Benzene is an organic chemical compound composed of six carbon atoms joined to each other forming a planar ring, and one hydrogen atom attached to each. The molecular formula for benzene is C6H6. Since it is made of only carbon and hydrogen atoms, benzene falls under the category of hydrocarbons. It is a clear, colorless, volatile, liquid aromatic hydrocarbon. Benzene is an innate part of crude oil, cigarette smoke, and gasoline.
So, Is Benzene Polar or nonpolar? Although C-H bonds are slightly polar, benzene is a nonpolar compound. This is because, benzene is a symmetric and planar molecule having a ring-like structure, so there are equal and opposite dipoles that cancel out each other. As a result, the net dipole moment of the benzene molecule is 0D.
Structure and Formation of Benzene
Benzene was discovered by one of the famous scientist named Michael Faraday in 1825 in the illuminating gas.
In 1834, the German chemist Eilhardt Mitscherlich heated benzoic acid with lime [Ca(OH) 2 ], resulting in the production of benzene. Then, German chemist A.W. von Hofmann isolated benzene from coal tar in 1845.
Structure of Benzene
The benzene ring is formed by a planar arrangement of carbon and hydrogen atoms. The molecular formula for benzene is C6H6, thereby signifying six C-H bonds. It has an average mass of about 78g.
The benzene structure is a six-carbon ring, with three double bonds and one hydrogen atom attached to each carbon. This structure is represented in a number of ways in organic chemistry.
Below is the image of three different ways of representing the Benzene molecule
Each double bond of the molecule is separated by a single bond in between i.e. there are alternate double and single carbon bonds.
Thus, benzene is said to have conjugated double bonds. The formulas and structures of benzene establish it as a purely aromatic hydrocarbon.
One of the benzene structures has a ring in the center of the hexagon, the ring depicts an aromatic compound.
What is Polarity?
A bond is known to be polar when the centers of positive and negative charges of the molecule do not coincide with each other.
This means that one of the two atoms forming the bond is somewhat more electronegative than the other. This leads to a shift in the charge cloud towards the more negative atom, resulting in a net dipole across the molecule.
The existence of this dipole indicates that the concerned molecule is polar.
Polar molecules have:
- An electronegative difference greater than 0.4
- High melting and boiling points
- Low vapor pressure
- A net dipole moment (> 0D)
- Charge build-up at the poles (positive and negative charges)
- No Symmetricity
- Good solubility in polar molecules and insoluble/sparingly soluble in non-polar molecules.
- Examples of polar molecules are SCl2, HBr, etc. Check out the polarity of SCl2.
On the contrary, non-polar molecules have:
- Electronegative difference less than 0.4
- Low melting and boiling points
- Comparatively higher vapor pressure
- No net dipole moment (net dipole = 0 D)
- No charge accumulation at poles
- No unshared electrons
- Examples of non-polar molecules are CS2, Cl2, etc. Check out the non-polarity of CS2.
Why is Benzene a nonpolar molecule?
As discussed, the benzene molecule is made up of 6 carbon and 6 hydrogen atoms connected in a ring-like structure in a planar arrangement.
There is a little difference between the electronegativity of carbon and hydrogen atoms. And, each dipole is directed towards hydrogen atom.
Due to the ring-like symmetric structure, all the dipoles are in their respective direction that cancels out each other making the molecule a nonpolar in nature.
Therefore, it is generally seen that nonpolar molecules are geometrically symmetric in shape.
Polarity of Benzene
Around the molecular ring of benzene, there are six carbon-hydrogen bonds spaced out in equal proportion.
There exists some electronegative difference between each pair of carbon and hydrogen atoms that results in some value of dipole moment in their respective directions.
Let us discuss the below parameters that contribute to check whether benzene is polar or nonpolar.
Electronegativity of carbon = 2.55
Electronegativity of hydrogen = 2.20.
The difference in electronegativity = |2.55-2.20| = 0.35 This electronegative difference is almost equal to 0.4 (condition for polar bonds), yet the overall benzene molecule is a non-polar molecule.
And, all the six bonds are in different directions in such a way that all dipoles cancel out each other leaving zero dipole moment of the overall molecule.
This is because benzene has a symmetrical molecular structure.
Symmetricity of Benzene
The Lewis structure of benzene clearly displays the symmetry of the molecule. Two forms of resonance exist in the benzene ring. First, due to alternating/conjugated double bonds.
And second, by virtue of the entire planar structure comprising of the six C-H bonds. The single-double bonds can switch places while being spaced out equidistantly.
The symmetricity of benzene has been substantiated using X-ray Crystallography, which has helped in establishing the fact that the bond lengths between the carbon atoms as well as the angle between the bonds is constant and does not change from one carbon atom to another.
Moreover, the intermolecular forces of hydrogen pull equally in all directions. Consequently, all the forces cancel out each other and there is no net force in one particular direction.
The Dipole Moment of Benzene
- In benzene, there are either carbon-carbon bonds or carbon-hydrogen bonds.
- The C-C bond has a net dipole of 0D because there is no electronegativity difference between the two carbon atoms.
- However, there is some difference in the electronegativity of carbon and hydrogen. This results in a small dipole moment across each C-H bond (close to 0.4D).
- As depicted in the figure, for the direction of each dipole there is a corresponding dipole that is equal in magnitude but opposite to cancel out each other.
- So, the dipole moment cancels out each other and the net dipole moment of benzene = 0D.
Formation of Benzene
There are several types of methods that are used for the formation of benzene.
1. The cyclic polymerization of ethyne (C 2 H 2 ) leads to the formation of the benzene molecule. During the polymerization process, ethyne passes through a red hot iron tube at 873K. The result so obtained is benzene.
2. Decarboxylation reaction performed on aromatic acids (ex: benzoic acid) can lead to the formation of benzene (along with a by-product).
3. The reduction of phenol also produces benzene. The phenol vapors are passed over the heated zinc (Zn) dust. The by-product thus formed in ZnO (zinc oxide).
Properties of Benzene
- Molecular weight = 78.11g
- IUPAC name = Cyclohexa-1,3,5-triene
- Density = 0.87 gcm -3 . Benzene is lighter than water.
- Melting point = 5.5˚C (non-polar molecules have a low melting point)
- Boiling point = 80.1˚C (b.p. is lower than water but not very low). It is said to have a moderate boiling point.
- Benzene is soluble in organic solvents but immiscible in water.
- It is a colorless liquid with an aromatic odor.
- Benzene is a highly inflammable compound and burns with a sooty flame.
- Printing: The products used for the cleaning and maintenance of printing equipment contain benzene.
- Painting: It is a constituent in many mixtures used for painting such as top and base coats, lacquers, and spray paints.
- Manufacturing: Benzene is used in the manufacturing of tires and rubber. Solvents containing benzene are used at various steps of manufacturing processes in general.
- Petroleum and Oil: In the petroleum and oil refineries, products such as gasoline, crude oil, kerosene, etc. are produced. These products contain benzene.
- Chemicals: Chemicals such as detergents, dyes, insecticides, pesticides and herbicides, and plastics such as nylon, styrene, etc. are manufactured using benzene.
Health effects of Benzene
Continuous and prolonged exposure to benzene can cause various nuisances for the human body, such as permanent damage to the DNA or benzene poisoning. The symptoms include:
Long term exposure can also lead to the development of cancers and even death. Hence, people who work in industries where benzene is frequently used, are at high health risks and must always take the prescribed safeguards.
The benzene molecule consists of 6 pairs of carbon and hydrogen atoms with each carbon connected in a ring-like structure. The C-H bond results in a little dipole moment due to the difference in their electronegativity.
But all the dipoles are in such directions that they cancel out each other. As a result, the benzene molecule becomes nonpolar.