Formic acid was isolated from the distillation of ant bodies in earlier times and it is also produced from methanol industrially. The molar mass of formic acid is 46.03 g/mol and its boiling point is nearly equal to the water molecule i.e., 100.8 °. Formic acid is a colorless liquid with a pungent and penetrating odor. It is highly soluble in water and polar solvents. It exists as a hydrogen-bonded dimer in the vapor phase as well as in hydrocarbons. Here, we will discuss the chemical bonding in the formic acid by drawing its Lewis structure, understanding its molecular geometry, and hybridization. Then, we will move towards the polar nature of formic acid. Let us start with the Lewis Structure of Formic acid, HCOOH.
HCOOH Lewis Structure
The Lewis structures or electron dot structures are the two-dimensional diagrams, which represent the bonding electron pairs between atoms of the molecule and lone pairs of electrons on an atom if present. The bonding and nonbonding electrons are valence electrons, which are present in the outermost shell of an atom. As per the Lewis rule or octet rule, an atom tends to bond with other atoms in such a way that every atom has eight electrons in its valance shell except hydrogen and helium. The drawing of Lewis structure consists of the following steps: Step 1: Count the total number of valence electrons in the molecule by writing down the electronic configuration of the atom. The electronic configuration of carbon, hydrogen, and oxygen atom is [He] 2s22p2, 1s1, and [He] 2s22p4, respectively. Therefore, the valence electrons in C, H, and O are 4, 1, and 6, respectively. There are two hydrogen atoms and two oxygen atoms in formic acid. Hence, the total number of valence electrons in formic acid is 4 + (1 *2) + (6 *2) = 18 electrons. Step 2: The least electronegative atom with the maximum number of group valence acts as a central atom. Group valence is the maximum number of bonds an atom can form with other atoms. The group valance of C, H, and O is 4, 1, and 2, respectively. Hence, the carbon atom will act as a central atom. In the skeletal structure of formic acid, the carbon atom will be surrounded by H, O, and OH groups. Step 3: Arrange all the valence electrons in the skeletal structure of the molecule. We have 18 valence electrons, which need to be arranged in the Lewis structure of the formic acid. First of all, start with the carbon atom as it is a central atom. The carbon atom will be surrounded by eight electrons as per the octet rule. It will complete the duplet of hydrogen, which is attached directly to the carbon atom. Now, the oxygen atom attached to the carbon atom will share four electrons or two bond pairs of the electron with the carbon atom. It will also have two lone pairs of electrons (four electrons) to complete its octet. The remaining six electrons will be distributed in the OH group bonded to the carbon atom. Therefore, the possible Lewis structure of formic acid would be: The two bonding electrons will form a single bond and four bonding electrons will form a double bond. Hence, the Lewis structure of formic acid can be drawn as:
The Lewis structure is a simple representation of the molecule. It does not give any information about the shape of the molecule and the hybridization of the atom in the molecule. For that, we need to study the valence shell electron pair repulsion (VSEPR) theory and the valance bond theory (VBT).
HCOOH Molecular Geometry
The molecular geometry or shape can be predicted by the VSEPR theory. It considers the repulsions between bonding and nonbonding (lone pair) valence shell electrons. According to the Lewis structure of formic acid, the carbon atom is a central atom and it has three bond pairs without any lone pair of electrons. In VSEPR theory, the double bond is treated as one bond pair for the prediction of the shape of the molecule. Hence, the shape of the formic acid can easily be predicted by the following table. Hence, formic acid will have a trigonal planar geometry around carbon atom and tetrahedral geometry around oxygen atom as it has two lone pair and two bond pairs. The trigonal planar geometry of formic acid around the carbon atom should lead to the bond angle (H-C-O or O-C-O) of 120° to minimize bond pair-bond pair repulsions around the carbon atom. However, the actual bond angle is slightly different from 120° because there will be larger repulsion between the double bond pair and single bond pair. Hence, the H-C=O and O=C-O bond angles are greater than 120°. Similarly, the bond angle around the oxygen atom i.e., C-O-H should be 109.5 ° being a tetrahedral geometry. But, the C-O-H bond angle is 106 ° to minimize the repulsion between two lone pairs present on the oxygen atom. The bond lengths, which are determined from x-ray diffractometer, are shown in the following figure:
The hybridization of the carbon and oxygen atom in the formic acid can be determined by the valence bond theory (VBT) and steric number.
HCOOH Hybridization
Hybridization is the process of mixing atomic orbital and forming hybrid orbitals of equivalent energy. The number of hybrid orbitals is equal to the number of atomic orbitals combined. The resultant hybrid orbital overlap with either the hybrid orbital of other atoms or atomic orbital to form the covalent bond. The hybridization of the carbon atom in formic acid can be determined in the following way: The ground state electronic configuration of the carbon atom is [He] 2s22p2. One of the electrons from the 2s orbital will excite to the 2p orbital of the carbon atom and lead to the excited state configuration of the carbon atom as [He] 2s12p3.
As the carbon atom form three sigma bonds with other atoms and hence, one 2s orbital and two 2p orbitals of the carbon atom will mix and form three sp2 hybrid orbitals and one of the p orbitals remains unhybridized, which form a pi bond with the oxygen atom. The orbital diagram of formic acid, which represents the sigma bonds, is shown below. We can observe from the orbital diagram is that the carbon atom is the sp2 hybridized and one of the oxygen atoms is also sp2 hybridized whereas another oxygen atom bonded to hydrogen and carbon atom, is sp3 hybridized. The hybridization and shape can also be determined from the methods explained above or directly from the steric number. The steric number is equal to the sum of the number of atoms bonded to the particular atom and the number of lone pairs of electrons at the same atom. If the steric number is 2, 3, and 4 and hybridization will be sp, sp2, and sp3, respectively. The hybridization of different atoms in the formic acid has been explained in the following table from the steric number.
HCOOH Polarity
The net dipole moment and distribution of charges decide the polarity of the molecule. The electronegativity values of the hydrogen atom, carbon atom, an oxygen atom are 2.5, 2.2, and 3.5, respectively. The carbon atom is the central atom in formic acid. The bond electronegativity difference for the C-H bond is 2.5 – 2.2 = 0.3, whereas for C-O and O-H bond is 1.0 and 1.3, respectively. Hence, the C-H bond is slightly polar. The O-H and C-O bonds are polar covalent bonds. It creates a dipole in C-O bonds with a partial positive charge on carbon and a partial negative charge on the oxygen atom. Similarly, the O-H bond is also dipole. It leads to the polar nature of the formic acid. The charges are also not evenly distributed in the formic acid and hence, formic acid is a polar molecule. It is highly soluble in water and most of the polar solvents being its polar nature.
Conclusion
Formic acid is an organic compound with a pungent smell. It is the first member of the carboxylic acid family. Here, we have studied the basic properties of formic acid concerning its bonding nature. In formic acid, the carbon atom is a central atom. All atoms except hydrogen follow the octet rule in the Lewis structure of formic acid. The molecular geometry at the central carbon atom and hybridization of the central atom is trigonal planar and sp2 hybridization, respectively. Formic acid is a polar molecule, which makes it soluble in polar solvents. Feel free to ask your queries regarding the bonding nature of formic acid. Thank You. Happy learning.
