Rules for Aromaticity

Rules for aromaticity 1. A cyclic planer structure and each atom must have a P orbital so that the electron cloud is not intterupted 2. It must contain an odd number of electron pairs ▪ i.e. the number of electrons must be 4n+2 where n=0,1,2,3,4……… ▪ so the number of electrons is 2,6,10,14,18……. ▪ this rule is called the Hückel’s Rule

• monocyclic hydrocarbons with alternating single and double bonds are called annulenes • annulenes are named as [n]-annulene, where n is the number of carbon atoms in their ring, for example cyclooctatetraene is named [8]-annulene • cyclobutadiene has 4 electron (2pairs) so it's not aromatic • cyclooctatetraene has 16 e (8 pairs) it also isn't planer (it has a tub shape) so it's not aromatic • sp3 hybridized atoms doesn't have P orbitals

not aromatic aromatic not aromatic

Not aromatic Not aromatic

Cycloheptatriene cyclopentadiene

• cyclopentadiene cation is not aromatic because it has only 2 pairs of electrons • cyclopentadiene anion is a planer cycle and has 3 pairs of electrons and the CH- has a P orbital (it’s sp2 hybridized) so it’s aromatic • Resonance in cyclopentadiene anion means that all atoms are equivalent (each atom has 1/5 of the negative charge)

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aromatic aromatic aromatic

Naphthaline phenanthrene chrysene

• Any compound that is formed from fused benzene rings is aromatic

Aromatic Heterocyclic Compounds

• Heterocyclic compounds are cyclic compounds with one or more atoms of elements other than carbon within the ring structure, such as sulfur, oxygen or nitrogen.

• Pyridine

o It is an aromatic 6-membered ring with 3 double bonds and one of the atoms is nitrogen

o All of the atoms are sp2 hybridized including the nitrogen

o The nitrogen has one lone pair

o The nitrogen has 4 orbitals (3 sp2 + 1 P)

▪ 2 sp2 orbitals form bonds with the adjacent 2 carbons

▪ The nitrogen’s lone pair is in the third sp2 orbital

▪ One electron in the P orbital and it forms the π bond

o Remember that for an electron to be involved in the aromaticity it must be in a P orbital, so the lone pair is not involved and only the one electron that is in the P orbital is involved, so the nitrogen donates one electron to the aromaticity

• Pyrrole

o It is an aromatic 5-membered ring with 2 double bonds and one of the atoms is nitrogen

o All of the atoms are sp2 hybridized including the nitrogen

o The nitrogen has 4 orbitals (3 sp2 + 1 P)

▪ 2 sp2 orbitals form bonds with the adjacent 2 carbons

▪ 1 sp2 orbital form the bond with the hydrogen

▪ The lone pair is in the P orbital and it forms the π bond (the nitrogen is involved in a π bond that can be noticed if we looked at the pyrrole resonance)

• Furan and thiophene

o They are both aromatic 5-membered rings with 2 double bonds in each

o In furan one of the atoms is oxygen while in thiophene one of the atoms is sulfur

o In both structures all of the atoms are sp2 hybridized

o Both the oxygen and the sulfur have two lone pairs each

o Both the oxygen and the sulfur has 4 orbitals (3 sp2 + 1 P)

▪ 2 sp2 orbitals form bonds with the adjacent 2 carbons

▪ 1 sp2 orbital contain one of the lone pairs

▪ The other lone pair is in the P orbital and it forms the π bond

• Some of the other aromatic heterocyclic compounds include:

o quinoline

o indole

o imidazole

o pyrimidine

o purine

• The nitrogen bases in DNA and RNA are aromatic heterocycles • They are 2 groups o Pyrimidines: the core structure is a pyrimidine cycle

Cytosine thymine uracil

o Purines: where the core structure is a purine cycle

Adenine guanine • Resonance (specially amide resonance) in these cycles is what makes it aromatic

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2 pairs of e
The CH- atom has a P orbital

1 pair of e
The CH+ atom has a P orbital

1 pair of e
The CH2 atom has no P orbital (sp3)

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2 pairs of e
The CH2 atom has no P orbital (sp3)

3 pairs of e
The CH2 atom has no P orbital (sp3)

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