Is aspirin polar or nonpolar? – (Polarity of aspirin)

Aspirin is a non-steroidal anti-inflammatory drug (NSAID) used to relieve pain, fever and inflammation. Chemically, it is acetylsalicylic acid.

The IUPAC name for acetylsalicylic acid (molar mass 180.16 g/mol) is 2-acetoxy benzoic acid, represented by the molecular formula C₉H₈O₄. It exists as an odorless, colorless, crystalline solid at r.t.p.

In this article, you will find everything you need to know about the polarity of aspirin (acetylsalicylic acid). 

So, continue reading to find out whether aspirin is polar or non-polar.

Is aspirin polar or non-polar?

Aspirin, chemically known as acetylsalicylic acid, is polar in nature.

It consists of a phenyl ring containing an ester (COO) and a carboxylic acid (COOH) functional group.

The four main types of chemical covalent bonds present in acetylsalicylic acid are C-C (or C=C), C-H, C-O (or C=O), and O-H bonds.

A carbon-carbon bond formed between two identical C-atoms is purely non-polar as zero or no electronegativity difference exists between the bonded atoms.

A C-H bond is weakly polar, having an electronegativity difference of only 0.35 units between the bonded atoms.

Contrarily, the C-O and O-H bonds are strongly polar as per the electronegativity differences of 0.89 units and 1.24 units between the bonded atoms, respectively.

The strong C-O and O-H dipole moments stay uncancelled, which leads to an asymmetric electron cloud distribution and, thus, a 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:

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

Now let’s explore in detail how the above three factors contribute to making aspirin a polar molecule definitely.

Factors affecting the polarity of aspirin

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.

Like all organic molecules, the three different types of elemental atoms present in acetylsalicylic acid 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. 

The Lewis dot structure of acetylsalicylic acid shows that a carboxylic acid (COOH) functional group is bonded to C-1 of a phenyl ring that also carries an ester (COO) functional group at C-2. Thus, the name 2-acetoxy benzoic acid is given to this molecule.

In aspirin, all 4 valence electrons of each C-atom get consumed in covalent bonding; therefore, there is no lone pair on any one C-atom in this structure.

Contrarily, all 4 O-atoms carry 2 lone pairs, respectively. This leads to distortion in the molecular shape of aspirin w.r.t each oxygen atom.

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 and C=O bonds are strongly polar.

Similarly, the O-H bond is 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 the structure of aspirin, the strong dipole moments of the C-O (or C=O) bonds point from C^δ+ to O^δ– while that of the O-H bond points from H^δ+ to O^δ–.

Refer to the figure drawn below.

Molecular geometry

As per the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, aspirin is an AX₃-type molecule w.r.t each C-atom (except the C-atom in the terminal methyl CH₃ group).

To one carbon atom at the center (A), three bond pairs (X) are attached, and there are no lone pairs of electrons (E) on the central C-atom. Thus, the molecular shape of aspirin w.r.t each C-atom is trigonal planar.

However, all the oxygen atoms possess lone pairs of electrons, which leads to lone pair-lone pair and lone pair-bond pair electronic repulsions. Hence, the shape of the molecule w.r.t each O-atom is bent, angular or V-shaped.

The strong C-O, C=O and O-H dipole moments stay uncancelled due to this bent shape. The charged electron cloud stays asymmetrically distributed over the molecule.

Consequently, aspirin has a resultant dipole moment greater than zero and it is overall polar.

Difference between polar and nonpolar?

FAQ

Summary

• Aspirin is polar in nature.
• The chemical name for aspirin is acetylsalicylic acid. It consists of a polar carboxylic acid (COOH) and a polar ester (COO) functional group bonded to a non-polar phenyl ring.
• The C-C and C=C bonds present in the phenyl rings are purely non-polar.
• The C-H bonds are slightly polar, almost non-polar, as per Pauling’s electronegativity scale.
• The C-O, C=O and O-H bonds present outside the phenyl ring are strongly polar as per the high electronegativity differences of 0.89 units and 1.24 units between the bonded atoms.
• The strong C-O, C=O and O-H dipole moments stay uncancelled due to the bent shape of the molecule w.r.t each oxygen atom, yielding a strongly polar molecule overall (net µ > 0), i.e., aspirin.

References

• Study.com. ‘Why is aspirin more polar than salicylic acid? ‘’. https://homework.study.com/explanation/why-is-aspirin-more-polar-than-salicyclic-acid.html
• The Chemistry of Aspirin (acetylsalicylic acid). https://www.aspirin-foundation.com/history/chemistry.