Difference Between Homocyclic and Heterocyclic Compounds

Main Difference – Homocyclic vs Heterocyclic Compounds

Cyclic compounds in chemistry are molecules having atoms bonded to each other forming a ring structure. In order to form a ring, there should be at least three atoms bonded to each other. If the ring only consists of carbon atoms, then it is an organic cyclic molecule. If none of the atoms in the ring are carbon atoms, they are inorganic compounds. If the atoms present in the ring belongs to the same element, they are called homocyclic compounds. But if there are both carbon and other atoms present in the ring, they are known as heterocyclic compounds. This is the main difference between homocyclic and heterocyclic compounds.

Key Areas Covered

1. What are Homocyclic Compounds
– Definition, Organic and Inorganic Homocyclic compounds, Examples
2. What are Heterocyclic Compounds
– Definition, Organic and Inorganic Heterocyclic Compounds, Examples
3. What are the Similarities Between Homocyclic and Heterocyclic Compounds
– Outline of Common Features
4. What is the Difference Between Homocyclic and Heterocyclic Compounds
– Comparison of Key Differences

Key Terms: Atom, Benzene, Carbocycles, Carbocyclic Compounds, Cyclic Compound, Heterocyclic, Homocyclic, Inorganic, Organic, Sulfur

What are Homocyclic Compounds

Homocyclic compounds are cyclic compounds having atoms of the same element as ring members. In organic chemistry, homocyclic compounds only consist of carbon atoms. These compounds are also known as carbocyclic compounds or carbocycles. They can be organic compounds as well as inorganic compounds. Inorganic compounds do not contain carbon atoms in the ring structure. They contain different elements such as sulfur, silicon, phosphorous, boron, etc. The number of atoms present in a homocyclic compound can vary from 3 to many more.

The common carbocycles include cycloalkanes (a a regular alkane with a ring or loop) and cycloalkenes (a type of alkene hydrocarbon which contains a closed ring of carbon atoms). The following image shows the first four cycloalkanes. These compounds contain carbon atoms connected to each other via sigma bonds (single bonds), and all the carbon atoms are sp³ hybridized and form a ring. In cycloalkenes, the carbon atoms are sp² hybridized and are connected to each other having one single bond and one double bond (along with one single bond between C and H). One of the most common cycloalkenes is benzene.

Difference Between Homocyclic and Heterocyclic Compounds

Figure 1: Cycloalkanes are Carbocycles

In inorganic chemistry, elements such as sulfur, silicon and phosphorous form some common cyclic compounds that are homocyclic compounds. For example, dodecasulfur is composed of only sulfur atoms forming a ring structure.

What are Heterocyclic Compounds

Heterocyclic compounds are cyclic compounds having atoms of different elements as ring members including carbon atoms. There should be at least two different elements present as members, and there should be at least 3 atoms to form a ring.

In organic heterocyclic compounds, elements such as oxygen, nitrogen or sulfur are present in place of a carbon atom in the ring structure. However, halogens can never be found in the ring. This is because, an atom should form at least two bonds to take part in a ring structure, but halogens can form only a single bond.

Main Difference - Homocyclic vs Heterocyclic Compounds

Figure 2: Some Heterocyclic Organic Compounds Containing Some Other Elements such as Oxygen, Sulfur and Nitrogen

These compounds resemble, a cycloalkane or a cycloalkane when atoms of other elements are replaced by carbon atoms.

Similarities Between Homocyclic and Heterocyclic Compounds

Both are cyclic structures.
Both contain atoms arranged in a ring structure.

Definition

Homocyclic Compounds: Homocyclic compounds are cyclic compounds having atoms of the same element as ring members.

Heterocyclic Compounds: Heterocyclic compounds are cyclic compounds having atoms of the different elements as ring members including carbon atoms.

Types of Atoms

Homocyclic Compounds: The ring of a homocyclic compound contains atoms of the same element.

Heterocyclic Compounds: The ring of a heterocyclic compound contains atoms of different elements.

Composition

Homocyclic Compounds: Homocyclic compounds contain atoms of the same element bonded to each other forming a ring.

Heterocyclic Compounds: Heterocyclic compounds contain atoms of at least two different elements bonded to each other forming a ring.

Examples

Homocyclic Compounds: Some examples of homocyclic compounds include benzene, cyclohexane, toluene, cyclohexanol, etc.

Heterocyclic Compounds: Some examples of heterocyclic compounds include pyran (contain oxygen), azocine (contain carbon and nitrogen), thiocane (contain carbon and sulfur), etc.

Conclusion

Heterocyclic and homocyclic compounds are composed of ring structures that have atoms bonded to each other forming an enclosed, cyclic structure. The main difference between homocyclic and heterocyclic compounds is that homocyclic compounds contain rings made of atoms of the same element whereas heterocyclic compounds contain rings made of atoms of different elements.

Reference:

1. Denisko, Olga V., and Alan Roy Katritzky. “Heterocyclic compound.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 8 Dec. 2014, Available here.
2. “Heterocyclic compound.” Wikipedia, Wikimedia Foundation, 19 Jan. 2018, Available here.
3. “Cyclic compound.” Wikipedia, Wikimedia Foundation, 19 Jan. 2018, Available here.

Image Courtesy:

1. “First four cycloalkanes” By Pete Davis – Own work (Public Domain) via Commons Wikimedia
2. “Heterocycle sample” By Peter Davis – Own work (Public Domain) via Commons Wikimedia