Trichloromethane, commonly known as chloroform, is a volatile organic compound in which one C-atom is covalently bonded to 3 Cl-atoms and 1 H-atom.
It is a colorless liquid with a characteristic odor.
At laboratory scale, it is prepared by chlorination of ethanol. Bleaching powder is often used as a chlorinating agent.
Earlier, it was used as an anesthetic during medical procedures. Nowadays, it is widely used as a solvent for organic compounds and in producing polymeric materials and refrigerants.
It is an industrial compound and sometimes irritates the eyes. It is non-flammable.
Chloroform is always stored in bottles of dark colors. It gets oxidized by sunlight to form phosgene (a poisonous gas) if kept in light-colored bottles.
In this article, we will understand the concept and procedure behind finding out the lewis structure, geometry, hybridization, and polarity of a given molecule.
CHCl3 Lewis Structure
Lewis Structure is a simple representation of how valence shell electrons are arranged in a molecule. It tells us that a bond exists between atoms but does not tell us anything about the type of bonds.
In the lewis structure, valence electrons are represented by dots, and 2 bonding electrons between two atoms are represented by a line.
The octet rule and formal charges need to be satisfied for verification of the so drawn lewis structure.
Octet Rule: Noble gases are considered stable, and they have 8 electrons in their valence shell.
Main group elements prefer to have electronic configuration resembling noble gas (8 electrons in valence shell) to attain stability.
If there are more or less than 8 electrons, the atom prefers to lose or gain electrons to achieve octet configuration.
In simple words, in a lewis structure, all atoms should have 8 electrons around them.
Formal Charge: It is not an actual charge; it is just a theoretical concept.
It is used to determine the charge on an individual atom in a polyatomic molecule.
The sum of formal charges on all atoms has to be equal to the net charge.
It is obtained by subtracting the number of electrons assigned to an atom in the lewis structure from the number of valence electrons of the atoms in their free state.
Steps to Draw the Lewis Structure of CHCl3
Step1: Calculate the total number of valence electrons in the molecule.
It is calculated by adding up the valence electrons on all atoms.
|Atom||Atomic Number||Group Number||Valence electrons according to group number||Electronic configuration (E.C.)||Valence shell from E.C.||Valence electrons from E.C.|
|One C||6||14||4||1s2 2s2 2p2||n=2||4|
|Three Cl||17||17||7||1s2 2s2 2p6 3s2 3p5||n=3||7|
|Total number of valence shell electrons= 4 + 1 + (7*3) =26|
Lewis structure for H, C, and Cl can be drawn using the number of valence shell electrons.
Step 2: Select the central atom.
The central atom is chosen so that it provides stability to the whole molecule, and electron density spread is facilitated.
Since the central atom has to share the electron density with the surrounding atoms, it should not be highly electronegative like Cl.
Chlorine will not share the electron density with other atoms. Out of C and H, H is very small and is not suitable as a central atom.
Hence, C is the central atom in the chloroform atom.
Step 3: Draw a skeletal diagram of the molecule.
Step 4: Place the valence electrons around individual atoms
The total valence shell electrons of the molecules are arranged according to the preferred bond formation.
Step 5: Complete the octet of atoms by forming bonds.
Chlorine and carbon atoms have attained octet configuration.
H has only 1 electron, which is shared with one electron of C to achieve a fully filled configuration. 1s can have a maximum of two electrons.
Step 6: Calculate the formal charge on all atoms.
The net charge on the molecule is zero, but this does not guarantee that the formal charge on all atoms is also zero.
The formal charge is calculated using the formula-
|Atom||Total number of valence electrons in a free atom||Total number of lone pairs||(Total number of bonding electrons)*0.5||Formal Charge|
The subscript in Cl indicates the first Cl atom, second Cl atom, and third chlorine atom.
The lewis structure so formed is correct. For a better understanding, one can go through this video.
One of the many drawbacks of Lewis structure is that it cannot tell us the molecular geometry of a covalent compound. VSEPR theory proves to be helpful in overcoming this drawback.
Molecular geometry is the representation of all atoms and bonds of a molecule in space.
VSEPR theory (Valence Shell electron pair repulsion theory) overcomes this drawback.
According to VSEPR theory-
• The valence shell electron pairs of atoms in a molecule repel each other, leading to instability.
• Repulsions have to be decreased in order to make the arrangement stable.
• Electrons align themselves so that the repulsion is the least, and the distance between them is maximum.
• The stable arrangement of the valence electron pairs of atoms helps in determining the molecular geometry.
Valence shell electrons that are involved in bonding are known as bonding pairs of electrons (bp), and those
valence shell electrons that are not involved in bonding are termed as lone pairs of electrons (lp).
Steps for Predicting Molecular Geometry using VSEPR
Count the valence shell electrons on the central atom (N).
For CHCl3, the total number of valence shell electrons on the central atom (C) is 4.
• Add one electron for each surrounding atom to N to get A. The electrons are also added to A for the negative charge on a molecule and subtracted for the positive charge on the molecule.
In chloroform, C has 4 surrounding atoms, therefore A=4+ (1*4)+0=8
• Divide A by 2 to get total domains or total pairs of electrons.
Here, we get total domains as 4 (8/2=4)
• If total domains= number of surrounding atoms, then lone pair is absent on the central atom.
Thus, in chloroform, the total number of electrons pair= the total number of bonding electron pair=4 and the lone pair=0.
• We can predict the shape using the following table.
Thus, the geometry and shape of CHCl3 are tetrahedral.
The concept of hybridization explains the geometrical shape and bonding in polyatomic molecules.
An orbital is a 3D region around the nucleus where the probability of finding an electron is maximum.
Hybridization can be defined as the mixing of pure atomic orbitals to form hybrid atomic orbitals. This mixing is feasible if the pure atomic orbitals are of similar shape and energy.
For instance, 2s and one 2p can form sp hybrid orbitals, but 2s and 5d cannot. Also, in C (carbon atom), one 2s and three 2p orbitals are mixed to form 4 equivalent sp3 hybrid orbitals.
C is the central atom in the chloroform molecule.
The ground state electronic configuration of C is 1s2 2s2 2p2. Only valence orbitals are used in hybridization.
In the excited state, one electron of 2s gets promoted to 2p orbital. Now, these 4 orbitals (one 2s and three 2p) undergo hybridization to form four sp3 orbitals, which will form bonds with the surrounding atoms.
In CHCl3, there are 4 surrounding atoms. Each atom forms bond with one sp3 hybrid orbital.
Three Cl-atoms and one H-atom form a sigma bond with sp3 hybrid orbitals. Thus, CHCl3 has sp3 hybridization.
The trick for calculating the type of hybridization.
In VSEPR theory, we calculated the total electron pairs in the last step. For CHCl3, it came out to be 4.
From the table below, we can predict hybridization using total electron pairs or steric numbers.
The bond angle in the CHCl3 molecule is around 109.5°, and hybridization is sp3.
The polarity of a compound depends on the following factors-
• Dipole moment
• The difference in electronegativity of two atoms
• Geometry and symmetry
• Charge Separation
The bonds are polar (dipole moment not equal to zero) if the two atoms forming the bond have different electronegativity.
There are two types of bonds in chloroform — C-Cl and C-H. Both are polar as the difference in electronegativity is 0.61 (3.16-2.55) and 0.35 (2.55-2.2), respectively.
Polar bonds do not ensure a polar compound. In a symmetrical molecule, all bonds are of the same type and dipole moments often cancel out.
For C-Cl bonds, the dipole moment vector is directed from C to Cl atom. The three C-Cl bond moment vectors are directed in a downward direction.
In the C-H bond, the dipole moment vector is directed from H to the C atom. The C-H bond moment vector is directed in an upward direction.
The magnitude of three C-Cl bond vectors is greater than the C-H bond vector, and the net dipole moment is downward.
Hence, the compound is polar.
Chloroform is a colorless liquid with a characteristic smell.
The C- atom forms one covalent bond with each Cl- atom and one with H-atom in chloroform.
The geometry and shape of CHCl3 are tetrahedral.
Its hybridization is sp3, and it is a non-polar molecule.
I hope you enjoyed the chemistry of chloroform. In case of any queries, ask them in the comments. I will be happy to answer them.