What is an Aromatic Substance?
A chemical that contains one or more rings with pi electrons that are delocalized around the entire ring or rings is aromatic.
The compound must have a cyclic structure. Every atom is conjugated in the compound. In other words, every atom must be attached to either a double bond or an atom with a pair of electrons that are unpaired.
The amount of pi electrons in a compound must equal 4n+2, where n can be any whole number. This is known as the Hückel rule.
Quantification of Pi Electrons
To count pi electrons, you must take into account the number of double bonds and lone pairs in the ring of the molecule.
Each double bond is composed of 2 pi electrons. Count the amount of double bonds, then multiply that number by 2. The product is the number of pi electrons present in the double bonds of the molecule. Only atoms within the ring structure should be considered for counting lone pairs.
If one of the atoms in the ring possesses a lone pair or two lone pairs, this is equivalent to 2 pi electrons. Count the total amount of pi electrons from double bonds and lone pairs. If the result equals 4n+2, the compound is aromatic.
Criteria for antiaromaticity
Antiaromatic chemicals resemble aromatic compounds, with the exception that they are extremely unstable. An antiaromatic molecule must satisfy three conditions:
Compounds must have a cyclic structure.
All atoms are conjugated.
4n equals the number of pi electrons, where n is any whole number.
Therefore, antiaromatic chemicals and aromatic compounds share the first two characteristics.
In antiaromatic compounds, the number of pi electrons is equal to 4n or multiples of 4.
Aromatic Polycyclic vs Heterocyclic Compounds
Multiple aromatic rings exist in a single polycyclic aromatic chemical. These are massive, voluminous aromatic compounds.
Heterocyclic aromatic compounds contain components other than carbon in their ring or rings; nitrogen is the most common of these elements. By possessing lone pairs of electrons, several elements can impart aromaticity to a ring.
Examples of Aromatic Compounds
You must learn the names and structures of common aromatic compounds for the MCAT. It will be a recognition test, therefore it is essential to become familiar with a few samples.
Benzyne is a six-membered ring containing six carbon atoms and three double bonds.
Toluene resembles benzene, but has a projecting stick that signifies the extra methyl group it possesses.
The structure of phenol resembles a benzyne ring with an attached hydroxy group.
Aniline appears to be a benzyne ring with an amino group (NH2) attached.
Characteristics of Aromatic Substances
Aromatic substances are nonpolar and water-insoluble. Aromatic compounds are quite stable and not highly reactive in terms of stability.
Due to the existence of conjugated pi electrons, aromatic compounds exhibit an intriguing fluorescence when exposed to UV light.
When stimulated by ultraviolet light, pi electrons are able to reach a high energy level more easily. After falling to a lower energy level, excited electrons produce photons. This phenomenon explains the luminous feature of aromatic compounds.
Finally, it is important to note that heterocyclic molecules containing nitrogen are basic. Aromatic rings containing nitrogen can link with hydrogen atoms or protons. Only when its lone pair of nitrogen electrons participates in the conjugated pi system is a nitrogenous heterocyclic molecule acidic.
Aromatic Rings in Biological Studies
Aromatic rings exist inside amino acids, DNA, and the electron transport chain.
There are three aromatic amino acids: tryptophan (two rings), tyrosine (one ring), and phenylalanine (1 ring). Due to cation-pi interactions, they are typically observed in protein-protein binding sites.
Positively charged amino acids can interact with aromatic rings in amino acids. A cation-pi contact is created, which maintains the proximity of these two groups.
DNA and RNA both include purines and pyrimidines as bases. Pyrimidines are aromatic heterocyclic chemicals. These include cytosine, thymine, and uracil, as examples.
Nitrogen is the other element found in the structure of heterocyclic compounds. These are also referred to as nitrogenous bases. Purines, including adenine and guanine, are aromatic polycyclic molecules. Purines, like pyrimidines, have nitrogen in their structure. Additionally, they are termed nitrogenous bases.
The electron transport chain contains aromatic molecules ubiquinone, NAD+, and FAD. Their aromatic rings allow them to readily lose or receive electrons without damaging their stability. They are regarded as excellent electron transporters.
Ubiquinone functions as an electron transport chain carrier. It is capable of accepting two electrons, transforming itself into ubiquinol. As it passes through the electron transport chain, it is oxidised and transformed back into ubiquinone.
NAD+ and FAD both transfer electrons to the electron transport chain. FAD is polycyclic while NAD+ is heterocyclic. When NAD+ gains two electrons, it transforms into NADH. When FAD is diminished, it becomes FADH2.