Is benzoic acid polar or nonpolar? - (Polarity of benzoic acid)
Benzoic acid is represented by the chemical formula C6H5COOH. It is an aromatic carboxylic acid. The carboxylic acid (COOH) functional group is attached to the phenyl ring.
Benzoic acid (molar mass = 122.12 g/mol) is a white, crystalline solid, a sweet-smelling compound that is predominantly used as a preservative in food, cosmetics, and pharmaceutical products owing to its valuable antimicrobial properties.
In this article, we have answered a pertinent question regarding the chemical nature of benzoic acid, i.e., is benzoic acid polar or non-polar?
Is Benzoic acid polar or nonpolar?
Benzoic acid (C6H5COOH) is a polar molecule.
The polarity of benzoic acid is accredited to the presence of a polar carboxyl (COOH) functional group in it. The C-O, C=O and O-H bonds present in the COOH group are strongly polar.
A high electronegativity difference of 0.89 units is present between a C-atom and an O-atom in the C-O or C=O bond.
Conversely, an even higher electronegativity difference of 1.24 units is present between an O-atom and an H-atom in the O-H bond, respectively.
The covalent chemical bonds present in the phenyl ring, i.e., C-C, C=C, and C-H bonds, are either non-polar or very weakly polar. Therefore, the phenyl ring of benzoic acid is non-polar.
However, the strong dipole moments of individually polar bonds in the COOH group stay uncancelled due to the bent, angular, or V-shape of benzoic acid w.r.t the OH-bonded O-atom.
The charged electron cloud stays non-uniformly spread over the molecule. Hence, benzoic acid is a polar molecule with a net dipole moment value greater than zero.
Name of molecule
Benzoic acid
Bond type
Polar covalent (C-O , C=O and O-H)
Molecular geometry
Trigonal planar (w.r.t each C-atom)
Bent, angular or V-shaped (w.r.t OH-bonded O-atom)
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 benzoic acid a polar molecule.
Factors affecting the polarity of benzoic acid
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 benzoic 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.
Atom
Electronic configuration
Valence electrons
Hydrogen (1H)
1s1
1
Carbon (6C)
1s2 2s2 2p2
4
Oxygen (8O)
1s2 2s2 2p4
6
The benzoic acid structure (as shown below) displays a phenyl (C6H5) ring and a carboxylic acid (COOH) functional group as its two primary components.
There are 3 C-C, 3 C=C and 5 C-H single covalent bonds in the phenyl ring. Conversely, there is 1 C-C, 1 C=O, 1 C-O and 1 O-H bond in the COOH group bonded to the phenyl ring.
Each of the C-atoms and O-atoms has a complete octet, while all the H-atoms possess a complete duplet electronic configuration in this structure.
There are no lone pairs of electrons on any of the C-atoms, while both the O-atoms contain 2 lone pairs of electrons each, in the benzoic acid molecule.
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 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 the structure of benzoic acid, the strong dipole moments of the C-O (or C=O) bonds point from Cδ+ to Oδ–while that of theO-H bond points from Hδ+ to Oδ–.
Refer to the figure drawn below.
Molecular geometry
As per the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, benzoic acid is an AX3-type molecule w.r.t each C-atom.
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 benzoic acid w.r.t each C-atom is trigonal planar.
However, both the oxygen atoms possess 2 lone pairs of electrons, which leads to strong lone pair-lone pair and lone pair-bond pair electronic repulsions. Thus, the shape of the molecule w.r.t the O-H bonded O-atom in the COOH group 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 spread over the molecule.
Consequently, benzoic acid has a resultant dipole moment (net µ = 1.78 D) and is overall polar.
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), methanol (CH3OH), acetone (C3H6O), phenol (C6H6O), benzophenone (C13H10O), benzoic acid (C6H5COOH), etc.
Examples include oxygen (O2), nitrogen (N2), methane (CH4), n-hexane (C6H14), benzene (C6H6), etc.
What is benzoic acid’s molecular structure? Is it polar or non-polar?
Benzoic acid is an aromatic carboxylic acid (a weak organic acid). It comprises a polar carboxyl (COOH) functional group bonded to a non-polar phenyl ring (C6H5).
The carbonyl (C=O) and hydroxyl (OH) bonds present in the COOH group are strongly polar as per the high electronegativity differences of 0.89 units and 1.24 units between the covalently bonded atoms, respectively.
The strong O-H dipole moment does not get canceled with the C-O and C=O dipole moments due to the bent molecular shape of the COOH group in benzoic acid w.r.t the O-atom.
So, benzoic acid is overall polar(net µ > 0).
Is benzoic acid soluble in water (a polar solvent)?
Benzoic acid is soluble in hot water. However, it is only sparingly soluble in cold water. Benzoic acid molecules can develop intermolecular H-bonding using their hydroxyl (OH) functional groups.
Under normal temperature conditions, H2O molecules cannot break this strong intermolecular force of attraction to form new H-bonds.
However, on being provided heat, polar H2O molecules gain enough kinetic energy to do so and get benzoic acid fully dissolved.
Is phenol more polar than benzoic acid?
No. Benzoic acid is more polar than phenol. Phenol comprises a single polar OH group directly attached to the phenyl ring.
In contrast, benzoic acid consists of a COOH group, i.e., a polar carbonyl (C=O) bond, in addition to the O-H bond.
The greater the number of polar covalent bonds present in a molecule, the higher the molecular polarity.
Which is more polar: benzoic acid or benzaldehyde?
Benzoic acid is more polar than benzaldehyde.
Benzaldehyde comprises a polar aldehyde (CHO) functional group attached to the phenyl ring.
The CHO group possesses a polar carbonyl (C=O) group having an electronegativity difference of 0.89 units between the bonded atoms.
The C=O dipole moment of the CHO group in benzaldehyde stays uncancelled.
So, benzaldehyde is overall polar. However, it is less polar than benzoic acid, which contains a strongly polar O-H bond in addition to the C=O bond.
Is benzoic acid more or less polar than aliphatic carboxylic acids?
Aliphatic carboxylic acids are acyclic carboxylic acids in which the COOH group is attached to a hydrocarbon chain.
Their examples include acetic acid (CH3COOH), butanoic acid (CH3CH2CH2COOH), 3-methyl-butanoic acid ((CH3)2CHCH2COOH), etc.
Benzoic acid is more polar than aliphatic carboxylic acids because it possesses a phenyl ring containing a delocalized pi-electron cloud as opposed to the purely non-polar hydrocarbon chains present in the latter.
The small C-H dipole moments get canceled due to the trigonal planar shape of C6H6 w.r.t each C-atom in the hexagonal ring arrangement of benzene to yield an overall non-polar molecule (net µ = 0).
Contrarily, benzoic acid is polar as it comprises a polar COOH group bonded to the non-polar phenyl ring.
Summary
Benzoic acid (C6H5COOH) is a polar molecule.
Benzoic acid consists of a polar carboxyl (COOH) functional group covalently bonded to a non-polar phenyl (C6H5) ring.
The C-O, C=O, and hydroxyl (O-H) bonds present in the COOH group of benzoic acid are strongly polar.
The dipole moments of individually polar bonds stay uncancelled due to the asymmetric bent shape of C6H5COOH w.r.t the O-atom in the COOH group, which leads to an overall polar benzoic acid molecule (net µ > 0).
References
PubChem. National Library of Medicine. Compound Summary. ‘Benzoic acid’’. https://pubchem.ncbi.nlm.nih.gov/compound/Benzoic-Acid
Vishal Goyal is the founder of Topblogtenz, a comprehensive resource for students seeking guidance and support in their chemistry studies. He holds a degree in B.Tech (Chemical Engineering) and has four years of experience as a chemistry tutor. The team at Topblogtenz includes experts like experienced researchers, professors, and educators, with the goal of making complex subjects like chemistry accessible and understandable for all. A passion for sharing knowledge and a love for chemistry and science drives the team behind the website. Let's connect through LinkedIn: https://www.linkedin.com/in/vishal-goyal-2926a122b/
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