Why Are Five Membered Rings With One Heteroatom Important in Chemistry?
All aromatic compounds are organic compounds. Most organic aromatic compounds that you have seen to date are six membered rings. These aromatic rings are called benzene rings comprising six carbon atoms with three double bonds in the structure. These compounds are called homocyclic compounds since all the atoms in the ring are the same.
Heterocyclic compounds are those that have different atoms taking part in the formation of aromatic rings. There can be heterocyclic compounds- five membered rings with one heteroatom. The parent aromatic compounds that belong to this family of hydrocarbons are furan, pyrrole, and thiophene. The structures for each of these compounds are given below.
[Image will be uploaded soon]
Structures of heterocyclic compounds- five membered rings with one heteroatom
Derivatives of Pyrrole Furan and Thiophene
There are different derivatives of the parent aromatic compounds consisting of five membered rings with one heteroatom. The saturated derivative of pyrrole is called pyrrolidine. Tetrahydrofuran is the saturated derivative of furan. Thiophene is the saturated derivative of thiophene. There are bicyclic derivatives of these parent aromatic compounds as well. They are indole (or isoindole for pyrrole), benzothiophene (for thiophene), and benzofuran (for furan).
Natural Occurrence of Such Heterocyclic Compounds
These heterocyclic compounds are present in the natural environment. The nitrogen-containing pyrrole is present in abundance in bone oil. This compound is formed upon the application of strong heat to the bones. The process of protein decomposition produces the aromatic compound due to strong heating.
Pyrrole rings are an integral component of different amino acids like proline and hydroxyproline. These amino acids form a major part of different proteins. Proline is considered as an essential amino acid. These proline and hydroxyproline-containing proteins are mainly found in high concentrations in collagen. Collagen is the chief structural constituent of skin, bones, ligaments, and tendons.
1. Natural Occurrence of Pyrrole Derivatives in Plants
Pyrrole derivatives are also found naturally. They are an important part of different alkaloids. Plants produce alkaloids, which are a huge group of different organic alkaline nitrogen-containing compounds. For example, nicotine, produced by tobacco plants, is a well-known alkaloid that contains pyrroles. Some examples of such alkaloids are given below.
[Image will be uploaded soon]
Different alkaloids containing heterocyclic rings
In plants, chlorophyll contains four pyrrole units joined together to form the porphyrin head, as observed in chlorophyll b.
2. Natural Occurrence of Pyrrole Derivatives in Animals
The chief constituent of our blood is hemoglobin. The heme group of hemoglobin is involved in transporting oxygen to different tissues of the body. The Heme group consists of a pyrrole ring. Similarly, myoglobin also contains a pyrrole ring in its structure. Vitamin B12 also contains pyrrole rings.
Porphyrin rings formed by the chain of four pyrrole rings are also found in bile pigments in the liver. One such bile pigment is bilirubin. It is the chief reason for the brownish-yellow color of our urine and feces. It is produced by the breakdown of heme obtained from dead and destroyed red blood cells.
Natural Occurrence of Indole Derivatives
If we look at the indole derivatives that occur naturally, the first example will be phthalocyanines. They are synthetic pigments that consist of four isoindole rings linked together. One example of this group is the phthalocyanine blue. The commercial name of this compound is Monastral Fast Blue.
There are several plant and animal compounds that contain one or multiple indole units in their structure. One such example is vat dye indigo. It contains two indole rings. This product was previously obtained from plants. Now it is chemically synthesized on large scales.
An important plant essential is indole-3-acetic acid. It promotes plant growth.
A closely related dye from animal species is Tyrian purple. This dye is obtained from snails. It was previously used during classical times. The chemical name of this compound is 6,6′-dibromoindigo. It has two bromine atoms bonded to 6 and 6’ carbon atoms.
Tryptophan is an essential amino acid in different organisms. It contains an indole ring in its structure. It is a constituent in several proteins that are involved in several vital processes. For example, the neurotransmitter serotonin, and Niacin, a B-complex vitamin, contain tryptophan. Serotonin is an essential regulator of human mood. Upon degradation, tryptophan produces Skatole, which can retain the indole ring. It is the major contributor to the strong odor found in mammalian feces.
FAQs on Five Membered Rings With One Heteroatom Explained
1. What exactly is a five-membered ring with one heteroatom?
A five-membered ring with one heteroatom is a type of heterocyclic compound. It features a ring structure made of five atoms, where four are carbon atoms and the fifth is a different element, known as a heteroatom. The most common heteroatoms found in these rings are nitrogen (N), oxygen (O), and sulfur (S).
2. What are the most common examples of five-membered heterocyclic rings?
The most fundamental examples are based on the single heteroatom they contain. These are:
- Pyrrole: Contains one nitrogen atom.
- Furan: Contains one oxygen atom.
- Thiophene: Contains one sulfur atom.
These three are key aromatic structures in organic chemistry.
3. Why are compounds like pyrrole, furan, and thiophene considered aromatic?
These rings are considered aromatic because they satisfy Hückel's rule for aromaticity. They are cyclic, planar, and have a continuous loop of p-orbitals. The heteroatom (N, O, or S) contributes a lone pair of electrons to the ring's pi system, which results in a total of 6 pi electrons. This number (4n+2, where n=1) gives the ring significant stability, much like benzene.
4. Where can we find five-membered rings with a nitrogen atom in nature?
The pyrrole ring is a vital component in many biological systems. For example, the heme group found in hemoglobin and myoglobin, which transports oxygen in our blood, is made of four pyrrole rings. The essential amino acid proline also features a ring structure derived from pyrrole.
5. What is an example of a fused ring system involving a five-membered heterocycle?
A very important fused ring system is indole, which consists of a pyrrole ring fused to a benzene ring. The indole structure is the core of the essential amino acid tryptophan. Tryptophan is crucial for producing the neurotransmitter serotonin and the vitamin niacin. Indole is also a key part of some plant hormones like auxin.
6. How does changing the heteroatom in furan, pyrrole, and thiophene affect their chemical properties?
The heteroatom's identity greatly influences the ring's stability and reactivity. Thiophene is the most aromatic because sulfur's electrons are held less tightly and are easily delocalised into the ring. Pyrrole is less aromatic than thiophene. Furan is the least aromatic because oxygen is highly electronegative and holds its lone pair of electrons more tightly, making them less available for delocalisation. This directly impacts how they behave in chemical reactions.
7. Are all five-membered rings containing one heteroatom aromatic?
No, not all of them are. Aromaticity requires the ring to be unsaturated (containing double bonds). For instance, tetrahydrofuran (THF) has an oxygen atom in a five-membered ring, but all its bonds are single bonds, making it saturated and non-aromatic. It's a common lab solvent. Similarly, pyrrolidine is the saturated, non-aromatic version of pyrrole.
