Polarity, as seen in compounds, is a condition where separation in electric charge results in the positive and negative pole of a compound.
This is produced due to the difference in the electronegativity (the ability of an atom in a chemical bond to pull electrons towards itself) of two or more atoms in a molecule or in other words, the unequal sharing of their valence electrons.
It involves the physical properties of the compounds such as boiling and melting points, solubility, surface tension and the interaction between the molecules.
So, Is CH3Cl polar or non-polar? Yes, Methyl chloride (CH3Cl) or Chloromethane is a polar molecule. The C-Cl covalent bond shows unequal electronegativity because Cl is more electronegative than carbon causing a separation in charges that results in a net dipole.
Polar molecules are those molecules that possess two ends, like two poles of a magnet, which vary completely in the nature of charge they carry.
For instance, in HCl ( Hydrogen Chloride) the chlorine exhibits higher electronegativity than hydrogen thereby strongly attracting electrons yielding a partial negative charge on itself and a partial positive charge on the other end, that is, on hydrogen.
The electronegativity of atoms sharing covalent bonds can be best understood by the Lewis structure and Valence Bond Theory.
Lewis electron-dot structure of CH3Cl
The Lewis structure is used to predict the properties of molecules and how they react with other molecules. It also throws light on the physical properties of molecules.
Determining the arrangement of atoms and the distribution of electrons around it is important to predict the molecule’s shape and explain its characteristics.
If you consider Lewis structure for CH3Cl, you will find that it is an asymmetrical molecule. The absence of symmetry is due to the unequal sharing of valence electrons.
When the structure is drawn, carbon is positioned at the center as the central atom with chlorine on one side and the hydrogen atoms on the other side.
If we look at the molecular geometry of the molecule, we can determine the polarity by drawing arrows of net dipole. Let’s learn the Lewis dot structure for CH3Cl.
For the Lewis structure, we need to calculate the total number of valence electrons for CH3Cl. As per the periodic table, carbon lies in group 14 and has 4 valence electrons, hydrogen belongs to group 1 and has only 1 valence electron and here, we have 3 hydrogen atoms.
Chlorine belongs to group 17 and has 7 valence electrons. Now, by adding all the valence electrons we get a total of 14 valence electrons.
Carbon being the central atom remains at the center. The hydrogen atoms are always positioned at the outside and chlorine which is highly electronegative will go on the outside as well.
Further, we need to distribute these electrons in the structure.
We have a total of 14 valence electrons out of which 2 have to be placed between each of the atoms to form a chemical bond. We used 8 valence electrons and after this, we are left with 6 valence electrons. Let’s check if we have filled the outer shells of all the atoms.
In the case of chlorine, we add 6 more valence electrons to complete the octet. Hydrogen needs only 2 valence electrons and it already has. Chlorine needs 8 valence electrons and it has 8. Now, that we have used all 14 valence electrons, the outer shells of each atom are filled.
As we know, chlorine is more electronegative than carbon since it lies closer to fluorine on the periodic table, a dipole arrow can be drawn from Carbon to Chlorine [ C-Cl ] with the cross at one end.
The cross is marked near the end of the molecule that is partially positive and the arrow-head lies at the partially negative end of the molecule.
The difference between electronegativity values of hydrogen and carbon is small and thus C-H bond is non-polar. Therefore, we do not draw any dipole arrow for C-H bonds.
Using Lewis structure we can infer that the C-Cl bond is polar and hence, the CH3Cl is polar and has a net dipole.
The magnitude of the polarity of a bond is termed as the dipole moment. The more the difference in the relative electronegativity of the atoms the higher is the dipole movement and the polarity.
Valence Bond Theory [VBT]
The Lewis electron dot structure reveals the arrangement of electrons in a molecule in a two-dimensional representation.
Whereas the Valence Bond Theory reflects on the different shapes of a molecule and the molecule model that result from the overlapping of atomic orbitals holding bonding and non-bonding electrons.
CH3Cl exhibits an Sp3 hybridization. How? Let’s understand.
The steric number in the case of CH3Cl is 4. The steric number is the number of bonds and lone pairs at the central atom. Three sigma bonds are present between carbon and hydrogen and one between carbon and chlorine.
Now, there is no lone pair of the electrons left since carbon has 4 valence electrons and all the 4 have formed bonds with 3 hydrogens and 1 chlorine atom.
Thus, the hybridization will be 1+3=4=Sp3 i.e., 1s and 3p.
sp³ hybridization and tetrahedral bonding
Let us have a closer look at the tetrahedral bonding. Here, we need to understand how carbon forms four bonds when it has only two half-filled p- orbitals available for bonding.
In order to explain this, we have to consider the orbital hybridization concept. This concept refers to the combination of atomic orbitals on a single atom that forms new hybrid orbitals with geometry appropriate for the pairing of electrons so as to form chemical bonds.
In the picture below, there are four valence orbitals of carbon i.e., one 2s and three 2p orbitals. These combine forming four equivalent hybrid orbitals.
Structure and properties of Chloromethane
Chloromethane belongs to the group of organic compounds called haloalkanes or methyl halides and has a tetrahedral structure with a bond angle of 109.5°.
The tetrahedral structure of chloromethane is a result of repulsion between the electron clouds on atoms around the central carbon atom. It has an asymmetrical geometry to avoid the canceling of dipoles which arise due to the opposing charges.
This molecule has a boiling point of -24°C (-11.2°F) and turns into liquid under its own pressure. It freezes at -97.6°C and is industrially used as a refrigerant.
It has a molecular mass of 50.49 g/mol and a density of 2.22 kg/m³. CH3Cl is soluble in both alcohol and water.
In laboratories, methyl chloride can be prepared by using methanol and hydrogen chloride. It can also be prepared by chlorination of methane.
In nature, methyl chloride is formed in oceans by marine phytoplankton. They are also formed from natural processes like the burning of biomass in grasslands and forests.
A mixture of chlorine and methane when subjected to ultraviolet light undergo a substitution reaction forming chloromethane.
In methyl chloride, one hydrogen is replaced by a chloro-group and it gives a mild sweet smell only when present in high concentration in the air or otherwise is difficult to detect.
Methyl chloride is a colorless, odorless (in low concentration), toxic and flammable gas. It is a weak electrolyte because of the polar covalent bond that allows the molecule to acts as a good conductor.
Polar molecules like CH3Cl tend to associate more due to the attraction between the positive and negative ends of the molecule.
This association leads to a decrease in the vapor pressure and an increase in the boiling point as more energy is required to vaporize the molecule. Chloromethane, like other polar molecules, interacts through dipole-dipole forces.
Industrial applications of methyl chloride
- Methyl chloride is a well-known refrigerant.
- It is used as a catalyst or solvent in the production of butyl rubber and elastomers.
- It is also widely used as a chlorinating agent.
- It is also used by petroleum refineries.
- In fields, it is used as a herbicide.
- In the production of silicone polymers, silicone fluids, resins, and methyl celluloses.
- It is used in the manufacturing of drugs and medicine.
- For medical purposes, It is used as a local anesthetic.
- Used as a raw material for the manufacturing of surfactants, pharmaceuticals, and dyes.
Hazards of exposure to methyl chloride
Methyl chloride is a highly flammable and also a hazardous chemical. Sources of exposure to methyl chloride include burning of wood, coal and some plastics, cigarette smoke, aerosol propellants.
Low concentration of this chemical is also present in lakes, streams and in drinking water.
In humans, a brief exposure to toxic levels of methyl chloride can have a serious impact on the nervous system and can cause coma, paralysis, convulsions, seizures and possibly death.
Effects involve dizziness, blurred vision, nausea, fatigue, vomiting, slurred speech, lung congestion. Some experience a problem in their heart rate, liver, and kidneys after inhaling the methyl chloride gas for a brief period.
It has been reported that it can cause frostbite and neurotoxicity depending on the route and concentration of exposure.
Important reactions involving chloromethane
CH4 + Cl2—🡪 CH3Cl (Chloromethane) + HCl
CH3Cl + Cl2—🡪 CH2Cl2 (Dichloromethane) + HCl