Formic acid has the chemical formula of HCOOH or CH2O2 where a hydrogen atom is attached to the -COOH group to form the simplest carboxylic acid. It is also known as methanoic acid.
Do you know that formic acid can naturally occur in several species of insect kingdom like ants and stingless bees?
We prepare HCOOH in industries via the below-mentioned equations:
CH3OH + CO ——> HCO2CH3 (1)
HCO2CH3 + H2O ——> HCOOH + CH3OH (2)
In laboratory preparation, the necessary chemical reactions are:
C2O4H2 ——-> CO2H2 + CO2
Pb(HCOO)2 + H2S ——-> 2HCOOH + PbS
Formic acid has a 47.018 g/mol molecular weight and a density of 1.220 g/ml. It appears as a colorless fuming liquid that bears a pungent, penetrating odor.
It is used as fuel cells and also as a component of the mobile phase of reverse-phase high-performance liquid chromatography methods.
Other than this, HCOOH can be used in leather production and dyeing industries, as miticides and anti-bacterial agents.
Let us now learn about the chemical bonding of this carboxylic acid compound.
CH2O2 Lewis Structure
If we want to find out the nature of chemical bonding inside any polyatomic molecule, we need to draw the Lewis Structure.
Lewis Structure gives us a step-by-step procedure to sketch the 2D schematic representation of a given molecule. Here, we use the concept of valence electrons to find out the type of bond formation.
In HCOOH or methanoic acid, we have two hydrogen atoms, one carbon atom, and one two oxygen atom.
Here, we have a picture of the modern Periodic Table.
As we can see, carbon has a valency of 4 and has 4 valence electrons. Oxygen belongs to group 16 (chalcogen family) and has 6 valence electrons.
Hydrogen belongs to group 1 and has only 1 electron in its outermost shell (valence electron).
Total number of valence electrons = 1*2 + 4*1 + 6*2 = 18.
Among the three elements, hydrogen has the least electronegativity value, carbon comes second and oxygen has the highest value.
According to the general rule, the least electronegative element must take the central position.
However, hydrogen tends to stay at the corners and does not become the central atom owing to having only one valence electron.
So, we will have carbon as the central atom in formic acid.
The atoms will look like this:
Now, we will put the valence electrons around each constituent atom and the diagram will look like this:
Here comes the role of a very necessary concept: the Octet rule. According to this rule, every element present in groups 1-17 (the main groups) tends to attain octet configuration in their valence shells like those of noble gas elements.
For example, Nitrogen will tend to have a Neon configuration and Chlorine will tend to have an Argon configuration.
You might have heard chemistry is full of exceptions and one such exception is that Hydrogen tends to achieve Helium configuration hence will need a total of 2 electrons to fulfill the valence shell.
In the above diagram, we can notice that carbon has not yet reached the octet fulfillment state. In order to do so, we will bring two more electrons around the atom.
The required sketch will be:
If we calculate the formal charges:
This is the formula for finding formal charge values:
Formal charge of C = 4 – 0.5*8 – 0 = 0.
Formal charge for each H = 1 – 0.5*2 – 0 = 0.
Formal charge for O bonded to H and C both = 6 – 0.5*4 – 4 = 0.
Formal charge for O bonded to C = 6 – 0.5*4 – 4 = 0.
Therefore, the atoms are present in their least formal charge values.
The most suitable Lewis Structure for methanoic acid is:
CH2O2 Molecular Geometry
VSEPR stands for Valence Shell Electron Pair Repulsion Theory. This model is used in chemistry to predict the molecular geometry of a given composition from its Lewis Structure.
While Lewis Structure provides us with a 2D representation of a chemical molecule, the VSEPR model offers us a basis to decipher the 3-dimensional molecular shape.
This is relevant for covalently bonded molecules. It considers the electron pairs (both bonded and unbonded) and talks about the concept of minimum repulsion.
According to VSEPR theory, the electrons create a negatively charged atmosphere around the atomic nuclei and this creates a repulsive force among the like charges.
The stability of the molecules is maintained via increasing the distance between electrons and therefore minimizing the repulsive force.
In VSEPR theory, we can predict the molecular geometry via VSEPR notations.
AXnEx is the VSEPR notation.
Here, A stands for the central atom, X stands for the surrounding atoms (in VSEPR, we also consider triple and double bonds to be one bonding group),
E represents the number of lone pairs attached to the central atom.
Here, Carbon is the central atom (A). X: 2 oxygen and 1 hydrogen atom,
∴ n = 3.
E: no lone pair of A, ∴ x = 0.
AX3E0 is the VSEPR notation for formic acid.
As we can see from the chart, the molecular geometry of HCOOH is trigonal planar.
One of the major important models to describe the nature of chemical bonding is orbital hybridization.
The concept of hybridization deals with atomic orbitals (AOs). The orbitals of the same atom having equivalent energies come together to combine and fuse and form hybrid orbitals. The valence shell electrons participate. These hybrid orbitals then are considered to take part in the formation of bonds.
Now, let us decipher the hybridization type for methanoic acid.
Steric number = Number of atoms bonded to central atom inside a molecule + Number of lone pair of electrons attached to the central atom
In HCOOH, the central atom carbon is double bonded to an oxygen atom, single bonded to another oxygen atom, and a hydrogen atom.
There is no lone or unbonded pair of electrons in formic acid.
∴ the steric number = 3 + 0 = 3.
We, therefore, have sp2 hybridization for carbon.
For the oxygen in C=O, we have sp2 hybridization since it has two lone pairs and one bonded atom whereas the oxygen in C-O, we have sp3 hybridization because of 2 lone pairs and 2 sigma bonds.
Let us now talk about polarity.
Polarity is an important characteristic or property of a chemical molecule. It is related to the distribution of electric charges among the constituent atoms inside a molecule.
When can you call a molecule polar? We can term a molecule to be polar in nature when the distribution of electrons among the atoms is not even i.e there is an asymmetric charge distribution within the molecular composition.
We can check this from the concept of electronegativity. A difference in electronegativity among two atomic elements in the range of around 0.4 – 2 is considered to be polar covalent bonds.
Here, we have a diagram of Pauling electronegativity chart:
Here, in C-H bond, the difference: 2.55 – 2.20 = 0.35.
In C and O bond, the difference: 3.44 – 2.55 = 0.89.
In the O-H bond, the difference: 3.44 – 2.20 = 1.24
As we can see the inclination of the bonds is towards polar nature. Along with this, we have an asymmetric structure so the hydrogen end is on the positive side whereas the oxygen end is on the negative.
HCOOH is polar in nature. It is miscible in water and polar organic solvents.
HCOOH is the simplest carboxylic acid and an important organic molecule. We have discussed chemical bonding in detail in this article.