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Aromaticity

Literal Meaning: The literal meaning of "aromaticity" is "fragrance," but the word has a special meaning in chemistry.
Definition: “Aromaticity is a property of conjugated cycloalkenes in which the stabilization of the molecule is enhanced due to the ability of the electrons in the π orbitals to delocalize.” This act as a framework to create a planar molecule.

“In organic chemistry, the term aromaticity is used to describe a cyclic (ring-shaped), planar (flat) molecule with a ring of resonance bonds that exhibits more stability than other geometric or connective arrangements with the same set of atoms.”
As a result of their stability, it is very difficult to cause aromatic molecules to break apart and to react with other substances.

Aryl group: “A functional group or other substituent that is aromatic is called an aryl group.”

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Characteristics of aromatic (aryl) compounds:

A short list of rules, discovered by Eric Huckel in the 1930’s, lists the properties of aromatic compounds. In order to be aromatic, a molecule must possess the following four structural characteristics:

Aromatic compounds are cyclic i.e. contributing atoms are arranged in one or more rings.

Aromatic compounds contain a delocalized conjugated π-electron system, most commonly in an arrangement of alternating single and double bonds. This configuration allows for the electrons in the molecule's pi system to be delocalized around the ring, increasing the molecule's stability.

They have a coplanar structure, with all the contributing atoms in the same plane.

They contain a number of π delocalized electrons that is even, but not a multiple of 4. That is, 4n + 2 π-electrons, where n = 0, 1, 2, 3, and so on. This is known as Hückel's rule.

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Example of an Aromatic Compound:

Benzene (C₆H₆) is the simplest aromatic hydrocarbon (or arene) and one of the most commonly encountered aromatic systems of compounds in organic chemistry is based on derivatives of benzene.

The word “aromatic” is occasionally used to refer informally to benzene derivatives.

Nevertheless, many non-benzene aromatic compounds also exist. In living organisms, for example, the most common aromatic rings are the double-ringed bases in RNA and DNA.

Benzene

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Example of a Heterocyclic (Non-Benzene) Aromatic Compound:

In furan, the oxygen atom is sp² hybridized.

One lone pair is in the π system (which participate in aromaticity) and the other in the plane of the ring (analogous to the C–H bond in the other positions).

There are 6 π-electrons (2 electrons in the form of a lone pair and 4 electrons in two of the respective π - bonds) i.e., it follows 4n + 2 rule, so furan is aromatic.

Furan

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Anti-Aromatic & Non-Aromatic Compounds

According to Huckel's rule, if a planar molecule has 4n + 2 π-electrons, it is aromatic, but if it has 4n π-electrons and has characteristics 1–3 above, the molecule is said to be antiaromatic.

Antiaromatic compounds are, in general, destabilized and more reactive than expected.

Examples of Aromatic, Antiaromatic and Non-aromatic Compounds:

In benzene each of the three double bonds contributes 2 pi-electrons over 6 atoms, for a total of 4 x 1 + 2 = 6 electrons, in a ring, in a pi-orbital that is planar. Therefore it is aromatic.

Cyclobutadiene is antiaromatic, since the number of π delocalized electrons is 4, which of course is a multiple of 4. The cyclobutadienide (2⁻) ion, however, is aromatic (6 electrons).

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Types of aromatic compounds

[1]. Simple Aromatic Compound Like Benzene.

[2]. Heterocyclics:

In heterocyclic aromatic compounds, one or more of the atoms in the aromatic ring is of an element other than carbon.

This can lessen the ring's aromaticity, and thus increase its reactivity.

Examples:

Note: The aromatic compounds which do not contain benzene ring in their structures are called as a non-benzenoid aromatics or non-benzenoid aromatic compound.

In all these examples, the number of π-electrons is 6, due to the π electrons from the double bonds as well as the two electrons from any lone pair that is in the p-orbital that is in the plane of the aromatic π system.

Pyridine

Pyrazine

Pyrazole

Thiophene

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[3]. Fused aromatics and polycyclics:

Polycyclic aromatic hydrocarbons are molecules containing two or more simple aromatic rings fused together by sharing two neighboring carbon atoms.

Examples:

[4]. Substituted Aromatics:

Many chemical compounds are aromatic rings with other functional groups attached.

Examples:

Naphthalene

Anthracene

Phenanthrene

Trinitrotoluene (TNT)

Acetylsalicylic Acid (Aspirin)

Paracetamol

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Importance of Aromatic Compounds

[1]. Many drugs like aspirin (NSAID), S-α-Methyldopa (anti-hypertensive drug), pyrazinamide (anti TB), morphine (opioid analgesic), paracetamol (NSAID) etc. have aromatic structures.

Acetylsalicylic Acid (Aspirin)

Paracetamol

Morphine

Pyrazinamide

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Aromatic molecules typically display enhanced chemical stability, compared to similar non-aromatic molecules. This extra stability has a profound effect on the chemistry of an aromatic molecule.

Aromatic compounds undergo electrophilic aromatic substitution and nucleophilic aromatic substitution reactions, but not electrophilic addition reactions as happens with simple carbon–carbon double bonds.

Aromatic compounds play key roles in the biochemistry of all living things. For example:

The four aromatic amino acids histidine, phenylalanine, tryptophan, and tyrosine each serve as one of the 20 basic building-blocks of proteins.
Further, all 5 nucleotides (adenine, thymine, cytosine, guanine, and uracil) that make up the sequence of the genetic code in DNA and RNA are aromatic purines or pyrimidines.

The molecule heme contains an aromatic system with 22 π-electrons.
Chlorophyll also has a similar aromatic system.

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