Organic compounds -classification of organic compounds

The compounds in solid, liquid, or gaseous states that contain carbon in their molecules are known as organic compounds. There are a large number of organic compounds and therefore a proper systematic classification was required. Organic compounds can be broadly classified as acyclic (open chain) or cyclic (closed chain). Organic compounds are seen in a number of formats, including Lewis structures, space-filled models, and structural formulas. It is not uncommon to view the hydrogens as lines or to leave them all together in a structural formula of an organic molecule. They are understood to be present in order to complete the 4-bonds provided by the carbon atoms.

JEE Main 2025: Chemistry Formula | Study Materials | High Scoring Topics | Preparation Guide

JEE Main 2025: Syllabus | Sample Papers | Mock Tests | PYQs | Study Plan 100 Days

NEET 2025: Syllabus | High Scoring Topics |  PYQs

This Story also Contains

1. Organic Compounds - Classification Of Organic Compounds
2. Cyclic or Closed-chain compounds
3. Some Solved Example
4. Conclusion

The major components of an organic compound have to be carbon and hydrogen, but some other elements such as nitrogen, oxygen, and phosphorus may also bind to the carbon atom to form these compounds. Also, not all carbon compounds are organic compounds. A few carbon-containing compounds are not part of the organic compound list. Carbon dioxide, carbon monoxide, cyanates, cyanides, and other ion compounds that contain carbon do not come under the category of organic compounds.

In this article, we will cover the topic (Organic Compounds - Classification Of Organic Compounds). This topic falls under the broader category of (Some Basic Principles of Organic Chemistry), which is a crucial chapter in (Class 11 Chemistry).

Organic Compounds - Classification Of Organic Compounds

Acyclic or Open-chain compounds

These are the compounds in which the carbon atoms are linked to each other in such a manner that the molecule is having an open-chain structure. The chain of the carbon atoms may be straight or branched. These compounds are also called as aliphatic compounds. The term aliphatic has been derived from the Greek word aleiphatos meaning fats, since the earliest compounds to be studied were fatty acids or compounds found in fats. Some examples include:

Cyclic or Closed-chain compounds

These are the compounds in which carbon atoms are linked to each other or to the atoms of other elements in such a manner that the molecule has a closed-chain or cyclic or ring structure. One or more close chains or rings may be present in the molecule. The compounds with only one ring of atoms in the molecule are known as monocyclic but those with more than one ring of atoms are termed as polycyclic. These are divided into two categories:

(a) Homocyclic compounds: These are the compounds that only have a ring or rings of carbon atoms in the molecule. The carbocyclic or homocyclic compounds may again be divided into two types, i.e,

• Alicyclic compounds: These are the compounds that contain rings of three or more carbon atoms. These resemble with aliphatic compounds than aromatic compounds in many respects. That is why these are named alicyclic, i.e, aliphatic cyclic. Some examples include,
  
• Aromatic compounds: These compounds consist of at least one benzene ring, i.e, a six-membered carbocyclic ring having alternate single and double bonds. Generally, these compounds have some fragrant odour and hence, named as aromatic.
  
  The above compounds are also known as benzenoid aromatics as their molecules consist of benzene ring or rings. However, there are aromatic compounds, which have structural units different from benzenoid type and are known as non-benzenoid aromatics.
  

(b) Heterocyclic compounds: These are cyclic compounds having ring or rings built up of more than one kind of atoms. The most common other atoms besides carbon are O, N and S. Some examples include,

Ethane

Acetic acid

Complete structural formula
Such a structural formula focuses on the electrons involved in bond formation. A single dash represents a single bond, the double dash is used for a double bond, and a triple dash represents a triple bond. Lone pairs of electrons on heteroatoms (e.g., oxygen, nitrogen, sulphur, halogens, etc.) may or may not be shown. Thus, C₂H₆ ethene C₂H4 , ethyne C₂H_2, and methanol CH₃OHcan be represented by the structural formulas as shown below. Such structural representations are called complete structural formulas.

Condensed structural formula
These structural formulas can be further abbreviated by omitting some or all of the dashes representing covalent bonds and by indicating the number of identical groups attached to an atom by a subscript. The resulting expression of the compound is called a condensed structural formula. Thus, ethane, ethene, and ethyne can be written as:

$\mathrm{CH}_3 \mathrm{CH}_3$ & $\mathrm{H}_2 \mathrm{C}=\mathrm{CH}_2$ & $\mathrm{HC} \equiv \mathrm{CH}$
Ethane & Ethene & Ethyne

Bond-line structural formula
In this bond-line structural representation of organic compounds, carbon and hydrogen atoms are not shown and the lines representing carbon-carbon bonds are drawn in a zig-zag fashion. The only atoms specifically written are oxygen, chlorine, nitrogen etc. The terminals denote methyl (–CH₃) groups (unless indicated otherwise by a functional group), while the line junctions denote carbon atoms bonded to an appropriate number of hydrogens required to satisfy the valency of the carbon atoms. For example, 3-methyloctane is represented as follows:

π (pi) bond

In a π (pi) bond formation, parallel orientation of the two p orbitals on adjacent atoms is necessary for a proper sideways overlap. Thus, in ${H}_2{C}={CH}_2$ molecule all the atoms must be in the same plane. The p orbitals are mutually parallel and both the p orbitals are perpendicular to the plane of the molecule. Rotation of one CH₂ fragment with respect to other interferes with maximum overlap of p orbitals and, therefore, such rotation about carbon-carbon double bond (C=C) is restricted. The electron charge cloud of the π bond is located above and below the plane of bonding atoms. This results in the electrons being easily available to the attacking reagents. In general, π bonds provide the most reactive centers in the molecules containing multiple bonds.

Recommended topic video on(Organic compounds -classification of organic compounds)

Some Solved Example

Q. 1 How many $\sigma$ and $\pi$ bonds in $\mathrm{HC} \equiv \mathrm{CCH}=\mathrm{CHCH}_3$ ?
1) (correct) $\sigma_{C-C}: 4, \sigma_{C-H}: 6, \pi_{C-C}: 3$
2) $\sigma_{C-C}: 3, \sigma_{C-H}: 7, \pi_{C \equiv C}: 2$
3) $\sigma_{C-C}: 5, \sigma_{C-H}: 5, \pi_{C \equiv C}: 2$
4) $\sigma_{C-C}: 4, \sigma_{C-H}: 6, \pi_{C \equiv C}: 4$

Solution:

As we have learned

The given structure is-

In this compound, there are 4 ${C}-{C}$, 6 ${C}-{H}$ boldsymbol{a}bonds and 3 pi bonds present.

Therefore, option (1) is correct.

Q.2 Which of the following compounds is not a heterocyclic compound?

(1)

(2)

(3)

(4)

Solution:

As we have learned

Heterocyclic compounds: These are cyclic compounds having rings or rings built up of more than one kind of atoms. The most common other atoms besides carbon are {O},{N}and S. Some examples include,

is not a heterocyclic compound, it is an example of a homocyclic compound as it contains carbon atoms in the cyclic chain.

Hence, the correct answer is Option (4)

Conclusion

We have seen how organic compounds are present all around us and are an inseparable part of our lives. Most of the compounds that are known are organic. They are valuable for humans as they have many uses. To study these vast numbers of compounds, we have divided them into various categories. Organic compounds are significant since carbon is found in all living species. They are the fundamental elements of many of Earth’s cycles; the carbon cycle, for example, involves the carbon exchange between plants and animals during photosynthesis and cellular respiration.