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Hybridization of Benzene (C₆H₆)

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What is Benzene?

  • We know that benzene is an organic chemical compound with the chemical formula C6H6

  • The benzene molecule comprises six carbon atoms joined in a ring with one hydrogen atom attached to each. Since it contains both Carbon and Hydrogen atoms, we classify benzene as a hydrocarbon.

  • It has an sp2 hybridization. 

  • During the hybridization process, each carbon atom forms different bonds with two other similar carbon atoms instead of just one.

  • Benzene has a Trigonal Planar geometry having a bond angle of 120°.

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  • Below is the structure of benzene:

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Benzene is the first aromatic compound which we study in organic chemistry. It has the following properties:

Properties of Benzene

S.No.

Property

Value/Name

1.

Molar Mass

78.11 g/mol

2.

Boiling Point

80.1°C

3.

IUPAC (International Union for Pure and Applied Chemistry) ID 

Benzene

4.

Density

876 kg/m³

5.

Soluble in

Alcohol, Acetone, Acetic Acid, Chloroform


Other Properties:

  • An electron cloud is present below and above the benzene ring.

  • Benzene allows addition and substitution reactions to occur on it easily.


Orbital Structure of Benzene

  • Benzene has a planar hexagonal structure (because of hexagonal shape) in which all the C-atoms are sp2 hybridized.

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  • If we look at its structure, all the Carbon-Carbon bonds are equal in length, while the remaining cyclic array of six p-orbitals (one on each C) overlap to generate six molecular orbitals, three bonding, and three anti-bonding.

  • To understand how this happens, let’s understand the hybridization of benzene.


Benzene Hybridization

  • To understand the hybridization of benzene, we must know its electronic configuration.    

  • The electronic configuration of C6H6. is 1s2 2s2 2px1 2py1.

  • We know that the electronic configuration Of 6C-atom is 1s2 2s2 2p2.

1⇂

1⇂

1

1


1s         2s      |-------2p ------------|

Here, 2p has two unpaired electrons. 

  • Ground state: Now, one electron from 2s migrates to the p-orbital, which is the ground state.

1⇂

1

1

1

1

1s         2s      |-------2p ------------|

In this state, 2p has three unpaired electrons.

  • Now, taking one s-orbital, and 2 p-orbitals, we get four sp2 orbitals with the same energy. 

1

1

1

  sp2          sp2                  sp2

The third p-orbital of 2p remains unhybridized.

  • These hybrid sp2  orbitals arrange in such a way around the central C-atom, shown below:

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The angle formed is 120°, and the shape is Trigonal Planar.

  • The unhybridized p-orbital (of 2p) stands below (shown in grey color) and above the central C-atom, as shown below:

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  • The electron standing below and above the C-atom shows 100% p-character. That’s why it has a dumbbell-shaped orbital.

  • sp2  has 33% s-character and 66% p-character. That’s why the shape is elongated and flattened, as shown in diagram (a). These elongated sp2 orbitals will have a partial s-character and p-character.                            

  • Now, let’s say, we have six C-atoms similar to that of the diagram (a) having sp2 orbitals and 6 H-atoms.

  • Since each C-atom requires 4 H-atoms, but for six C-atoms we require 24 H-atoms which can’t be possible.

  • Now, each C-atom forms a bond with H-atom, as shown below:

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  • In diagram (a), all the green orbitals are sp2 hybridized, where each  sp2 orbital forms a bond with the H-atom.

  • The unhybridized p-orbital stands perpendicularly above and below each C-atom, as shown in diagram (b).

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  • In diagram (b), we can see the following things:

  1.  p-orbital of C-atom forms a σ-bond with its neighboring C-atoms.

  2. The third orbital forms a σ-bond with H-atom, and 

  3. The unhybridized orbital stands perpendicularly to the central C-atom.

  • We can see that central C-atom has formed three σ-bonds, but it should make four bonds, so for this, an unhybridized p-orbital of this C-atom overlaps with p-orbital of its neighboring C-atom, and forms a double bond (shown with parallel lines), as shown below:

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  • So, around the central C-atom, we get a σ-bond (formed between C-atom and the unhybridized p-orbital) and a π-bond between two p-orbitals. This means that a double bond forms between the two C-atoms.

  • As we can see, two orbitals already formed a π-bond, so none of these can form a further bond with other p-orbitals. So what happens here is, the remaining four p-orbitals form the same arrangement, i.e., double bond.

  • This is how three double bond forms between three C-atoms and the other three σ-bonds were already formed with H-atoms. We get the Benzene molecule, as shown below:

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  • If we look carefully, at this structure, the double bond formation between two p-orbitals can occur in two ways, as shown in diagram (c).  This means we can get an alternate double-bond structure for Benzene.

  • So, in this phenomenon, p-orbital can form a double bond with any of its neighboring p-orbitals. This means that the location of bond formation isn’t fixed or the electron isn’t fixed. We call it a delocalized electron, while the electron in the σ-bond is fixed or localized.

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We know this structure as Kekule’s explanation of the structure of benzene.

  • So, we can see that the molecular structure (position of atoms) remains the same, but only the double bond formation between p-orbitals varies, resulting in the formation of two or more bonds. This phenomenon is called resonance.

FAQ (Frequently Asked Questions)

Q1: Write the IUPAC Name of Benzene.

Ans: Look at two structures of benzene to determine the IUPAC name:

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  1. The structure of benzene is cyclic, so the primary prefix is cyclo.  

  2. The structure is hexagonal so the root word becomes hex, and because of three double bonds, the root word changes to Hexa and the suffix as triene.

  3. If we look at the left benzene structure, these three double bonds are at 1, 3, and 5th C-atoms. So, the IUPAC name becomes Cyclohexa-1,3,5-triene.

  4. Similarly, the structure on the right, the double bonds are at 2, 4, and 6th carbon atoms, so the IUPAC name is Cyclohexa-2,4,6-triene.

Q2: What Contains Benzene?

Ans: The products containing benzene are listed below:


S.No.

Product’s Name

1.

Paints and varnish removers

2.

Gasoline 

3.

Thinners

4.

Furniture Wax

5.

Detergents

6.

Industrial cleaning & degreasing formulations

Q3: Write the Names of Industries that Use Benzene.

Ans: The industries that use benzene are:


S.No.

Industries Name

1.

Petrochemical manufacturing & Petroleum refining

2.

Coke and coal chemical manufacturing

3.

Shoe manufacturing

4.

Plastics, rubber and rubber tire manufacturing

5.

Gasoline storage, shipment and retail operations

Q4: Write Examples of Simple Benzene Derivatives.

Ans: The examples of simple benzene derivatives are:

       

Benzene Derivatives

Chemical Formula

Phenol

PhOH

Toluene

PhMe

Aniline

PhNH₂