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

Benzophenone is a diphenyl ketone, represented by the chemical formula C₁₃H₁₀O. The IUPAC name for benzophenone is diphenylmethanone. It is frequently used in the chemical industry as a flavoring agent, a fragrance enhancer, a UV-light protectant in cosmetics, etc.

There is a lot of confusing information about the polarity of benzophenone on the web. Hence, in this article, we have cleared all the questions you may have about benzophenone polarity.

So, is benzophenone polar or non-polar, and why? Let’s find out.

Is Benzophenone polar or non-polar?

Benzophenone (C₁₃H₁₀O) is a polar molecule overall. It consists of a polar C=O bond lying in between two non-polar benzene rings, one on each side.

C-C, C=C, C-H and C=O are the four main types of chemical covalent bonds present in a benzophenone molecule.

The C-C or C=C bond is purely non-polar as zero, or no electronegativity difference is present between two identical carbon atoms.

A C-H bond is slightly polar (almost non-polar) as per Pauling’s electronegativity scale, as an electronegativity difference of only 0.35 units is present between a carbon and a hydrogen atom.

Contrarily, a high electronegativity difference of 0.89 units is present between the covalently bonded carbon and oxygen atoms in the C=O bond of benzophenone. Thus, the C=O bond (also called the carbonyl functional group) is strongly polar.

Oxygen being highly electronegative, attracts the nearby C-C and C-H electrons as well, in addition to attracting the C=O bonded electrons.

The charged electron cloud stays non-uniformly spread over benzophenone overall. Thus, benzophenone is a polar 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 benzophenone a polar molecule overall.

Factors affecting the polarity of benzophenone 

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 different types of atoms present in a benzophenone molecule 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 benzophenone drawn below shows two benzene rings bridged with the help of a carbonyl (C=O) functional group.

All 4 valence electrons of each C-atom get involved in covalent bonding; thus, there is no lone pair on any one C-atom in benzophenone. However, the C=O bonded oxygen atom carries 2 lone pairs of electrons respectively.

In this way, all the C-atoms and the O-atom achieve a stable, octet electronic configuration via chemical bonding and lone pairs in the benzophenone molecule while each H-atom completes its duplet.

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.

However, as the C-H bond is formed between two dissimilar atoms, therefore a slight polarity effect may exist.  

Oxygen is a strongly electronegative element. An electronegativity difference of 0.89 units is present between a carbon and an oxygen (E.N = 3.44) atom. Therefore, the C=O bond present in benzophenone is strongly polar.

Oxygen being highly electronegative, attracts not only the C=O bonded electrons but also the C-C and C-H bonded electrons to some extent.

Oppositely charged poles develop in the benzophenone molecule as the O-atom gains a partial negative charge (δ^–) while all the C and H-atoms obtain partial positive charges (δ⁺).

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 charges 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 benzophenone, the C-C and C=C bonds have no dipole moment values, while the small dipole moment of each C-H bond points from H^δ++ to C^δ+.

In contrast, the strong C=O dipole moment points from C^δ+ to O^δ- as shown below.

Molecular geometry

According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, benzophenone is an AX₃-type molecule.

 To one C-atom at the center (A), three bond pairs (X) are attached, i.e., 2 C-atoms and an H-atom or 2 C-atoms and 1 O-atom, while the central C-atom has no lone pairs of electrons (E). Thus, the shape of benzophenone w.r.t each C-atom is trigonal planar, identical to its ideal electron pair geometry.

It is due to this symmetrical trigonal planar shape and hexagonal arrangement of C-atoms in the benzene rings that the C-H dipole moments get canceled equally.

The electron cloud stays uniformly distributed; thus, the two sideways benzene rings are purely non-polar in benzophenone.

However, the strong C=O dipole moment stays uncancelled, which induces a net dipole moment in the benzophenone molecule. So, it is overall a polar molecule (net µ > 0).

Difference between polar and nonpolar?

FAQ

Summary

• Benzophenone is polar in nature.
• Benzophenone consists of two benzene rings with a carbonyl (C=O) functional group present as a bridge in between.
• The C-C and C=C bonds present in the benzene rings are purely non-polar, while the C-H bonds are very weakly polar.
• The C=O bond is strongly polar with an electronegativity difference of 0.89 units.
• The small C-H dipole moments get canceled equally due to the symmetrical trigonal planar shape of benzophenone w.r.t each C-atom in the hexagonal ring arrangement.
• The strong C=O dipole moment stays uncancelled to yield an overall polar benzophenone molecule (net µ >0).

References

1. Wiley Online Library. ‘Benzophenone’’. Sujan K. Sarkar and Manabu Abe (Feb,30^th, 2021). https://onlinelibrary.wiley.com/doi/abs/10.1002/047084289X.rn02302.
2. National Library of Medicine. PubChem. ‘Benzophenone’’. https://pubchem.ncbi.nlm.nih.gov/compound/Benzophenone.
3. Study.com. ‘Is benzophenone soluble in hexane?’’. https://homework.study.com/explanation/is-benzophenone-soluble-in-hexane.html.