This is the notes for aromaticity and benzene.
Aromaticity
Aromaticity is a property of cyclic (ring-shaped), planar (flat) structures with a ring of resonance bonds that gives increased stability compared to other geometric arrangements with the same set of atoms. Aromatic molecules are very stable, and do not break apart easily to react with other substances.
Conditions/Characteristics of aromatic compounds:
There are four conditions, a molecule must meet in order to be aromatic. If any of these conditions are violated, no aromaticity is possible.
- The contributing atoms must be arranged in one or more rings (i.e., must be cyclic).
- There must be a delocalized conjugated π system, most commonly an arrangement of alternating single and double bonds.
- The molecule must have a number of π delocalized electrons. That is, (4n + 2) π-electrons, where n = 0, 1, 2, 3, and so on. This is known as Huckel's rule. 4.
- The molecule must have coplanar structure, with all the contributing atoms in the same plane (flat).
Huckel’s rule states that, “a conjugated cyclic ring system which consists of delocalized (4n+2) π-electrons is considered to be aromatic (where n= 0,1,2,3,…)”.
If n=0, then (4n+2)= 2 π-electrons
If n=1, then (4n+2)= 6 π-electrons
If n=2, then (4n+2)= 10 π-electrons
If n=3, then (4n+2)= 14 π-electrons and so on….
In other words, if a conjugated cyclic ring system consists of 2,6,10,14,…. delocalized π-electrons , the molecule will show aromatic character.
The numbers 2,6,10,14,….. are also called Magic numbers or Huckel’s numbers.
• Kekule was the first to suggest a sensible structure for benzene. The carbons are arranged in a hexagon, and he suggested alternating double and single bonds between them. Each carbon atom has a hydrogen attached to it.
Problems with the Kekule structure
Although the Kekule structure was a good attempt in its time, there are serious problems in chemistry, shape and stability of benzene.
Actual structure of benzene
Preparation of benzene:
Post a Comment