Aromaticity

Introduction

Eilhardt Mitscherlich correctly determined benzene’s molecular formula (C6H6 and decided to call it benzin because of its relationship to benzoic acid, a known substituted form of the compound.

Later its name was changed to benzene.

Compounds like benzene, which have relatively few hydrogens in relation to the number of carbons, are typically found in oils produced by trees and other plants.

Early chemists called such compounds aromatic compounds because of their pleasing fragrances.

In this way, they were distinguished from aliphatic compounds, with higher hydrogen-to-carbon ratios, that were obtained from the chemical degradation of fats.

The chemical meaning of the word "aromatic" now signifies certain kinds of chemical structures.

We will now examine the criteria that a compound must satisfy to be classified as aromatic.

Benzene is a planar, cyclic compound with a cyclic cloud of delocalized electrons above and below the plane of the ring.

Because its πelectrons are delocalized, all the bonds have the same length—partway between C-C the length of a typical single and a typical double bond.

We also saw that benzene is a particularly stable compound because it has an unusually large resonance energy (36 kcal>mol or 151 kJ>mol).

Most compounds with delocalized electrons have much smaller resonance energies.

Compounds such as benzene with unusually large resonance energies are called aromatic compounds.

How can we tell whether a compound is aromatic by looking at its structure?

In other words, what structural features do aromatic compounds have in common?

To be classified as aromatic, a compound must meet both of the following criteria:

a) It must have an uninterrupted cyclic cloud of π electrons (often called a π cloud) above and below the plane of the molecule.

Let's look a little more closely at what this means:

- For the π cloud to be cyclic, the molecule must be cyclic.

- For the π cloud to be uninterrupted, every atom in the ring must have a p orbital.

- For the π cloud to form, each p orbital must overlap with the p orbitals on either side of it.

Therefore, the molecule must be planar.

b) The cloud must contain an odd number of pairs of π electrons. Benzene is an aromatic compound because it is cyclic and planar, every carbon in the ring has a p orbital, and the π cloud contains three pairs of π electrons.

The German chemist Erich Hückel was the first to recognize that an aromatic compound must have an odd number of pairs of π electrons.

In 1931, he described this requirement by what has come to be known as Hückel’s rule, or the 4n+2 rule.

The rule states that for a planar, cyclic compound to be aromatic, its uninterrupted π cloud must contain 4n+2 π electrons, where n is any whole number.

According to Hückel’s rule, then, an aromatic compound must have 2(n=0), 6(n=1), 10(n=2), 14(n=3), 18(n=4) etc, π electrons.

Because there are two electrons in a pair, Hückel’s rule requires that an aromatic compound have 1, 3, 5, 7, 9, etc., pairs of electrons.

Thus, Hückel's rule is just a mathematical way of saying that an aromatic compound must have an odd number of pairs of π electrons.

RULES :

A) Aromatic Compounds

1. Planar, cyclic compound

2. Delocalized electrons 4n+2 where n stand’s for……1,2,3,4,5,6,7,……etc

3. Required sp2 carbons

Example :

B) Anti-Aromatic Compounds

1. Planar, cyclic compound

2. Delocalized (Conjugted) electrons 4n where n stand’s for ... 1,2,3,4,5,6,7, ... etc

3. Required sp2 carbons

Example :

C) Homo-Aromatic Compounds

1. Cyclic compound

2. Delocalized (Conjugted) electrons (4n) Or 4n+2 where n stand’s for ... 1,2,3,4,5,6,7,...etc

3. Required atleast one sp3 carbon

Example :

D) Aromatic Heterocyclic Compounds:

A compound does not have to be a hydrocarbon to be aromatic.

Many heterocyclic compounds are aromatic.

A heterocyclic compound is a cyclic compound in which one or more of the ring atoms is an atom other than carbon.

A ring atom that is not carbon is called a heteroatom.

The name comes from the Greek word heteros, which means "different"

The most common heteroatoms found in heterocyclic compounds are N, O, and S.

Pyridine is an aromatic heterocyclic compound.

Each of the six ring atoms of pyridine is sp2 hybridized, which means that each has a p orbital; and the molecule contains three pairs ofπ electrons.

Don’t be confused by the lone-pair electrons on the nitrogen; they are not π electrons.

Because nitrogen is sp2 hybridized, it has three sp2 orbitals and a p orbital.

The p orbital is used to form π the bond.

Two of nitrogen's sp2 orbitals overlap the sp2 orbitals of adjacent carbon atoms, and nitrogen's third sp2 orbital contains the lone pair.

E) Membered Heterocycles are Aromatic:

c) Quinoline, indole, imidazole, purine, and pyrimidine are other examples of heterocyclic aromatic compounds.

Chemical Consequences of Aromaticity:

If compound having 4π electron system with anion that are Aromatic compounds

As a result of its aromaticity, the cyclopentadienyl anion is an unusually stable carbanion.

This is why cyclopentadiene has an unusually low .In other words, it is the stability conveyed by the aromaticity of the cyclopentadienyl anion that makes the hydrogen much more acidic than hydrogens bonded to other carbons.

Carbocations are also Aromatic compounds:

Another example of the influence of aromaticity on chemical reactivity is the unusual chemical behavior exhibited by cycloheptatrienyl bromide.

Recall from that alkyl halides tend to be relatively nonpolar covalent compounds they are soluble in nonpolar solvents and insoluble in water.

Cycloheptatrienyl bromide, however, is an alkyl halide that behaves like an ionic compound it is insoluble in nonpolar solvents, but readily soluble in water.

Combine Example :