Heterocyclic Compound

The chemistry of heterocyclic compound or the heterocyclic meaning is also referred to as heterocycle, which is any of a major class of organic chemical compounds that are characterized by the fact that either some or all of the atoms in their molecules are joined in the rings that contain at least one atom of an element other than the carbon atom(C). The cyclic part of heterocyclic represents that at least one ring structure is present in such a compound, whereas the prefix hetero- refers to the noncarbon or heteroatoms atoms in the ring. This is the simple heterocyclic compounds definition.

Structure of Heterocyclic Compounds

In general structure, heterocyclic compounds resemble the cyclic organic compounds that incorporate only the carbon atoms in the rings. For example, benzene with a six-carbon-atom (6 membered heterocyclic compounds or six-membered heterocyclic compounds) ring or cyclopropane with a three-carbon-atom ring, but the presence of the heteroatoms gives the heterocyclic compounds physical properties and chemical properties that are often distinct from those of the all-carbon-ring analogues.

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Comparison with Carbocyclic Compounds

The organic chemical compound molecules are built up from a backbone or framework of the carbon atoms to which are attached oxygen, hydrogen (H), or the other heteroatoms. Carbon atoms contain the unique property of being able to link with one another to produce the chains of atoms. When the ends of these chains are joined together into a ring, the cyclic compounds result; often, such substances are referred to as either alicyclic or carbocyclic compounds. Substitution of either one or more of the ring carbon atoms present in the molecules of a carbocyclic compound with a heteroatom produces a heterocyclic compound.

A typical carbocyclic compound is given as cyclopentane (C5H10), which is the molecular structure of which is indicated by the formula,

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where the chemical symbols represent the atoms of elements, and the lines represent the bonds (such as a covalent bond) between the atoms. For convenience, all such formulas are often written in a simplified polygonal form, like

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for cyclopentane, where each and every corner of the polygon will represent a carbon atom (it is also being understood that, as per the requirement, the hydrogen atoms are joined to the carbon atoms).

When one of the carbon atoms of cyclopentane is replaced with an atom of nitrogen, the compound pyrrolidine produces a chemical relative of pyrrole. The structural formula of the pyrrolidine is given as follows:

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The other heterocyclic compounds are envisioned the same as derivatives of the cyclopentane by substitution with other carbocyclic compounds or other heteroatoms by substitution with nitrogen or other heteroatoms.

Nomenclature of Heterocyclic Compounds

Naming the heterocyclic compounds is not easier because of the existence of several common names in addition to the internationally agreed-upon systematic nomenclature.

Nature of Heteroaromaticity

Aromaticity represents the significant stabilization of the ring compound by a system of alternating both single and double bonds, which are called a cyclic conjugated system, where six π electrons participate, in general. A nitrogen atom in a ring carries either a positive charge or a negative charge, or it can be in neutral form also. A sulfur or oxygen atom present in a ring can either carry a positive charge or be in neutral form.

One of the fundamental distinction is generally made between those heteroatoms, participate in a cyclic conjugated system by means of an unshared, or lone, pair of the electrons that are present in an orbital, which is perpendicular to the plane of the ring, and those heteroatoms that do so due to they are connected to another atom with the help of a double bond.

An example for the first type of heteroatom atom is given as the nitrogen atom in pyrrole, linked by single covalent bonds to both one hydrogen atom and two carbon atoms. Nitrogen contains an outermost shell of 5 five electrons (also called five-membered heterocyclic compounds), 3 of which can enter into 3 covalent bonds with the other atoms. After the bond formation takes place, as in the case of pyrrole, there exists a remaining unshared electron pair that can engage in the cyclic conjugation. Heterocyclic amino acids, which are also called HCAs are chemical compounds that have at least 1 heterocyclic ring.

An example for the second type of heteroatom is given as the nitrogen atom in pyridine, linked by the covalent bonds to only 2 carbon atoms. In addition, pyridine has a sextet of π-electrons, with the nitrogen atom contributing only one single electron and each of the 5 carbon atoms in the ring contributing one extra electron.

Conversion of One Heterocyclic Compound into Another

Although there are several reactions of theoretical importance, where one heterocyclic ring is converted into the other, a few are of practical use. The pyridine preparation from the ammonia and tetrahydrofurfuryl alcohol and the conversion of pyrylium salts into the pyridinium salts are some good examples of such transformations. In addition, the rearrangement of ring-atom, or "shuffling," can be brought about with light (which means the photochemical reaction) in both five-membered heteroaromatic compounds and six-membered heteroaromatic compounds and the ring contraction by extrusion of an atom or a group can take place under certain conditions.