Difference Between Structural Isomers and Stereoisomers

The key difference between structural isomers and stereoisomers is that the structural isomers have the same chemical formula, but different atomic arrangements, whereas the stereoisomers have the same chemical formula and atomic arrangement, but different spatial arrangements.

Isomerism is a chemical concept that describes the occurrence of chemical compounds with the same chemical formula and different chemical and physical properties. Moreover, the two major categories of isomers are structural isomers and stereoisomers.

CONTENTS

1. Overview and Key Difference
2. What are Structural Isomers
3. What are Stereoisomers
4. Side by Side Comparison – Structural Isomers vs Stereoisomers  in Tabular Form
5. Summary

What are Structural Isomers?

Structural isomers or constitutional isomers are the organic molecules having the same chemical formula, but different atomic arrangements. In other words, the atoms of the molecule are bound to each other in different ways. There are three major categories of these molecules as follows:

Figure 1: Structural Isomers of the Same Compound

Chain isomers have differently arranged carbon chains; Ex: C5H12. Position isomers have the functional groups attached to different carbon atoms in the same carbon chain. Furthermore, functional group isomers have the same chemical formula, but a different functional group. In addition, there are two more classes of structural isomers as metamerism and tautomerism.

What are Stereoisomers?

Stereoisomers are organic compounds having the same chemical formula and atomic arrangement, but they have different spatial arrangements. Moreover, there are two groups of stereoisomers as geometric isomers and optical isomers.

Figure 2: Stereoisomers of Octane

Geometric isomers are what we call cis-trans isomers. There are always two isomers as cis isomer and trans isomer. Therefore, they always occur as a pair. Moreover, an organic compound must have double bonds in order to have geometric isomers. Here, the isomers are different from each other according to the attachment of functional groups to the vinyl carbon atoms (carbon atoms in the double bond).

Furthermore, optical isomers are molecules with chiral carbon atoms. A chiral carbon atom has four different functional groups attached to the same carbon atom. Therefore, one isomer differs from the other according to the arrangement of four different groups; here, one isomer acts as the non-superimposable mirror image of the other.

What is the Difference Between Structural Isomers and Stereoisomers?

The two major groups of isomers are structural isomers and stereoisomers. The key difference between structural isomers and stereoisomers is that the structural isomers have the same chemical formula, but different atomic arrangements whereas the stereoisomers have the same chemical formula and atomic arrangement, but different spatial arrangements. Moreover, structural isomers have different arrangements of atoms while stereoisomers have the same arrangement of atoms.

Furthermore, the above-said differences lead to an important difference between structural isomers and stereoisomers. That is; the structural isomers have very different chemical and physical properties whereas stereoisomers have relatively close chemical and physical properties.

Summary – Structural Isomers vs Stereoisomers

Isomers are the organic compounds having the same chemical formula, but the atomic arrangements are different from each other. The key difference between structural isomers and stereoisomers is that structural isomers have the same chemical formula, but different atomic arrangements, whereas stereoisomers have the same chemical formula and atomic arrangement, but different spatial arrangements.

Reference:

1. “Structural Isomer.” Wikipedia, Wikimedia Foundation, 5 Sept. 2018, Available here.
2. “Chirality and Stereoisomers.” Chemistry LibreTexts, National Science Foundation, 26 Nov. 2018, Available here.

Image Courtesy:

1. “Structural isomers” By V8rik – (CC BY-SA 3.0) via Commons Wikimedia
2. “Octane stereo isomers 10 3 4 dimethylhexane” By Steffen 962 – Own work (Public Domain) via Commons Wikimedia

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