7.4 Aromaticity

Aromatic compounds are those that meet the following criteria

• The structure must be cyclic and planar, containing conjugated π-bonds.
  
  
• Each atom in the ring must have an unhybridized p-orbital.
  
  
• The unhybridized p-orbitals must overlap to form a continuous ring of parallel orbitals.
  
  
• Delocalization of the π-electrons over the ring must lower the electronic energy.
  
  
• It should follow the Huckel’s rule. The rule states that aromatic compounds must contain (4n+2) π-electrons, where n is any whole number. If it contains (4n) π-electrons, the compounds are anti-aromatic compound.
  
  
• Aromatic systems have 2, 6, or 10 π-electrons, for n = 0, 1, or 2 and antiaromatic systems have 4, 8, or 12 π-electrons, for n = 1, 2, or 3.
  

• Monocyclic hydrocarbons with alternating single and double bonds are called annulenes. A prefix in brackets denotes the number of carbons in the ring. Benzene is [6]-annulene, cyclic and planar, with a continuous ring of overlapping p-orbitals. Huckel’s rule predicts that benzene is an aromatic compound as it has (4n+2) π-electron system. Cyclobutadiene ([4]-annulene) cyclic and it has a continuous ring of overlapping p-orbitals. But it has (4n) π-system so Huckel’s rule predicts that cyclobutadiene is an antiaromatic compound.

  

• By Huckel’s rule one can predict that cyclooctatetraene ([8]-annulene) would be an antiaromatic as it has (4n) π-system. But it does not apply for cyclooctatetraene because it has the flexibility to adopt nonplanar “tub” shaped conformation. There is no continuous overlapping of p-orbital. Huckel’s rule applies to a compound only if there is a continuous ring of overlapping p-orbitals. So the compound is nonaromatic.

  

• Like cyclooctatetraene, larger annulenes such as [12]-annulene and [16]-annulene have (4n) π-systems and do not show antiaromaticity because they have the flexibility to adopt nonplanar conformations.

  

• [10]-Annulene would be aromatic as it has (4n+2) but it is a nonaromatic compound. [10]-Annulene that has only cis double bonds cannot have the planar conformation because of angle strain. [10]-Annulene that has two trans double bonds cannot adopt a planar conformation either, because two hydrogen atoms interfere with each other.

  

• Some larger annulenes with (4n+2) π-electrons such as [14]-annulene and [18]-annulene can achieve planar conformations to have aromatic properties.

  

• Cyclopropene is not aromatic because one of its ring atoms is sp³ hybridized so it does not fulfill the criterion for aromaticity. But the cyclopropenyl cation is aromatic because it has an uninterrupted ring of p-orbital and (4n+2) π-system. The cyclopropenyl anion is antiaromatic as it has (4n) π-system.

  

Cyclopentadiene is not aromatic because of the presence of sp³ hybridized carbon atom. The cyclopentadienyl anion is aromatic because it has an uninterrupted ring of p-orbital and (4n+2) π-system. The cyclopentadienyl cation is antiaromatic as it has (4n) π-system.

Cycloheptatrienyl cation and cyclooctatetraene dianion are aromatic compounds because they have uninterrupted ring of p-orbital and (4n+2) π-system.

Cyclopropenone and cycloheptatrienone are stable aromatic compounds. We know that the double bond in carbonyl (C=O) group is polarized to give partial positive charge on the carbon atom and partial negative charge on the oxygen atom. So cyclopropenone and cycloheptatrienone are considered to be aromatic as it obeys (4n+2) π-rule. But the same reason makes cyclopentadienone to be antiaromatic and it is unstable, rapidly undergoes a Diels-Alder dimerization.