The Creation Of Aromatic Orbital: A Chemical Theory Explained

Aromatic compounds, originally named for their fragrant properties, are a group of unsaturated chemical compounds with one or more planar rings of atoms joined by covalent bonds of two different kinds. The unique stability of these compounds is referred to as aromaticity. The term aromatic was first used in 1855 by August Wilhelm Hofmann, though the chemical structure of benzene, the simplest aromatic hydrocarbon, was not recognized until the 19th century by Joseph Loschmidt and August Kekulé. Aromatic compounds are now defined as cyclic compounds satisfying Hückel's Rule.

Benzene and its derivatives

Aromatic compounds, also called arenes, are organic compounds that typically refer to those "with a chemistry typified by benzene" and "cyclically conjugated." The word "aromatic" comes from the past grouping of molecules based on their sweet smell, before their general chemical properties were understood. Today, aromatic compounds are defined as cyclic compounds that satisfy Hückel's Rule.

Benzene, C6H6, is the least complex and the first compound defined as an aromatic hydrocarbon. Its bonding nature was first recognized independently by Joseph Loschmidt and August Kekulé in the 19th century. Each carbon atom in the hexagonal cycle has four electrons to share. One electron forms a sigma bond with the hydrogen atom, and one is used in covalently bonding to each of the two neighboring carbons. This leaves six electrons, shared equally around the ring in delocalized pi molecular orbitals the size of the ring itself.

Benzene derivatives have from one to six substituents attached to the central benzene core. Examples of benzene compounds with just one substituent are phenol, which carries a hydroxyl group, and toluene with a methyl group. When there is more than one substituent present on the ring, their spatial relationship becomes important for which the arene substitution patterns ortho, meta, and para are devised.

The phenyl group is formed by removing one hydrogen from benzene to create the fragment C6H5. Different R groups on the phenyl group allow different benzene derivatives to be formed. Phenol, Ph-OH, or C6H5OH, for example, is formed when an alcohol (-OH) group displaces a hydrogen atom on the benzene ring.

Polycyclic aromatic hydrocarbons

PAHs are formed by the incomplete combustion of organic matter, such as in engines and incinerators, or when biomass burns in forest fires. They are also created and released into the environment by burning organic material such as fuels, garbage, and wood. PAHs are considered possible starting materials for abiotic syntheses of materials required by the earliest forms of life.

PAHs are toxic, mutagenic, and/or carcinogenic. They are highly lipid-soluble and thus readily absorbed from the gastrointestinal tract of mammals. They are rapidly distributed in a wide variety of tissues with a marked tendency for localization in body fat. PAHs do not accumulate in the body.

PAHs can be detected and quantified through various methods, including gas chromatography-mass spectrometry, liquid chromatography with ultraviolet-visible or fluorescence spectroscopic methods, and rapid test PAH indicator strips.

Aromaticity

The term "aromatic" was first used in an article by August Wilhelm Hofmann in 1855, though it was not until 1925 that the term "aromatic sextet" was coined by Sir Robert Robinson to describe a group of six electrons that resists disruption. The concept can be traced back to Henry Edward Armstrong, who in 1890 described at least four modern concepts, including the electron, which was discovered by J.J. Thomson seven years later.

The rules for aromaticity are as follows:

• The molecule must be cyclic.
• Every atom in the ring must be conjugated.
• The molecule must have 4n+2 pi electrons.
• The molecule must be flat.

Aromatic compounds are important in industry, with about 35 million tonnes produced worldwide every year. They are also vital to the biochemistry of all living things, with three of the twenty amino acids used to form proteins being aromatic compounds. Aromatic compounds are also used in the creation of perfumes, hence the term "aromaticity".

Sources of aromatic compounds

Aromatic compounds, also known as aromatics or arenes, are organic compounds that were originally grouped by their distinctive odour. However, the term 'aromatic' in chemistry now refers to compounds with particular electronic, structural, or chemical properties. Aromatics are hydrocarbons, consisting exclusively of carbon and hydrogen, derived from crude oil and coal. They are characterised by one or more planar rings of atoms joined by covalent bonds of two different kinds.

The largest group of aromatic compounds are those related to benzene, which is the simplest aromatic hydrocarbon. Benzene derivatives include toluene and benzoic acid. Polycyclic aromatic compounds are assemblies of benzene rings that share a common side, for example, naphthalene. Heterocyclic aromatic compounds contain at least one atom other than carbon within the ring, such as nitrogen, oxygen, or sulphur. Examples include pyridine and purine.

Aromatic compounds are widely distributed in nature and are produced by plants and microorganisms. They are also found in the atmosphere, with car exhaust from gasoline-powered vehicles being a major source.

Aromatic compounds are used in a wide range of consumer products, including medicine, hygiene, transport, telecommunications, fashion, and sports. They are also used in perfume manufacturing, with plant sources such as flowers, blossoms, roots, and seeds being the largest resource for fragrant compounds.

Reactions of aromatic compounds

Aromatic compounds are defined by their great stability, which is due to the complete delocalization of pi electrons across their ring systems. This stability means that aromatic compounds do not undergo addition reactions, but instead react by electrophilic aromatic substitution.

Electrophilic Aromatic Substitution Reactions

In electrophilic aromatic substitution, a C-H bond is broken and a new C-E bond is formed, where E is an electrophilic atom such as Cl, Br, or N. Electrophilic aromatic substitution reactions share a common mechanism, which can be summarised as follows:

• An electrophile is generated.
• The electrophile attacks the pi electron system of the benzene ring, forming a nonaromatic carbocation.
• The positive charge on the carbocation is delocalized throughout the molecule.
• Aromaticity is restored by the loss of a proton from the atom to which the electrophile has bonded.
• The proton reacts with the catalyst to form the product.

Examples of Electrophilic Aromatic Substitution Reactions

Bromination of Benzene

The bromination of benzene is a classic example of electrophilic aromatic substitution. The reaction proceeds as follows:

• The Br+ ion is generated by the reaction of the bromine molecule with ferric bromide, a Lewis acid.
• The Br+ ion attacks the pi electron system of the benzene ring.
• The positive charge on the resulting carbocation is delocalized.
• Aromaticity is restored by the loss of a proton from the carbon to which the bromine atom has bonded.
• The proton reacts with the FeBr4- ion to regenerate the FeBr3 catalyst and form the product, HBr.

Nitration of Benzene

Nitration of benzene is another example of an electrophilic aromatic substitution reaction. The reaction proceeds in six steps:

• Sulfuric acid ionizes to produce a proton.
• Nitric acid accepts the proton in an acid-base reaction.
• The protonated nitric acid dissociates to form a nitronium ion.
• The nitronium ion acts as an electrophile and is attracted to the pi electron system of the benzene ring.
• The positive charge on the resulting nonaromatic carbocation is delocalized around the ring.
• Aromaticity is restored by the loss of a proton from the carbon to which the nitro group is attached.

Sulfonation of Benzene

The reaction of benzene with concentrated sulfuric acid produces benzenesulfonic acid. The mechanism for this reaction is as follows:

• The sulfuric acid reacts with itself to form sulfur trioxide, the electrophile.
• The sulfur trioxide is attracted to the pi electron system of the benzene molecule.
• The charge around the ring is delocalized.
• Aromaticity is restored by the loss of a proton.

Frequently asked questions