CH2Cl2 is the chemical formula for dichloromethane. It is a colorless and volatile liquid having a molar mass of 84.93 g/mol. It is used as a solvent in the manufacturing of various pharmaceutical products.
To find out whether the dichloromethane (CH2Cl2) molecule is polar or non-polar, continue reading the article.
Is CH2Cl2 polar or non-polar?
Dichloromethane CH2Cl2 is a polar molecule. CH2Cl2 consists of one carbon (C) atom, two hydrogens (H) atoms, and two chlorine (Cl) atoms. The carbon is kept at the central position, and the other atoms are at the surrounding positions, making a regular tetrahedral molecular shape.
An electronegativity difference of 0.61 units exists between a carbon and a chlorine atom in the C-Cl bond in the CH2Cl2 molecule. Thus, both C-Cl covalent bonds are individually polar in the CH2Cl2 molecule and possess a specific dipole moment value (symbol µ).
Although the CH2Cl2 molecule is ideally tetrahedral in shape, still the strong dipole moment of two C-Cl bonds will not cancel out with the small dipole moments of two C-H bonds that are only weakly polar because of a small electronegativity difference. As a result, CH2Cl2 has a permanent dipole moment and is overall polar (net µ =1.67 D).
A molecule is polar if there is a non-uniform charge distribution present in it. If the charge distribution gets equally balanced in different parts, then that molecule or molecular ion is considered non-polar.
The following three factors mainly influence the polarity of a molecule.
The electronegativity difference between two or more covalently bonded atoms
Dipole moment
Molecular geometry or shape
Now, let us discuss the effect of the above three factors one by one to prove that dichloromethane CH2Cl2 is overall a polar molecule.
Factors affecting the polarity of CH2Cl2
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 bonded atoms in a molecule, the higher the bond polarity.
Carbon (C) is present in Group IV A of the Periodic Table. The electronic configuration of carbon is 1s2 2s2 2p2. As per this electronic configuration, a C-atom has a total of 4 valence electrons. It is thus short of 4 more electrons that are required so that the carbon atom can achieve a complete octet electronic configuration.
Hydrogen (H) is present in Group IA of the Periodic Table. The electronic configuration of a hydrogen atom is 1s1. According to this electronic configuration, an H-atom has 1 electron in its only shell and lacks one more electron to complete its duplet.
Conversely, Chlorine (Cl) is present in Group VII A of the Periodic Table. The electronic configuration of chlorine is 1s2 2s2 2p6 3s2 3p5, which translates as a Cl-atom having a total of 7 valence electrons. It thus has a deficiency of one electron to complete its stable octet configuration.
Hence in CH2Cl2, the central C-atom is bonded with H-atoms and Cl-atoms via four single covalent bonds. The structure contains two C-H bonds and two C-Cl bonds in a regular tetrahedral molecular geometry.
In this way, all bonded atoms attain a completely stable electronic configuration via chemical bonding in CH2Cl2.
All four valence electrons of the central carbon atom get consumed in bond formation; there is no lone pair of electrons on the central C-atom in CH2Cl2 to distort its molecular geometry or shape.
Atom
Electronic configuration
Valence electrons
Carbon (6C)
1s2 2s2 2p2
4
Hydrogen (1H)
1s1
1
Chlorine (17Cl)
1s2 2s2 2p6 3s2 3p5
7
The electronegativity difference between C-atom (E. N= 2.55) and H-atoms (E. N= 2.20) in each C-H bond is 0.35 units. Due to this small electronegativity difference, the C-H bonds are considered only weakly polar in the CH2Cl2 molecule.
Contrarily, the highly electronegative Cl-atom (E. N= 3.16) strongly attracts the shared electron cloud away from the central C-atom (E. N= 2.55) in each C-Cl bond present in CH2Cl2. There is an electronegativity difference of 0.61 units between the bonded C and Cl atoms in each C-Cl bond.
Thus, the chlorine atoms present in the CH2Cl2 molecule gain a partial negative (Clδ-) charge, while central carbon obtains a partial positive (Cδ+) charge. The terminal H-atoms also gain strong partial positive charges (Hδ++),as shown in the figure below.
The Cl-atoms not only attract C-Cl bonded electrons but also attracts the shared electron cloud of the C-H bonds.
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 a polar covalent bond conventionally points from the positive center to the center of the negative charge. So in CH2Cl2, the dipole moment of each polar C-Cl bond points from Cδ+ to Clδ-(as shown below).
Molecular geometry
As discussed already, the CH2Cl2 molecule consists of a central C-atom. It is single covalently bonded to 2 H-atoms and 2 Cl-atoms. There are a total of 20 valence electrons in the CH2Cl2 Lewis structure, including 4 bond pairs and 6 lone pairs.
3 lone pairs of electrons are situated on each terminal Cl-atom while there is no lone pair of electrons on the central C or any of the two H-atoms.
According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, CH2Cl2 is an AX4-type molecule. To one C-atom at the center (A), four bonded atoms are attached (two H-atoms and two Cl-atoms), and the central C-atom contains no lone pair (E).
So, the molecular geometry or shape of CH2Cl2 is identical to its ideal electron pair geometry, i.e., tetrahedral. To minimize the repulsions and to attain stability in the tetrahedral structure of CH2Cl2, the approximate bond angles are ∠H-C-H = 112°, ∠Cl-C-Cl = 112.2°, and ∠Cl-C-H = 108°.
Although, due to an absence of any lone pair of electrons on the central C-atom, the CH2Cl2 molecule has a tetrahedral shape, identical to its ideal electron pair geometry; still the net dipole moment of three downwards pointing bonds (2 C-H and 1 C-Cl) do not get canceled equally with the dipole moment of an upwards pointing C-Cl bond.
Consequently, in CH2Cl2 there is a permanent dipole moment and the molecule is overall polar (net µ = 1.67 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), dichloromethane (CH2Cl2), chloroform (CHCl3), etc.
Examples include oxygen (O2), nitrogen (N2), methane (CH4), carbon disulfide (CS2), etc.
CH2Cl2 molecule has two C-H and two C-Cl bonds. C-H bonds have weak dipole moments than C-Cl bonds due to the electronegativity difference of bonded atoms in molecules.
CH2Cl2 has a tetrahedral shape and geometry, still, the dipole moment of individually polar bonds does not get canceled equally on each side of the molecule.
The weak dipole of C-H bonds is unable to cancel out the strong dipole of C-Cl which results in some permanent dipole moment in the CH2Cl2 molecule which is 67 Debye.
Thus, CH2Cl2 is overall a polar molecule with the net dipole moment of µ = 1.67 D.
How CH2Cl2 molecule has both polar and non-polar bonds?
According to Pauling’s electronegativity scale, a truly polar covalent bond must have an electronegativity difference greater than 0.5 units between the bonded atoms.
CH2Cl2 has two C-H bonds and two C-Cl bonds. In C-H bonds, the electronegativity difference is 0.35 units between C-atom (E. N= 2.55) and H-atom (E. N= 2.20). Hence, a C-H bond present in CH2Cl2 is weakly polar or almost non-polar.
However, in C-Cl bonds, the electronegativity difference is 0.61 units between C-atom (E. N= 2.55) and Cl-atom (E. N= 3.16) which results in high polarity of C-Cl bonds.
According to VSEPR theory, what is the shape of the CH2Cl2 molecule?
According to the valence shell electron pair repulsion (VSEPR) theory, dichloromethane CH2Cl2 is an AX4E0 type molecule.
To one C-atom at the center (A), four bonded atoms are attached (two H-atoms and two Cl-atoms), and the central C-atom contains no lone pair (E).
Hence, it forms a tetrahedral molecular shape as well as tetrahedral electron geometry.
Why is CH2Cl2 polar while CH4 non-polar although both have a tetrahedral shape?
The CH4 molecule consists of four identical C-H bonds. There is a small electronegativity difference of 0.35 units between the bonded atoms in each C-H bond.
The small dipole moments of the C-H bonds get canceled equally in the symmetrical tetrahedral shape of CH4 thus it is overall non-polar (net µ=0).
Contrarily, the CH2Cl2 molecule consists of two C-H bonds and two C-Cl bonds. Chlorine is a highly electronegative halogen atom. It strongly attracts the shared electron cloud of each C-H bond in addition to attracting C-Cl bonded electrons.
The C-H and C-Cl dipole moments do not get canceled equally in the tetrahedral CH2Cl2 molecule. Thus, it is overall polar (net µ > 0).
Which of the two (CH2Cl2 and CHCl3) is more polar?
Chloroform (CHCl3) consists of three Cl-atoms; thus, there are three strongly polar C-Cl bonds and one weakly polar C-H bond. The three Cl-atoms strongly attract the shared electron cloud from each C-Cl bond in addition to attracting C-H bonded electrons.
The electron cloud stays non-uniformly distributed. Each C-Cl bond has a higher dipole moment that significantly cancels the dipole moments of other polar bonds.
So the net dipole moment of the molecule is smaller as compared to CH2Cl2, which has two C-H and two C-Cl bonds. Consequently, CH2Cl2 (net µ = 1.67 D) is more polar than CHCl3 (net µ = 1.08 D).
What are the formal charges present on bonded atoms in CH2Cl2?
Formal charge on an atom = [ valence electrons – non-bonding electrons- ½ (bonding electrons)]
All atoms in the CH2Cl2 molecule including carbon, hydrogen, and chlorine obtain a formal charge equal to zero, hence overall charge present on the dichloromethane molecule is Zero.
Summary
Dichloromethane (CH2Cl2) is a polar molecule.
It consists of two weakly polar C-H bonds with an electronegativity difference of 0.35 units and two strongly polar C-Cl bonds having an electronegativity difference of 0.61 units.
Dichloromethane CH2Cl2 has a symmetrical tetrahedral shape with bond angles ∠H-C-H = 112°, ∠Cl-C-Cl = 112.2°, and ∠Cl-C-H = 108°.
Due to the absence of any lone pair of electrons on the central C-atom, the CH2Cl2 molecule has a planar tetrahedral shape.
The weak dipole of C-H bonds is unable to cancel the strong dipole moments of C-Cl bonds. Consequently, in CH2Cl2 there is a permanent dipole moment of 1.67 Debye in the molecule.
The net dipole moment in CH2Cl2 is 1.67 Debye, so it is an overall polar molecule.
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