CH2O is the chemical formula for methanal. The commercially common name for methanal is formaldehyde. It is a colorless, strong-smelling chemical that easily catches fire.
Formaldehyde is perhaps the simplest member of the organic chemistry family. Thus, it is very important for us to know its chemical properties such as its polarity.
So, is formaldehyde (CH2O) polar or non-polar? To find that out, continue reading the article.
Is CH2O polar or non-polar?
Formaldehyde (CH2O) is a polar molecule. It is made up of three different atoms i.e., a carbon (C) atom in the center is bonded to 2 hydrogens (H) atoms and an oxygen (O) atom by single and double covalent bonds respectively.
There is a small electronegativity difference between a C and an H atom in a C-H bond. However, there is a large electronegativity difference between an O and a C atom in a C=O bond.
Oxygen strongly attracts the shared electron cloud from the entire molecule. Oppositely charged poles develop which leads to an overall unequal charge distribution in the molecule. So, CH2O is polar with a net dipole moment value of 2.33 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 of the molecule then the molecule is called a non-polar molecule.
The polarity of a molecule is mainly gets affected by the following three factors:
The electronegativity difference between two or more covalently bonded atoms
Dipole moment
Shape or geometry of the molecule
Now, let’s discuss each of the three factors one by one to prove the polarity of CH2O.
Factors affecting the polarity of CH2O
Electronegativity
Electronegativity 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.
A small electronegativity difference of 0.35 units does exist between C (E.N= 2.55) and H (E.N= 2.20 ) atoms. Carbon has a slightly better hold on the shared electron pair of each C-H bond as compared to hydrogen.
However, according to the Pauling scale, the bonded atoms should have an electronegativity difference of at least greater than 0.5 units for a covalent bond to be polar.
In accordance with the above statement, there is a high electronegativity difference of 0.89 units between a C and an O (E.N=3.44) atom. Oxygen belongs to group VII A (or group 16) of the Periodic Table. It is short of only 2 electrons to complete its octet electronic configuration.
Thus, oxygen has a high affinity for electrons, and it strongly attracts the shared electron cloud from the C=O bond.
Atom
Electronic configuration
Valence electrons
Carbon (6C)
1s2 2s2 2p2
4
Hydrogen (1H)
1s1
1
Oxygen (8O)
1s2 2s2 2p4
6
Due to this high electronegativity difference, oxygen does not only attract the C=O electrons but also the shared electrons from both C-H bonds. O gains a partial negative charge (Oδ-) due to this excess of electrons while each of the C and H atoms gains partial positive charges (Cδ+ and Hδ++) on account of their slight electron deficiencies.
As a result, oppositely charged poles develop in the CH2O molecule.
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 charge separation (r). The dipole moment is expressed in a unit called Debye (D).
The dipole moment of an individual C-H bond point from Hδ++ to C δ+ because H is more electron deficient as compared to C. Thus, if we see a C-H bond in isolation then C acts as the negative pole while H becomes the positive pole.
For the C=O bond, dipole moment points from Cδ+ to Oδ-. For the entire CH2O molecule, the net dipole moment points towards Oδ-. The dipole moment values of individual C-H and C=O bonds add up to give a net dipole moment value of µ= 2.33 D.
Molecular geometry or shape
It is often believed that polar molecules have an asymmetric shape that leads to an unbalanced charge distribution in the molecule overall. But CH2O is apparently a symmetric trigonal planar molecule, how is it then that it is still a polar molecule?
According to the Valence Shell Electron Pair Repulsion (VSEPR) theory of chemical bonding, CH2O is an AB3 – type molecule. Around the central carbon atom (A) there are three bond pairs (B) and no lone pair.
Carbon belongs to group IV A (or group 14) of the Periodic Table. It has 4 valence electrons. It uses all of its valence electrons for chemical bonding in the CH2O molecule by forming two single covalent bonds with 1 hydrogen atom on each side and a double covalent bond with an oxygen atom at the center.
Each of the carbon and oxygen atoms achieves a stable octet electronic configuration while hydrogen achieves a duplet configuration. There is no lone pair of carbon, but two lone pairs of electrons are present on the oxygen atom.
Even though the molecule has a trigonal planar shape (120° bond angle) but it is polar because the electron cloud density in it is not symmetrical. The negative charge is centralized on oxygen.
The pull of the electron cloud from the entire molecule towards oxygen leads to an enhanced dipole moment effect. So, CH2O is polar with a net dipole moment value of 2.33 D.
Why is CH2O a polar molecule even though it has a symmetrical trigonal planar shape?
CH2O is a polar molecule because of the high electronegativity of an oxygen atom bonded to the central carbon. Oxygen strongly attracts the shared electron cloud from each of the C-H and C=O bonds.
An unbalanced electron cloud distribution develops in the molecule.
The charge distribution does not get balanced even in the symmetrical shape of the molecule.
Therefore, dipole moments do not get canceled which makes the overall CH2O molecule polar with net µ > 0.
Both BF3 and CH2O are trigonal planar molecules but why is CH2O polar while BF3 is non-polar?
BF3 is non-polar because the dipole moments of polar B-F bonds get canceled equally.
The net dipole moment of two B-F bonds pointing upwards (red arrow) gets canceled with the dipole moment of a downward-pointing B-F bond (blue arrow).
Thus BF3 is a non-polar molecule overall with net µ=0.
Contrarily, the electron cloud density does not get balanced in the CH2O molecule overall even in its trigonal planar shape because oxygen strongly attracts the electrons from the entire molecule.
During CH2O molecule formation, one 2s electron of C gets excited to an empty 2p orbital which leads to a valence shell configuration of 2s1 2px1 2py1 2pz1.
The 2s orbital mixed with two 2 p orbitals to yield three sp2 hybrid orbitals, each containing a single electron only.
Similarly, the electronic configuration of oxygen is 1s2 2s2 2p4. One 2s electron gets excited to a 2p orbital thus the valence shell configuration becomes 2s1 2px2 2py2 2pz1. One 2s and two 2 p orbitals hybridize to give three sp2 hybrid orbitals.
The sp2 hybrid orbital containing one electron forms a sigma bond with the sp2 hybrid orbital of C while the other two sp2 orbitals containing two electrons each are situated as lone pairs on oxygen.
Similarly, the remaining two sp2 hybrid orbitals of carbon form sigma bonds with the s orbitals of hydrogen. While the unhybridized 2p orbitals of carbon and oxygen overlap side by side to form a pi (π) bond.
Summary
Formaldehyde (CH2O) is a polar molecule.
There is a small electronegativity difference between C and H atoms, but a high electronegativity difference exists between a C and an O atom.
Oxygen attracts the shared electron cloud of the entire molecule.
CH2O has a trigonal planar shape with a 120° bond angle but the overall charge distribution is not balanced in the molecule, so it is polar (µ = 2.33 D).
The C=O bond length is approx. 121 pm while the C-H bond lengths are 111 pm in the CH2O 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/
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