OCS or COS is the chemical formula for carbonyl sulfide. It is a colorless, flammable gas with an unpleasant odor. OCS has a molar mass of 60.07 g/mol. Carbonyl sulfide is used as an intermediate in the manufacture of certain herbicides. It is primarily used in small-scale chemical syntheses.
This article will discuss a very important property of carbonyl sulfide (OCS or COS), i.e., its polarity. So, is OCS polar or non-polar?
Continue reading to find out.
Is OCS polar or non-polar?
Carbonyl sulfide (COS or OCS) is a polar molecule. It consists of one oxygen (O) atom, one carbon (C) atom, and one sulfur (S) atom. The carbon atom is present at the center of the molecule, while oxygen and sulfur atoms occupy terminal positions, one on each side, making a linear molecular shape.
An electronegativity difference of 0.03 units exists between a carbon and a sulfur atom in the C=S bond; thus, it is only slightly polar, almost non-polar as per Pauling’s electronegativity scale.
In comparison, an electronegativity difference of 0.89 units exists between the bonded carbon and oxygen atoms in the C=O bond in the OCS molecule. Hence, C=O is a strongly polar covalent bond and possesses a high dipole moment value (symbol µ).
It is due to the varying electronegativity differences that the high dipole moment of the C=S bond does not get canceled with the very weak dipole moment of the C=O bond in the OCS molecule, even in its linear shape.
The electron cloud stays non-uniformly spread over the molecule. Thus, OCS or COS is a polar molecule overall (net µ = 0.71 D).
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 carbonyl sulfide (COS) is overall a polar molecule.
Factors affecting the polarity of OCS
Electronegativity
It is defined as the ability of an atom to attract a shared pair of electrons from a covalent chemical bond.
Electronegativity decreases down the group in the Periodic Table of elements while it increases across a period.
Greater the electronegativity difference between bonded atoms in a molecule, the higher the bond polarity.
Carbon (C) is present in Group IV A (or 14) 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.
Sulfur (S) belongs to Group VI A (or 16) of the Periodic Table. The electronic configuration of a sulfur atom is 1s2 2s2 2p6 3s2 3p4. Hence, each S-atom in OCS has a deficiency of 2 more valence electrons to obtain a complete octet electronic configuration.
Oxygen (O) also belongs to Group VI A (or 16) of the Periodic Table. The electronic configuration of an oxygen atom is 1s2 2s2 2p4. So each O-atom has a deficiency of 2 more valence electrons for it to complete its octet.
Hence, in the OCS molecule, a carbon atom at the center is double-covalently bonded to an oxygen and a sulfur atom, one on each side. All 4 valence electrons of the central carbon consumed in covalent bonding denote there is no lone pair of electrons present on it.
Contrarily, both S-atom, and O-atom contain two lone pairs of electrons each.
In this way, all atoms in the molecule attain a complete octet electronic configuration in their respective shells, via lone pairs and chemical bonding in the OCS molecule.
Due to the absence of any lone pair of electrons on the central carbon (C) atom, there is no distortion in the shape of the molecule and the OCS molecule attains a linear molecular shape.
Atom
Electronic configuration
Valence electrons
Carbon (6C)
1s22s22p2
4
Sulfur (16S)
1s22s22p63s23p4
6
Oxygen (8O)
1s22s22p4
6
The electronegativity of carbon is 2.55, and sulfur is 2.58. Thus, the electronegativity difference between the bonded carbon and sulfur atoms is only 0.03 units. Although it is less than that required for a covalent bond to be polar as per Pauling’s electronegativity scale, still a purely non-polar covalent bond is only formed between two identical atoms.
Thus the C=S bond is considered weakly polar. Sulfur being slightly more electronegative, attracts the C=S bonded electrons to a greater extent than carbon. Partial positive (Cδ+) and partial negative (Sδ-) charges appear in the molecule.
On the other hand, the electronegativity difference of the C=O bond between the bonded C-atom (E.N = 2.55) and O-atom (E.N = 3.44) is 0.89 units. This implies that the C=O bond is strongly polar in the OCS molecule. Therefore, the oxygen atom gains a partial negative (Oδ-) charge, while the central carbon maintains its partial positive (Cδ+) charge.
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 OCS, the dipole moment of C=O bond points from Cδ+ to Oδ-while the small dipole moment of C=S bond points from Cδ+ to Sδ-(as shown below).
Molecular geometry
According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, OCS is an AX2E0 or simply AX2-type molecule. To one C-atom (A), two (one C=S and one C=O) bond pairs (X) are attached, and there is no lone pair of electrons (E)present on the central atom.
So, the molecular geometry or shape of OCS is identical to its ideal electron pair geometry, i.e., linear. Due to the absence of any lone pair of electrons on the central C-atom, the molecule gets influenced to adopt a planar, linear shape with a mutual O=C=S bond angle of 180°.
Although the OCS molecule is linear in shape still as the C=S dipole moment is much smaller than the C=O dipole moment due to different electronegativity differences; therefore, the two dipole moments do not get canceled overall in the molecule.
Oxygen being more electronegative than the other two atoms attracting C=S electrons in addition to attracting C=O bonded electrons to a large extent.
The charged electron cloud stays non-uniformly spread over the molecule. Consequently, carbonyl sulfide (OCS or COS) is a polar molecule overall (net µ = 0.71 Debye).
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), nitric oxide (NO), carbonyl sulfide (OCS or COS), etc.
Why is carbonyl sulfide (COS or OCS) a polar molecule?
OCS has a slightly polar, almost non-polar C=S bond due to an electronegativity difference of just 0.03 units between the bonded carbon (E.N = 2.55) and sulfur (E.N = 2.58) atoms.
However, the molecule also possesses a strongly polar C=O bond owing to an electronegativity difference of 0.89 units between the bonded carbon (E.N = 2.55) and oxygen (E. N= 3.44).
Therefore, the OCS molecule attains a linear shape with a non-uniform distribution of charge. The dipole moments generated in the C=O and C=S bonds do not cancel each other, making it a polar molecule.
Consequently, carbonyl sulfide (COS or OCS) is overall polar with a net dipole momentµ = 0.71 D
CS2 and OCS both have a linear structure, but how CS2 is non-polar while OCS is a polar molecule?
CS2 has a linear molecular geometry (S=C=S) with two identical C=S bonds. Each C=S bond has an electronegativity difference of 0.03 units which results in an overall equal distribution of charges among the bonded atoms.
The small dipole moment of one C=S bond gets neatly canceled with an identical dipole moment of the other C=S bond.
Consequently, CS2 has zero dipole moment (net µ = 0), and the molecule is non-polar overall.
OCS also has a linear molecular geometry (O=C=S). But, it has one C=S and one C=O bond. The electronegativity difference between bonded atoms in the C=O bond is 0.89 units, while that in the C=S bond is 0.03 units.
Hence, the molecule has an unequal distribution of charges among the bonded atoms. The large dipole moment of the O=C bond does not get canceled equally with the small C=S dipole moment.
Consequently, OCS has a net non-zero dipole moment (net µ = 0.71 Debye), and it is overall a polar molecule.
Is there a formal charge on the bonded atoms in the OCS molecule?
Formal charge of an atom = [ valence electrons – non-bonding electrons- ½ (bonding electrons)]
For the central C-atom
Valence electrons = 4
Bonding electrons = 8
Non-bonding electrons = 0
∴ Formal charge on the central C-atom = 4 – 0 – 8/2 = 4 – 0 – 4 = 4 – 4 = 0
Zero formal charges are present on each of the C-atom, S-atom, and O-atom. Thus, the overall charge present on the carbonyl sulfide OCS molecule is (0) + (0) + (0) = 0.
Summary
Carbonyl sulfide (OCS or COS) is a polar molecule.
It consists of one slightly polar C=S bond having an electronegativity difference of 0.03 units between the bonded C-atom and S-atom.
OCS also possesses a strongly polar C=O bond with an electronegativity difference of 0.89 units between the bonded C-atom and O-atom.
Carbonyl sulfide (COS) possesses a linear shape with a 180° bond angle.
Due to the unequal dipole moment values of the C=S and the C=O bonds, these dipole moment does not get canceled overall.
The charged electron cloud stays asymmetrically spread over the molecule, which leads to a net dipole moment in carbonyl sulfide (OCS or COS) of µ = 0.71 Debye, so it is overall a polar molecule.
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