41,353 views
35 votes
35 votes
Why benzene is called aromatic compound according to Huckel’s rule?​

User Piyush Chauhan
by
3.1k points

2 Answers

17 votes
17 votes

Answer:

It obeys rule

Step-by-step explanation:

Benzene is an aromatic hydrocarbon because it obeys Hückel's rule.

User Yoohoo
by
2.5k points
22 votes
22 votes

Answer:

Benzene is an aromatic compound because it is planar, cyclic, conjugated and has (4n +2) π electrons, where n= 1.

Step-by-step explanation:

Hückel's rule states that a molecule is aromatic if it is planar, cyclic, conjugated and has (4n +2) π electrons.

Breaking it down into 4 requirements, we have:

  1. planar
  2. cyclic
  3. conjugated
  4. (4n +2) π electrons

Let's look at each of the 4 parts!

What is planar?

Planar refers to the molecule being flat. When an atom is sp² hybridized, it is in a trigonal planar geometry. Let's look at the benzene molecule. Each carbon is attached to 3 other atoms, namely 2 carbons and 1 hydrogen. This means that each carbon is sp² hybridized, making the molecule planar.

Cyclic

This refers to the molecule having a closed ring shape. Benzene fulfils this criteria.

Conjugation

This refers to a system of connected p orbitals, which allows π electrons to delocalize. Each carbon atom in benzene has one 2p orbital, making benzene a conjugated molecule. For molecules that are not cations or anions, conjugated molecules are those that have alternating single and double bonds.

(4n +2) π electrons

n is an integer (e.g. 0, 1, 2, 3) thus for the molecule to be aromatic, it can have 2, 6 or 10 π electrons (non exhaustive). This separates an aromatic compound from an antiaromatic compound, which has 4n π electrons instead.

Benzene has 6 π electrons, so n has a value of 1 in this case.

Thus, benzene is an aromatic compound because it is planar, cyclic, conjugated and has (4n +2) π electrons.

_______

Counting the number of π electrons

Let's look at π bonds!

A double bond is made up of 1 σ bond and 1 π bond, and each π bond is made up of 2 π electrons. We do not count the number of single bonds since single bonds are made up of 1 σ bond only.

There are 3 double bonds in the Kekulé structure of benzene. Thus, the number of π electrons in benzene is 3(2)= 6.

Attached diagrams

  1. Kekulé structure of benzene
  2. sp² hybridized carbon
  3. 6 p orbitals in benzene ring
Why benzene is called aromatic compound according to Huckel’s rule?​-example-1
Why benzene is called aromatic compound according to Huckel’s rule?​-example-2
Why benzene is called aromatic compound according to Huckel’s rule?​-example-3
User Vobject
by
2.9k points