Hybridization of Benzene (C₆H₆)

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

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.

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.

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. 

  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.