Are hydrocarbons polar or nonpolar? – (Polarity of hydrocarbons)

Hydrocarbons are the simplest organic compounds. As their name implies, a hydrocarbon is composed of carbon and hydrogen atoms only.

These are further categorized into aliphatic and aromatic hydrocarbons, possessing acyclic and cyclic structures, respectively. But in either case, the presence of only two types of elemental atoms (C and H) stays unchanged.  

So are these organic compounds polar or non-polar in nature, and why? Let’s find out through this article.

Are hydrocarbons polar or non-polar? And why?

Hydrocarbons are typically non-polar in nature.

Carbon (C) and hydrogen (H) are two different types of elemental atoms present in a hydrocarbon molecule.

Therefore, there are two main types of bonds present in a hydrocarbon chain, i.e., C-C (or C=C) and C-H single covalent bonds.

A C-C (or C=C) is purely non-polar, comprising identical carbon atoms with no or zero electronegativity differences.

A C-H bond is very weakly polar (almost non-polar as per Pauling’s electronegativity scale) due to an electronegativity difference of just 0.35 units between a carbon and a hydrogen atom.

The small C-H dipole moments get canceled uniformly in the symmetrical tetrahedral (or trigonal planar) shape of a straight-chain hydrocarbon molecule w.r.t each C-atom.

The electron cloud stays uniformly distributed to yield an overall non-polar hydrocarbon molecule (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:

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

Now let us discuss how the above three factors make hydrocarbons non-polar molecules overall.

Factors affecting the polarity of hydrocarbons

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.

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.  

The general structure of a straight-chain, aliphatic hydrocarbon molecule consists of C-C (C=C or C ≡ C) and C-H single covalent bonds.

All the C-atoms achieve a stable octet electronic configuration, while the H-atoms complete their duplets via chemical bonding in hydrocarbons.

All 4 valence electrons on all the C-atoms get consumed in covalent bonding; thus, there is no lone pair of electrons on any of the carbon atoms present in the hydrocarbon chain.

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.

In a hydrocarbon molecule, the carbon-carbon bond is purely non-polar as it is formed between two identical C-atoms having a zero or no electronegativity difference. Thus, the shared electron cloud stays uniformly spread between the two C-atoms.

In contrast, in a C-H bond, an 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. So, the C-H bond is also non-polar, as per Pauling’s electronegativity scale.

However, as it is formed between two dissimilar atoms, thus the C-atom being slightly more electronegativity, attracts the C-H electron cloud to a minutely greater extent than the corresponding H-atom.

The C-atoms thus gain a partial negative charge (δ^–) while the H-atoms obtain slight positive charges (δ⁺) in all C-H bonds in the hydrocarbon chain, 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 the hydrocarbon chain, the C-C bonds possess no dipole moment, while the small dipole moment of each C-H bond points from H^δ+ to C^δ-.

Molecular geometry

According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, the hydrocarbon 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, including 2 H-atoms and 2 other C-atoms, single-covalently bonded at the sides, and there is no lone pair of electrons on the central C-atom.

Thus, the shape of a hydrocarbon molecule w.r.t each C-atom is tetrahedral.

For a C-C=C bonded carbon atom, the shape is trigonal planar w.r.t the central C-atom as it is covalently bonded to two other C-atoms and 1 H-atom only.

In either case, the small C-H dipole moments get cancelled equally to yield an overall non-polar hydrocarbon molecule possessing a symmetrical electron cloud distribution (net µ = 0).

Similarly, cyclic non-aromatic hydrocarbons such as cyclohexane (C₆H₁₂), as well as aromatic hydrocarbons such as benzene (C₆H₆) and methylbenzene or toluene (C₇H₈), are all non-polar molecules with a uniformly distributed electron cloud and zero resultant dipole moment.

FAQ

Summary

• Hydrocarbons are non-polar in nature.
• Hydrocarbons are the simplest organic compounds containing carbon (C) and hydrogen (H) atoms only.
• A C-C or C=C bond is purely non-polar, having zero or no electronegativity difference between identical carbon atoms.
• A C-H bond is very weakly polar, possessing an electronegativity difference of 0.35 units between the bonded atoms.
• The small C-H dipole moments get canceled equally due to the symmetrical molecular shape to yield an overall non-polar hydrocarbon chain, ring, or aromatic compound (net µ = 0).

References

1. ‘Why are hydrocarbons insoluble in water?’’. (June, 30^th, 2022). https://psiberg.com/why-are-hydrocarbons-insoluble-in-water/
2. Toppr Answr. ‘Why are the molecules of hydrocarbons non-polar?’’. https://www.toppr.com/ask/question/why-are-the-molecules-of-hydrocarbons-nonpolar/
3. Elsevier. Science Direct. ‘Chapter One: The Preparation and Properties of Heteroarylazulenes and Hetero-Fused Azulenes’’. Shoji et.al., (2018). https://www.sciencedirect.com/science/article/abs/pii/S006527251830014X
4. ‘Hydrocarbons’’. David W. Ball and Jessie A. Key. https://ecampusontario.pressbooks.pub/introductorychemistry/chapter/hydrocarbon-2/