Methyl bromide is an important colorless, odorless and non-flammable hydrofluorocarbon. The chemical formula CH3Br represents this ozone-depleting gas. The IUPAC name of CH3Br is bromomethane.
CH3Br is extensively used as a fumigant insecticide and has a potential role as a fire-extinguishing agent. It is a highly toxic chemical, and prolonged exposure can even be fatal.
So what is the polarity of this highly toxic gas bromomethane (CH3Br)? Is it a polar or a non-polar molecule? Let’s find out in this article.
Is CH3Br polar or non-polar?
Methyl bromide (CH3Br) is a polar molecule. The central carbon (C) atom in the CH3Br molecule is surrounded by three hydrogens (H) and one bromine (Br) atom via single covalent bonds, forming a tetrahedral molecule.
The electronegativity of the bromine (Br) atom is greater than the carbon (C) and hydrogen (H) atoms. The higher electronegative Br atom attracts the shared electron clouds with more influence in the CH3Br molecule.
Thus, all the bonds are individually polar and possess a specific dipole moment value.
The asymmetric arrangement of atoms in the tetrahedral-shaped CH3Br molecule further enhances the polarity effect as the dipole moments of the bonds is not canceled in the molecule overall. Thus, CH3Br is a polar molecule with a net dipole moment value > 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 cancelled due to the asymmetrical shape of the molecule, the molecule will be polar, such as CH3Br.
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 factors control the polarity of the CH3Br molecule.
Factors affecting the polarity of CH3Br
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 1s22s22p63s23d104s24p5, indicating the presence of 7 valence electrons.
The three H-atoms and one Br-atom thus form a single covalent bond with the central C-atom on each side of the CH3Br molecule.
Atom
Electronic configuration
Valence electrons
Hydrogen (1H)
1s1
1
Bromine (35Br)
1s22s22p63s23d104s24p5
7
Carbon (6C)
1s22s22p2
4
The electronegativity bromine atom is more electronegative than the carbon and hydrogen atoms (E.N of Br = 2.96, E.N of C = 2.55, E.N of H = 2.2).
Due to this electronegativity difference, the Br-atom strongly attracts the shared electron cloud from the bonds. The bonded electrons are held significantly close to the bromine atom in the CH3Br molecule.
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 CH3Br 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 non-uniform overall. Thus, CH3Br 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 CH3Br molecule leads to dipoles pointing from Cδ+and Hδ+ to Brδ-.
Thus, each bond in the CH3Br molecule is polar, with a net dipole moment greater than 0.
Molecular geometry
As discussed earlier, a methyl bromide (CH3Br) molecule consists of three single C-H covalent bonds and one C-Br covalent bond. There are a total of 14 valence electrons in the overall molecule.
According to the Valence Shell Electron Pair Repulsion Theory (VSEPR) theory of chemical bonding, CH3Br 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 methyl bromide (CH3Br) molecule adopts a tetrahedral geometry with an H-C-H and H-C-Br bond angle of 112.5° and 107.7°, respectively.
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 methyl bromide (CH3Br) molecule. There is an unequal distribution of electronic charge over the molecule.
In conclusion, CH3Br is a polar molecule with a net dipole moment (µ = 1.83 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), methyl bromide/bromomethane (CH3Br), bromine pentafluoride (BrF5), etc.
Examples include oxygen (O2), nitrogen (N2), methane (CH4), carbon disulfide (CS2), etc.
Due to a great electronegativity difference between the bonded bromine, hydrogen, and carbon atoms, polar covalent bonds are generated in the CH3Br molecule.
Due to the unsymmetrical arrangement of atoms in the tetrahedral molecule, the individual dipole moments of the bonds are not canceled in the CH3Br molecule.
Thus, CH3Br is a polar molecule.
The polarity of CH3Br is less than CH3Cl. Why?
The greater the electronegativity difference between the bonded atoms, the higher the bond polarity, resulting in a high dipole moment value.
Chlorine ( E.N = 3.16) is more electronegative than the bromine atom (E.N = 2.96).
Thus, the chlorine atom will attract the shared electron pairs in the CH3Cl molecule with more influence than the bromine atom in the CH3Br molecule.
As a result, the C-Cl bond will be more polar than the C-Br bond.
Thus, the polarity of CH3Br is less than CH3Cl.
Compare the polarity of CH3Br and CH3I molecules.
Greater the electronegativity difference between the bonded atoms in any molecule, the higher the bond polarity
Bromine ( E.N= 2.96) is more electronegative than the iodine atom (E.N = 2.66).
As a result, bromine will attract the shared electron clouds in the CH3Br molecule more than the iodine atom in the CH3I molecule.
Thus, the bond polarity of the C-Br bond will be greater than the C-I bond.
Thus, CH3Br is more polar than the CH3I molecule with a higher dipole moment value.
Is there a formal charge on the bonded atoms in the CH3Brmolecule?
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 atom
Valence electrons = 7
Bonding electrons = 2
Non-bonding electrons = 6
∴ The formal charge on the bromine atom = 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 methyl bromide (CH3Br) molecule.
Summary
Methyl bromide/bromomethane (CH3Br) is a polar molecule.
Due to an electronegativity difference between the bonded bromine, hydrogen, and carbon, polar covalent bonds are generated.
The bromine atom strongly attracts the shared electron cloud from the CH3Br molecule.
The electronegativity difference between the bonded atoms leads to dipoles pointing from C δ+and Hδ+ to Brδ-.
The AX4 type CH3Br molecule adopts a tetrahedral geometry.
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.
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/
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