Dibromomethane is a colorless, non-flammable liquid with the chemical formula CH2Br2. It has a sweet, pleasant odor and is toxic if inhaled. CH2Br2 is widely used to synthesize organic compounds like fragrances, agrochemicals, and pharmaceuticals.
Due to its insolubility as a solution in water, dibromomethane (CH2Br2) is often considered non-polar, which is not true. It is a polar molecule.
If you are curious, how? Continue reading this article to find out.
Is CH2Br2 polar or non-polar?
Dibromomethane (CH2Br2) is a polar molecule. The central carbon (C) atom in the CH2Br2 molecule is surrounded by two hydrogens (H) and two bromine (Br) atoms via single covalent bonds, forming an asymmetric tetrahedral molecule.
The electronegativity of the bromine (Br) atoms is greater than the carbon (C) and hydrogen (H) atoms. The higher electronegative Br atoms attract the shared electron pairs with more influence in the CH2Br2 molecule.
Thus, all the bonds are individually polar and possess a specific dipole moment value.
The asymmetric arrangement of atoms in the tetrahedral shape of CH2Br2 further enhances the polarity effect as the dipole moments of the bonds do not get canceled in the molecule overall. Thus, CH2Br2 is a polar molecule with a net dipole moment value greater than 0.
In chemistry, a molecule with unequal charge distribution between different centers of bonded atoms is a polar molecule.
It is formed by the covalent bond between two different atoms leading to an asymmetric electron density.
In this case, the atoms acquire partial positive (δ+) and partial negative (δ–) charges.
If the dipole moments of individually polar bonds are not canceled due to the asymmetrical shape of the molecule, the molecule will be polar, such as CH2Br2.
Hence a polar molecule has an unequal distribution of the electronic charge. Contrarily, if the electronic charge is evenly distributed over the molecule, in that case, it will be a non-polar molecule overall.
The following three factors influence the polarity of any covalent molecule:
Electronegativity.
Dipole moment.
Molecular geometry or shape.
In the next section, we will discuss how these three factors influence the polarity of the CH2Br2 molecule.
Factors affecting the polarity of CH2Br2
Electronegativity
It is defined as the ability of an atom to attract a shared pair of electrons from a covalent chemical bond.
Electronegativity decreases down the group in the Periodic Table of elements while it increases across a period.
Greater the electronegativity difference between the bonded atoms in a molecule, the higher the bond polarity.
Hydrogen belongs to group 1-A (or 1) of the Periodic Table. The electronic configuration of hydrogen is 1s1, so it has 1 valence electron.
Carbon belongs to group IV-A (or 14) of the Periodic Table. The electronic configuration of carbon is 1s2 2s2 2p2, so it has 4 valence electrons available for bonding.
On the other hand, bromine belongs to group VII-A (or 17) of the Periodic Table. The electronic configuration of bromine is [Ar] 3d104s24p5, which indicates that it has 7 valence electrons.
The two H-atoms and two Br-atom thus form a single covalent bond with the central C-atom on each side of the CH2Br2 molecule.
Atom
Electronic configuration
Valence electrons
Hydrogen (1H)
1s1
1
Bromine (35Br)
1s22s22p63s23d104s24p5
7
Carbon (6C)
1s22s22p2
4
The electronegativity of bromine is more than the carbon and hydrogen atoms (E.N of F = 2.96, E.N of C = 2.55, E.N of H = 2.2).
Due to this electronegativity difference, the Br-atoms strongly attract the shared electron cloud from the bonds in the CH2Br2 molecule. The bonded electrons are held significantly close to the bromine atom.
The central C-atom and H-atoms thus gain a partial positive charge (Cδ+ and H δ+), while the bromine atom being more electronegative, obtains a partial negative (Brδ-) charge. In this way, oppositely charged poles develop in the CH2Br2 molecule.
The Br atoms not only attract the shared electron cloud of each C-Br bond but also attracts C-H electrons. As a result, the electron cloud distribution in the molecule is unequal overall. Thus, CH2Br2 is a polar molecule.
Dipole Moment
The dipole moment is the product of electrical charge (Q) and bond length (r) between two bonded atoms. It is a vector quantity expressed in Debye (D) units.
It is represented by a Greek symbol µ and measures the polarity of a bond.
The dipole moment of any molecule depends on the difference in electronegativity between the bonded atoms. The greater the electronegativity difference, the higher the bond polarity, resulting in a high dipole moment value.
It points from the partial positive (δ+) center to the partial negative (δ–) center of a bond or molecule.
The difference in electronegativity between the bonded atoms in the CH2Br2 molecule leads to dipoles from Cδ+and Hδ+ to Brδ-.
Thus, each bond in the CH2Br2 molecule is polar, with a net dipole moment greater than 0.
Molecular geometry
As discussed earlier, a dibromomethane (CH2Br2) molecule consists of two C-H and two C-Br covalent bonds. There are 20 valence electrons in the overall molecule (4 bond pairs and 6 lone pairs of electrons).
According to the Valence Shell Electron Pair Repulsion Theory (VSEPR) theory of chemical bonding, CH2Br2 is an AX4-type molecule. Around the central carbon atom (A) are four bond pairs of electrons (X).
To minimize the electronic repulsions between the atoms, the dibromomethane (CH2Br2) molecule adopts an tetrahedral geometry with a Br-C-Br bond angle of 113.4° and an H-C-H bond angle of 112.5°.
As a result of the asymmetric arrangement of atoms around the central carbon, the individual dipole moments of the bonds do not get canceled in the dibromomethane (CH2Br2) molecule. There is an unequal distribution of electronic charge over the molecule.
In conclusion, CH2Br2 is an overall polar molecule with a net dipole moment (µ = 1.7 D).
Difference between polar and nonpolar?
Polar molecule
Non-polar molecule
Atoms must have a difference in
electronegativity
Atoms may have the same or different electronegativity values
Unequal charge distribution overall
Equal charge distribution overall
Net dipole moment greater than zero
Net dipole moment equals to zero
Examples include water (H2O), ethanol (CH3CH2OH), ammonia (NH3), sulfur dioxide (SO2), bromine trifluoride (BrF3), dibromomethane (CH2Br2), bromine pentafluoride (BrF5), etc.
Examples include oxygen (O2), nitrogen (N2), methane (CH4), carbon disulfide (CS2), etc.
CH2Br2 is polar, yet it is only partially soluble in water (a polar solvent). Why?
CH2Br2 is sparingly soluble in water due to its polar ends but cannot form hydrogen bonding with H2O molecules.
Thus, CH2Br2 does not completely dissolve in water. For H-bonding, hydrogen atoms must be directly bonded to the electronegative atoms.
Another reason for its sparingly soluble nature is its higher density (2.4 g/mL) than water (1 g/mL).
Is there a formal charge on the bonded atoms in the CH2Br2 molecule?
Formal charge of an atom = [ valence electrons – non-bonding electrons- ½ (bonding electrons)]
For hydrogen atoms
Valence electrons = 1
Bonding electrons = 2
Non-bonding electrons = 0
∴ The formal charge on the hydrogen atoms = 1-0-2/2 = 1-1 = 0
For bromine atoms
Valence electrons = 7
Bonding electrons = 2
Non-bonding electrons = 6
∴ The formal charge on the bromine atoms = 7-6-2/2 = 1-1 = 0
For carbon atom
Valence electrons = 4
Bonding electrons = 8
Non-bonding electrons = 0
∴ The formal charge on the central carbon atom = 4-0-8/2 = 4-4 = 0
Thus, no formal charge is present on the dibromomethane (CH2Br2) molecule.
Summary
Dibromomethane/methyl dibromide (CH2Br2) is a polar molecule.
Due to a great electronegativity difference between the bonded bromine, carbon, and hydrogen atoms, polar covalent bonds are generated.
The bromine atoms strongly attract the shared electron cloud from the bonded atoms in the CH2Br2
The difference in electronegativity between the bonded atoms leads to dipoles from C δ+and Hδ+ to Br δ-.
The AX4 type CH2Br2 molecule adopts a tetrahedral geometry with a Br-C-Br bond angle of 113.4° and an H-C-H bond angle of 112.5°.
Due to the unsymmetrical arrangement of atoms in the tetrahedral molecule, the individual dipole moments of the bonds are not canceled in the molecule overall. Thus, CH2Br2 is a polar molecule (μ = 1.7 D).
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/
Topblogtenz is a website dedicated to providing informative and engaging content related to the field of chemistry and science. We aim to make complex subjects, like chemistry, approachable and enjoyable for everyone.
