Aromatic hydrocarbons are unsaturated hydrocarbons that exhibit ring structures having single and double bonds. Benzene is a highly flammable and volatile compound having a gasoline-like smell. In Oil-refining processes, benzene is found in crude oil as a side product. It is widely used as a solvent or as an intermediate for the synthesis of various chemicals in the chemical industry. It is also used in the preparation of dyes, plastics, detergents, pesticides, drugs, etc. The molar mass of benzene is 78.11 g/mol. It’s melting and boiling points are 5.53 °C and 80.1 °C respectively. The Flashpoint of benzene is less than 0°F, therefore it is less dense than water. Hence, slightly soluble in water. Now, let us focus on the properties such as structure, geometry, hybridization, and MO diagram of benzene.
Benzene Lewis Structure
The lewis structure is a demonstration of valence shell electrons in a molecule. It shows how individual electrons are arranged in a molecule. It is also known as an electron dot structure where each bond is shown as two dots between two atoms. Let us consider the Lewis structure of benzene step by step- Step 1– The first step is the determination of the total no. of valence electrons of each atom present in benzene (C6H6), i.e; carbon and a hydrogen atom. The no. of valence electrons in carbon – 4 The no. of valence electrons in hydrogen – 1 Total no. of valence electron in carbon – 6 X 4 = 24 Total no. of valence electron in hydrogen – 6 X 1 = 6 Step 2– Determination of total no. of valence electron present in benzene molecule which is as follows- Total no. of valence electrons in C6H6 = No. of valence electron in 6 carbon atoms + no. of valence electron in 6 hydrogen atom = 24 + 6 = 30 electrons Step 3– Determination of no. of electrons required by carbon and hydrogen atoms to complete their octet. Each carbon atom will require 4 more electrons whereas a hydrogen atom will require 1 more electron in its outermost shell to complete its octet. Therefore, each carbon atom will form a single bond with one hydrogen atom.
Now, all the hydrogen atoms are paired whereas each carbon atom will require 3 more electrons in its outermost shell. Step 4– Determination of no. of electrons required for benzene to form a stable configuration. 3 more electrons are required by each carbon atom to complete their octet, hence, benzene will require 18 electrons to form a stable configuration. The remaining valence electrons are placed in such a way so that octet of carbon will complete. In the structure given below, electrons involved in bond formation are represented by a dot. Therefore, 2 dot means 2 electrons, hence a single bond whereas 4 dot means 4 electrons, hence a double bond.
Step 5– Lewis structure of benzene Therefore, benzene comprised of six carbon atoms attached in a planar ring structure with alternate single and double bonds, and each carbon atom is attached to a hydrogen atom by a single bond.
Benzene Molecular Geometry
Valence Shell Electron Pair Repulsion Theory (VSEPR) theory is used to determine the molecular geometry of a molecule. According to this theory, the geometry and shape of the molecule depend upon minimizing the repulsion between valence shell electron pairs in a molecule. The shape and geometry of a molecule can be explained as follows: As we discussed in the lewis structure, that each carbon atom forms 3 bonds, 2 with neighboring carbon atoms and 1 with the hydrogen atom. Therefore, carbon has 3 bonding electron pair which corresponds to Trigonal planar geometry. Hence, the 3D structure of benzene will be as below.
The trigonal planar geometry of benzene corresponds to all C-C-C and H-C-C bond angles of 120° and C-C bond length of 139 pm. Now we will discuss hybridization in benzene molecules.
Benzene Hybridization
The concept of hybridization was proposed by Pauling and Slater. According to them, the mixing of different atomic orbitals having similar energies leads to a set of new orbitals known as “Hybrid orbitals”. The number of hybrid orbitals formed should be equal to the number of atomic orbitals mixed. The concept of hybridization will be better understood by valence bond theory (VBT). According to this theory, a chemical bond is formed between two atoms when incompletely filled atomic orbitals are overlapped. The hybridization of benzene will be explained as- The electronic configuration of carbon is 1s2 2s2 2p2 whereas the hydrogen atom has 1s1 in the ground state. On excitation, the electronic configuration of carbon atom becomes 1s2 2s1 2px1 2py1 2pz1 having 4 unpaired electrons in its valence shell. These unpaired electrons will participate in bond formation. In benzene, not all 4 carbon orbital are used for bond formation. Each carbon atom has 3σ bonds with 2 other neighboring carbon atoms and a hydrogen atom and 1π bond with one of the neighboring carbon atoms, therefore, each carbon atom is sp2 hybridized lying in a single plane at an angle of 120°. The remaining unhybridized 2pz-orbital will lie perpendicular to the plane of the hybridized orbital. This 2pz orbital has two lobes, one above the plane and one below the plane. This orbital will form sideways overlapping with 2pz orbital of adjacent carbon atoms which further leads to the formation of π-bond. The stability of the benzene molecule is due to the delocalization of π-electrons.
Benzene Molecular Orbital (MO) Diagram
A molecular orbital (MO) diagram explains the chemical bonding in molecules by energy level diagrams. They were proposed by Robert S. Mulliken and Friedrich Hund in 1928. The postulates of MO theory are as follows- By drawing molecular orbital diagrams of a molecule, one can predict the stability, bond order, and magnetic character of a molecule. Now, let us have a look at the MO diagram of benzene. The p-orbitals on each carbon atom could overlap to form six molecular orbitals, three bonding orbitals (ψ1 to ψ3) and three antibonding orbitals (ψ4 to ψ6) as shown in the figure. The bonding MO ψ1 is of lowest energy and most stable containing all six carbon atoms, i.e; it is delocalized. The two more bonding MO ψ2 and ψ3 are degenerate, i.e; have the same energy. They both lie above ψ1, i.e; they have more energy than ψ1. The antibonding orbitals ψ4 and ψ5 are degenerate lying above bonding orbitals ψ2 and ψ3. The remaining antibonding orbitals ψ6 lie above ψ4 and ψ5 and are of the highest energy. The stability of benzene implies that all electrons are present in bonding orbitals and all the π electrons are paired. These characteristics lead to the closed shell of delocalized π electrons.
Polarity of Benzene
Benzene is considered a nonpolar molecule. The reason behind this is its symmetric ring-like shape due to which the charge distribution on the overall charge remains neutral. Symmetric molecules always tend to be non-polar in nature. For detailed information, you must also go through the article written on the polarity of Benzene.
Conclusion
The characteristics such as lewis structure, molecular geometry, hybridization, and molecular orbital diagram of benzene molecule make it a special molecule. Its structure displays such properties of benzene which are different than other ring structures. The benzene ring is present in many fragrant oils, therefore, these compounds are called aromatic. Most of the available details about benzene are discussed herein in simple words. After this, one can easily go through any other concept related to benzene. This article covers all the basic properties of benzene. I hope at the end of this article you get the answers to your long searched queries. Please feel free to ask any questions related to benzene. Thank you for reading this article.
