure of phosgene (COCl2) consists of three different elemental atoms. The carbon (C) atom is present at the center of the molecule. It is bonded to an oxygen (O) atom at the center and two atoms of chlorine (Cl), one on each side.
There are a total of 3 electron density regions around the central C-atom in the Lewis structure of COCl2. All three electron density regions are constituted of bond pairs which mean there is no lone pair on the central C-atom in COCl2.
Drawing the Lewis structure of COCl2 is not a difficult task at all. So, grab a paper and pencil and draw this Lewis structure with us using the following simple steps.
Steps for drawing the Lewis dot structure of COCl2
1. Count the total valence electrons in COCl2
The Lewis dot structure of a molecule is referred to as a simplified representation of all the valence electrons present in it. Therefore, the very first step while drawing the Lewis structure of COCl2 is to count the total valence electrons present in the concerned elemental atoms.
There are three different types of atoms present in a phosgene (COCl2) molecule. We can find the valence electrons present in each element by identifying them in the Periodic Table.
Carbon (C) is present in group IV A of the Periodic Table, so it has a total of 4 valence electrons. Oxygen (O) is situated in group VI A, so it has a total of 6 valence electrons. Conversely, chlorine (Cl) is a halogen present in group VII A which denotes it has a total of 7 valence electrons.
∴ The COCl2 molecule is made up of 1 carbon atom, 1 oxygen atom, and 2 atoms of chlorine. So, the total valence electrons available for drawing the Lewis dot structure of COCl2 = 4+6+2(7) = 24 valence electrons.
2. Choose the central atom
Electronegativity is defined as the ability of an atom to attract a shared pair of electrons from a covalent chemical bond. Thus, the least electronegative atom is most likely to share its electrons with other atoms in its surroundings.
Carbon (C) is less electronegative as compared to both oxygen (O) and chlorine (Cl) in the phosgene molecule. Therefore, carbon is placed at the center of the Lewis structure of COCl2 while both O and Cl atoms are spread around it, as shown in the figure below.
3. Connect outer atoms with the central atom
In this step, we need to connect the outer atoms with the central atom of the Lewis structure using single straight lines. As O and Cl atoms are the outer atoms in COCl2 so the O atom as well as the two Cl atoms are joined to the central C atom using straight lines, as shown below.
Each straight line represents a single covalent bond i.e., a bond pair containing 2 electrons. There are a total of 3 straight lines in the above structure which means 3(2) = 6 valence electrons are used so far, out of the 24 initially available.
Total valence electrons available – electrons used till step 3 = 24-6 = 18 valence electrons.
This means we still have 18 valence electrons available to be accommodated in the Lewis dot structure of COCl2.
4. Complete the octet of outer atoms
One oxygen (O) and two chlorine (Cl) are the outer atoms in COCl2.
All these atoms require a total of 8 valence electrons in order to achieve a stable octet electronic configuration.
The C-O bond at the center represents 2 electrons. Thus, oxygen requires 6 more electrons to complete its octet. These 6 valence electrons are placed as 3 lone pairs around the oxygen atom.
Similarly, both the C-Cl bonds at the sides of the molecule represent 2 electrons each which mean both the Cl atoms require 6 more electrons to achieve a stable octet configuration. These 6 electrons are placed as 3 lone pairs around each Cl atom as shown below.
5. Complete the octet of the central atom and make a covalent bond if necessary
Total valence electrons used till step 4 = 3 single bonds + electrons placed around O-atom + 2 (electrons placed around each Cl-atom) = 3(2) + 6 + 2(6) = 24 valence electrons.
Total valence electrons available – electrons used till step 4 = 24 – 24 = 0 valence electrons.
All the valence electrons available are now used but the problem here is that the central carbon (C) atom still has an incomplete octet. There are a total of 6 valence electrons (3 single bonds) around the central C-atom. Consequently, it is short of 2 valence electrons to complete its octet.
To solve this problem, we can convert one lone pair present on the O-atom into a covalent bond between C and O atoms.
Now, the central C-atom has a complete octet (2 single bonds +1 double bond). The octet of the outer O-atom is also complete (1 double bond + 2 lone pairs) in addition to a complete octet of each Cl-atom (1 single bond + 3 lone pairs).
The final step is to check the stability of this Lewis structure using the formal charge concept.
6. Check the stability of the COCl2 Lewis structure using the formal charge concept
The less the formal charge on the atoms of a molecule, the better the stability of its Lewis structure.
The formal charge can be calculated using the formula given below.
What are the electron and molecular geometry of COCl2?
The phosgene (COCl2) molecule has an identical electron pair geometry and molecular geometry or shape i.e., trigonal planar. The three atoms bonded to the central carbon atom lie in a planar arrangement. There is no lone pair of electrons on the C-atom thus no distortion is witnessed in the geometry and shape of the molecule.
Molecular geometry of COCl2
The COCl2 molecule has a trigonal planar shape and molecular geometry. The three atoms bonded to the central C atom lie at the vertices of an equilateral triangle. C=O, C-Cl, and C-Cl, C-Cl bond pair-bond pair repulsions exist in the molecule which makes the O and Cl atoms occupy positions as far away from one another as possible.
However, there are negligible bond pair-lone pair and lone pair-lone pair repulsions with respect to the central atom in the molecule because there is no lone pair on the central C-atom. As a result, the molecule maintains a planar shape and geometry, identical to its ideal electron pair geometry.
Electron geometry of COCl2
According to the valence shell electron pair repulsion (VSEPR) concept of chemical bonding, the ideal electronic geometry of a molecule containing 3 regions of electron density around the central atom is trigonal planar.
The C=O double bond is considered one region of electron density. The C=O bond along with two C-Cl bonds represent a total of 3 electron density regions around the central C-atom in COCl2. Thus, its electron geometry is also trigonal planar.
An easy way to find the shape and geometry of the molecule is to use the AXN method.
AXN is a simple formula to represent the number of atoms bonded to the central atom in a molecule and the number of lone pairs present on it.
It is used to predict the geometry or shape of a molecule using the VSEPR concept.
AXN notation for COCl2 molecule:
A in the AXN formula represents the central atom of the molecule. As carbon (C) is marked as the central atom in the COCl2 molecule so A=C for COCl2.
X represents the number of atoms directly attached to the central atom. 1 O and 2 Cl atoms are bonded to the central C-atom in COCl2 thus X= 1+2 = 3 for COCl2.
N denotes the number of lone pairs present on the central atom. No lone pair of electrons is present on the central C-atom in COCl2 so N=0.
In conclusion, the AXN generic formula for the COCl2 molecule is AX3.
Now, you may quickly go through the VSEPR chart given below to find the electron and molecular geometries assigned against AX3.
The VSEPR chart reaffirms that the ideal electron geometry and molecular geometry or shape of a molecule with AX3 generic formula are identical i.e., trigonal planar, as we already noted down for the COCl2 molecule.
Hybridization of COCl2
The central C atom is sp2 hybridized in the COCl2 molecule.
The electronic configuration of carbon (C) is 1s2 2s2 2p2.
During chemical bonding, the 2s electrons of carbon get unpaired. One of the two 2s electrons gets excited and shifts to the empty 2p atomic orbital of carbon. This leaves behind half-filled 2s and 2p atomic orbitals.
The 2s orbital mixes with two 2p orbitals to yield three equivalent sp2 hybrid orbitals, each containing 1 electron. Each sp2 hybrid orbital has a 33.3 % s-character and a 66.7% p-character.
Two sp2 hybrid orbitals of carbon form C-Cl sigma (σ) bonds, on each side of the COCl2 molecule via sp2-p overlap with the pz orbitals of chlorine atoms.
The third sp2 hybrid orbital of carbon forms a sigma (σ) bond with the O-atom by sp2-p overlap with the p orbital of the oxygen atom. The unhybridized p-orbital of carbon forms the pi (π) bond with the oxygen atom in C=O via p-p overlap.
A short trick for finding the hybridization present in a molecule is to memorize the table given below and use the steric number of the central atom against this table.
Steric number
Hybridization
2
sp
3
sp2
4
sp3
5
sp3d
6
sp3d2
The steric number of central carbon in COCl2 is 3 so it has sp2 hybridization.
The COCl2 bond angle
The ideal bond angle of a trigonal planar molecule is 120°. But there are two different types of bonds present in the COCl2 molecule i.e., a C=O bond and a C-Cl bond. Due to this difference in the electron density regions around the central C-atom, there are two different bond angles present in COCl2. The Cl-C=O bond angle is 124.5° while the Cl-C-Cl bond angle is 111.8°.
The C=O double bond has a larger electron density domain than the C-Cl single bonds, so they exert a stronger bond pair-bond pair repulsive effect.
Consequently, the bond angle involving double-bonded atoms increases while those involving single-bonded atoms decrease as compared to the ideal bond angle (i.e., 120° in this case).
There are also two different bond lengths in the COCl2 molecule. The C=O bond length is 118 pm while the C-Cl bond length is 174 pm, as shown in the figure below. A double bond is stronger than a single bond and thus it exhibits a shorter bond length than the single bond.
According to Pauling’s electronegativity scale, a covalent bond is considered polar if its bonded atoms possess an electronegativity difference between 0.5 and 1.6 units.
Both chlorine (E.N = 3.16) and oxygen (E.N = 3.44) atoms are more electronegative than the central carbon (E.N = 2.55) in COCl2. An electronegativity difference of 0.61 units exists between the C and Cl atoms in a C-Cl bond while an electronegativity difference of 0.89 units exists between the C and O atoms in the C=O bond.
Thus, both C-Cl and C=O bonds are polar in nature in the COCl2 molecule. These polar bonds possess a specific dipole moment value (symbol μ).
The net dipole moment of the downwards pointing C-Cl bonds does not get canceled with the dipole moment of the upwards pointing C=O bond. Thus, the COCl2 molecule is polar. The electron cloud stays non-uniformly distributed in the molecule overall with net μ > 0.
The Lewis structure of phosgene (COCl2) consists of a carbon atom at the center. It is bonded to one oxygen atom and two atoms of chlorine at the sides.
There are a total of 24 valence electrons i.e., 24/2 = 12 electron pairs displayed in the COCl2 Lewis structure.
Out of the 12electronpairs, there are 4bondpairs and 8lonepairs. No lone pair is present on the central C-atom. 3 lone pairs are present on each Cl-atom while 2 lone pairs are placed on the O-atom, as shown below.
What is the molecular shape of COCl2?
The molecular geometry or shape of COCl2 is trigonal planar. It is identical to its electron pair geometry because there is no lone pair on the central C-atom. The central C-atom is sp2 hybridized.
The molecule (COCl2) has an AX3 generic formula according to the VSEPR concept. All the bonded atoms lie in a planar arrangement, forming an equilateral triangle.
Why do the bond angles in the COCl2 molecule deviate from the ideal 120°?
The ideal X-A-X bond angle in a trigonal planar arrangement is 120°. But there are two different types of bonds around the central C-atom in COCl2. There is a C-Cl single bond and a C=O double bond.
The double bond compresses the Cl-C-Cl bond angle from the ideal 120° to 111.8°. Contrarily, the Cl-C=O bond angle becomes greater than the ideal 120° i.e., 124.5°.
Vishal Goyal is the founder of Topblogtenz, a comprehensive resource for students seeking guidance and support in their chemistry studies. He holds a degree in B.Tech (Chemical Engineering) and has four years of experience as a chemistry tutor. The team at Topblogtenz includes experts like experienced researchers, professors, and educators, with the goal of making complex subjects like chemistry accessible and understandable for all. A passion for sharing knowledge and a love for chemistry and science drives the team behind the website. Let's connect through LinkedIn: https://www.linkedin.com/in/vishal-goyal-2926a122b/
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