COCl2 is the chemical formula for carbonyl dichloride, also known as phosgene. It is a colorless to yellow-colored gas at room temperature and atmospheric pressure. It has a molar mass of 129.839 g/mol.
Phosgene is used in the production of adhesives, glues, and sealants. It is also used in electroplating.
To find out whether the phosgene COCl2 molecule is polar or non-polar, continue reading this article.
Is COCl2 polar or non-polar?
Phosgene (COCl2) is a polar molecule. It consists of one carbon (C) atom, one oxygen (O) atom, and two chlorine (Cl) atoms. The carbon atom is kept at the central position, and the other atoms are at the surrounding positions, making a trigonal planar molecular shape.
An electronegativity difference of 0.61 units exists between a carbon and a chlorine atom in the C-Cl bond, while an electronegativity difference of 0.89 units exists between a carbon and an oxygen atom in the C=O bond in the COCl2 molecule.
Thus, the C=O bond and both C-Cl bonds are individually polar in the COCl2 molecule, and each possesses a specific dipole moment value (symbol µ).
The dipole moment of an upwards-pointing C=O bond does not get canceled with the net dipole moment of two downwards-pointing C-Cl bonds, even in the trigonal planar shape of the molecule. Thus, COCl2 is overall polar (net µ > 0).
A molecule is polar if there is a non-uniform charge distribution present in it. If the charge distribution gets equally balanced in different parts, then that molecule or molecular ion is considered non-polar.
The following three factors mainly influence the polarity of a molecule:
The electronegativity difference between two or more covalently bonded atoms
Dipole moment
Molecular geometry or shape
Now, let us discuss the effect of the above three factors one by one to prove that the phosgene (COCl2) molecule is overall a polar molecule.
Factors affecting the polarity of COCl2
Electronegativity
It is defined as the ability of an elemental atom to attract a shared pair of electrons from a covalent chemical bond.
Electronegativity increases across a period in the Periodic Table while it decreases down the group.
Greater the electronegativity difference between bonded atoms in a molecule, the higher the bond polarity.
Carbon (C) is present in Group IV A of the Periodic Table. The electronic configuration of carbon is 1s2 2s2 2p2. As per this electronic configuration, a C-atom has a total of 4 valence electrons. It is thus short of 4 more electrons that are required so that the carbon atom can achieve a complete octet electronic configuration.
Oxygen (O) is present in Group VI A of the Periodic Table. The electronic configuration of oxygen is 1s2 2s2 2p4. According to this electronic configuration, an O-atom has 6 electrons in its valence shell, and it lacks 2 more electrons to complete its outer electron shell.
Conversely, Chlorine (Cl) is a halogen present in Group VII A of the Periodic Table. The electronic configuration of chlorine is 1s2 2s2 2p6 3s2 3p5, which translates as a Cl-atom having a total of 7 valence electrons. There is thus a deficiency of one electron to complete its stable octet configuration.
Hence, in COCl2, the central C-atom is bonded with an O-atom via a double covalent bond and with two Cl-atoms via single covalent bonds.
The Lewis dot structure contains one C=O bond and two C-Cl bonds forming a trigonal planar molecular geometry.
All 4 valence electrons of the C-atom consumed in covalent bonding denote there is no lone pair of electrons on the central C-atom in COCl2. However, the O-atom and two Cl-atoms contain 2 lone pairs and 3 lone pairs of electrons, respectively.
In this way, all the bonded atoms attain a completely stable electronic configuration via lone pairs and chemical bonding in COCl2.
All the valence electrons of the carbon atom get consumed in bond formation; there is no lone pair of electrons on the central C-atom in COCl2. Hence, no distortion is witnessed in the geometry and shape of the molecule.
Atom
Electronic configuration
Valence electrons
Carbon (6C)
1s22s22p2
4
Oxygen (8O)
1s2 2s2 2p4
6
Chlorine (17Cl)
1s2 2s2 2p6 3s2 3p5
7
The electronegativity difference between a C-atom (E. N = 2.55) and an O-atom (E. N= 3.44) in the C=O bond is 0.89 units. In comparison, the electronegativity difference between a C-atom (E. N= 2.55) and a Cl-atom (E. N= 3.16) in each C-Cl bond is 0.61 units.
The more electronegative Cl-atoms and O-atom strongly attract the shared electron cloud away from the central C-atom in C=O, and each C-Cl bond is present in COCl2.
Thus, the central carbon atom (Cδ+) gains a partial positive charge, while the chlorine atoms (Clδ-) and an oxygen atom (Oδ-) obtain partial negative charges.
As a result, with an electronegativity difference of 0.89 units and 0.61 units, C=O and both C-Cl bonds are individually polar in COCl2, respectively.
Dipole Moment
Dipole moment (μ) is a vector quantity that points from the positive pole to the negative pole of a bond or a molecule.
It is mathematically calculated as a product of the magnitude of charge (Q) and charges separation (r). The dipole moment is expressed in a unit called Debye (D).
The dipole moment of a polar covalent bond conventionally points from the positive center to the center of the negative charge.
So in COCl2, the dipole moment of each polar C-Cl bond points from Cδ+ to Clδ- while the dipole moment of polar C=O bond points from Cδ+ to Oδ-(as shown below).
Molecular geometry
According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, COCl2 is an AX3E0 or simply AX3-type molecule. To one C-atom at the center (A), three bonded atoms are attached (one O-atom and two Cl-atoms), and the central C-atom contains no lone pair (E).
So, the molecular geometry or shape of COCl2 is identical to its ideal electron pair geometry, i.e., trigonal planar.
To minimize the repulsions and to attain stability in the trigonal planar structure of COCl2, the approximate bond angles are ∠Cl-C-O = 124.5° and ∠Cl-C-Cl = 111.8°.
Although due to the absence of any lone pair of electrons on the central C-atom, the COCl2 molecule has a trigonal planar shape; the net dipole moment of two downwards-pointing C-Cl bonds does not get canceled with the dipole moment of an upwards-pointing C=O bond.
The electron cloud stays non-uniformly spread over the molecule. Consequently, the COCl2 molecule is overall polar (net µ > 0).
Difference between polar and nonpolar?
Polar molecule
Non-polar molecule
Atoms must have a difference in electronegativity
Atoms may have the same or different electronegativity values
Unequal charge distribution overall
Equal charge distribution overall
Net dipole moment greater than zero
Net dipole moment equals to zero
Examples include water (H2O), ethanol (CH3CH2OH), ammonia (NH3), sulfur dioxide (SO2), bromine trifluoride (BrF3), phosphorus trifluoride (PF3), phosgene (COCl2), etc.
Examples include oxygen (O2), nitrogen (N2), methane (CH4), carbon disulfide (CS2), ethane (C2H6), propane (C3H8), etc.
In COCl2, an electronegativity difference of 0.61 units exists between the bonded C-atom (E. N= 2.55) and Cl-atom (E. N= 3.16) in each C-Cl bond, while an electronegativity difference of 0.89 units exists between the C-atom (E. N= 2.55) and O-atom (E. N= 3.44) in C=O bond.
Thus, both C-Cl and a C=O bond possess a specific dipole moment value.
The dipole moments of the C-Cl and C=O bonds do not get canceled uniformly.
Hence, phosgene (COCl2) is a polar molecule overall (net μ > 0).
COCl2 and BF3 are both trigonal planar molecules, but why is COCl2 polar while BF3 is non-polar?
The electron cloud density in COCl2 does not get balanced overall, even in its trigonal planar shape, because of the different electronegativity differences between the C-Cl (0.61 units) and C=O (0.89 units) bonds of the molecule.
Hence, COCl2 is polar with netμ > 0.
Contrarily, BF3 is non-polar because the dipole moments of polar B-F bonds get canceled equally.
All atoms in the COCl2 molecule including carbon, oxygen, and chlorine obtain a formal charge equal to zero.
Hence, the overall charge present on the phosgene molecule is zero.
Summary
Phosgene (COCl2) is a polar molecule.
It consists of a polar C=O bond with an electronegativity difference of 0.89 units and two polar C-Cl bonds with an electronegativity difference of 0.61 units between the bonded atoms.
Phosgene COCl2 has a trigonal planar molecular shape with bond angles ∠ Cl-C-O = 124.5° and ∠ Cl-C-Cl = 111.8°.
Due to the absence of any lone pair of electrons on the central C-atom, the COCl2 molecule has a trigonal planar molecular geometry or shape identical to its ideal electronic geometry.
The net dipole moment of two downwards-pointing C-Cl bonds does not get canceled with the dipole moment of an upwards-pointing C=O bond.
The electron cloud stays non-uniformly distributed in the molecule overall with a permanent dipole.The net dipole moment μ in COCl2 is > 0, so it is overall a polar molecule.
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/
Topblogtenz is a website dedicated to providing informative and engaging content related to the field of chemistry and science. We aim to make complex subjects, like chemistry, approachable and enjoyable for everyone.
