Is ethylene glycol polar or nonpolar? – (Polarity of ethylene glycol)

Ethylene glycol is a diol, represented by the chemical formula C₂H₆O₂. Its IUPAC name is ethan-1,2-diol implying that two alcohol (OH) functional groups are attached at adjacent carbon atoms.

Ethylene glycol (molar mass = 62.07 g/mol) exists as a clear, sweet-smelling, slightly viscous liquid at r.t.p and is frequently used as an antifreeze agent.

Do you want to know whether it is polar or non-polar? If yes, then this article is for you, so continue reading!

Is ethylene glycol polar or non-polar?

Ethylene glycol (ethan-1,2-diol) is non-polar (net µ = 0).

It comprises two hydroxyls (OH) functional groups attached at adjacent carbon (C) atoms. The four main types of chemical covalent bonds present in ethylene glycol are C-C, C-H, C-O, and O-H bonds.

The C-C bond is purely non-polar, while the C-H bonds are weakly polar. In contrast, the C-O and O-H bonds present in ethylene glycol are strongly polar, possessing high electronegativity differences of 0.89 units and 1.24 units between the respective atoms.

However, the equal, oppositely directed C-O and O-H dipole moments get canceled uniformly in the bent shape of the molecule w.r.t each oxygen atom.

This leads to a balanced electron cloud distribution and, thus, a non-polar molecule overall (net µ = 0).

What makes a molecule polar or non-polar?

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 is considered non-polar.

The following three factors mainly influence the polarity of a molecule:

• Electronegativity difference between two or more covalently bonded atoms
• Dipole moment
• Molecular geometry or shape

Now let’s see how the above three factors make ethylene glycol a non-polar molecule overall.

Factors affecting the polarity of ethylene glycol

Electronegativity

It is defined as the ability of an elemental atom to attract a shared pair of electrons from a covalent chemical bond.

Electronegativity increases across a period in the Periodic Table while it decreases down the group.

Greater the electronegativity difference between bonded atoms in a molecule, the higher the bond polarity.

The three main elemental atoms present in ethylene glycol are carbon, hydrogen and oxygen.

Carbon (C) belongs to Group IV A (or 14) of the Periodic Table of elements. Its electronic configuration is 1s² 2s² 2p². It has a total of 4 valence electrons which means it is still deficient in 4 more electrons in order to gain a stable octet electronic configuration.

Hydrogen (H) lies at the top of the Periodic Table in Group I A (or 1). Its electronic configuration is 1s¹, which implies that it lacks 1 more electron to complete its duplet.  

Oxygen (O) belongs to Group VI A (or 16). Its electronic configuration is 1s² 2s² 2p⁴. It has a total of 6 valence electrons. It thus needs 2 more electrons in order to complete its octet. 

As per the Lewis dot structure of ethylene glycol drawn below, it consists of 1 C-C bond, 4 C-H bonds, 2 C-O bonds and 2 O-H bonds, respectively.

As all four valence electrons of the central C-atoms get consumed in covalent bonding, hence there is no lone pair on any of the carbon atoms. Contrarily, both the O-atoms contain lone pairs of electrons.

In this way, both the C-atoms and O-atoms complete their octets via chemical bonding and lone pairs in ethylene glycol while each H-atom attains a complete duplet electronic configuration.

As per Pauling’s electronegativity scale, a polar covalent bond is formed between two dissimilar atoms having an electronegativity difference between 0.4 to 1.6 units.

A C-C or C=C bond is purely non-polar as zero, or no electronegativity difference is present between two identical C-atoms. Thus, the covalently-bonded electron cloud stays equally shared between the two carbon atoms.

In a C-H bond, only a small electronegativity difference of 0.35 units is present between a carbon (E.N = 2.55) and a hydrogen (E.N = 2.20) atom. It is less than 0.4 units; therefore, the C-H bond is also considered non-polar as per Pauling’s electronegativity scale.

In contrast, a high electronegativity difference of 0.89 units exists between a carbon and an oxygen (E.N = 3.44) atom. Thus, the C-O bonds are strongly polar.

Similarly, the two O-H bonds present at the sides are extremely polar as per an electronegativity difference of 1.24 units between the bonded atoms.

Oxygen, being strongly electronegative, attracts the shared electron cloud largely towards itself and gains a partial negative charge (δ^–), while the corresponding covalently bonded C and H-atoms obtain partial positive charges (δ⁺), as shown below.

Dipole moment

Dipole moment (μ) is a vector quantity that points from the positive pole to the negative pole of a bond or a molecule.

It is mathematically calculated as a product of the magnitude of charge (Q) and charge separation (r). The dipole moment is expressed in a unit called Debye (D).

The dipole moment of a polar covalent bond conventionally points from the positive center to the center of the negative charge.

In ethylene glycol, the strong dipole moments of the C-O bonds point from C^δ+ to O^δ– while that of the O-H bonds points from H^δ+ to O^δ–.

Refer to the figure drawn below.

Molecular geometry

According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, ethylene glycol is an AX₄-type molecule w.r.t each C-atom.

To a carbon atom at the center (A), four bond pairs (X) are attached, i.e., another C-atom, 2 H-atoms and an O-atom, and it has no lone pairs of electrons (E). Therefore, the shape of the molecule w.r.t each carbon atom is tetrahedral.

On the other hand, the molecular shape of ethylene glycol w.r.t each O-atom is bent, angular or V-shaped. The 2 lone pairs of electrons on the O-atoms lead to strong lone pair-lone pair and lone pair-bond pair repulsions, which distorts the overall molecular shape.

However, in this bent shape, the oppositely directed O-H dipole moments get cancelled equally.

Also, the equal and opposite C-O dipole moments get cancelled uniformly to yield an overall non-polar molecule with net zero dipole moment value, i.e., ethylene glycol (net µ = 0).

Difference between polar and nonpolar?

FAQ

Summary

• Ethylene glycol is a non-polar molecule.
• It is the simplest dihydric alcohol, i.e., ethan-1,2-diol.
• The four main types of covalent chemical bonds present in ethylene glycol are C-C, C-H, C-O, and O-H.
• The C-C and C-H bonds are non-polar with negligible dipole moment values.
• The C-O and O-H bonds are strongly polar due to the high electronegativity differences between bonded atoms, i.e., 0.89 units and 1.24 units.
• The strong C-O and O-H dipole moments get canceled equally in opposite directions in the bent, angular, or V-shape of the molecule w.r.t each O-atom, yielding a non-polar Ethylene glycol molecule overall (net µ = 0).

References

1. Britannica. ‘Ethylene glycol’’. https://www.britannica.com/science/ethylene-glycol.
2. toppr. ‘Ethylene Glycol’’. https://www.toppr.com/guides.chemistry/chemical-reactions-and-equations/ethylene-glycol/
3. Wikipedia. The Free Encyclopedia. ‘Ethylene glycol’’. https://en.wikipedia.org/wiki/Ethylene_glycol
4. Simon Cotton. University of Birmingham. Molecule of the month. June 2018. ‘Ethylene Glycol (Ethan-1,2-Diol). www.chm.bris.ac.uk/motm/ethylene-glycol/glycolh.htm