Constitutional Isomers vs Stereoisomerism: Difference and Comparison

Isomers are the organic chemistry phenomenon with two or more compounds and the same quantitative and qualitative composition; instead, it has different biological, chemical, and physical properties.

The various properties occur because of the constitution or stereoisomerism of the organic molecules.

Key Takeaways

1. Constitutional isomers are compounds with the same molecular formula but different structures, while stereoisomers have the same molecular formula and structure but different spatial arrangements.
2. Constitutional isomers differ in the order of attachment of atoms or groups, while stereoisomers differ in the orientation of atoms or groups in three-dimensional space.
3. Constitutional isomers can have different physical and chemical properties, whereas stereoisomers have identical physical and chemical properties.

Constitutional Isomers vs Stereoisomerism

Constitutional isomers have the same molecular formula but differ in their connectivity of atoms and can be differentiated by their functional groups and the way they are connected. Stereoisomers have the same molecular formula and connectivity of atoms but differ in their spatial arrangement.

Constitutional isomers are also known as structural isomers. It is a form of isomer where molecules with the properties of the same molecules create a bond in different orders to oppose stereoisomerism. It is a compound with the same molecular formula yet a different structural formula.

Stereoisomers can also be known as spatial isomers. It has quantitative, qualitative, and functional structures rather than having a different spatial orientation of molecules or their parts. Spatial isomers are different because they have different spatial symmetries of molecules.

Comparison Table

Parameter of Comparison	Constitutional isomers	Stereoisomers
Meaning	Constitute isomers have a different structure but the same molecular formula.	Stereoisomers are the same molecular functional and formula structure with different spatial orientations of molecules.
Types
Chirality	In constitutional isomers, chirality is absent	In stereoisomers, chirality is present.
Arrangement	In constitutional isomers, the arrangement of an atom is different.	In stereoisomers, the arrangement of an atom is the same.
Properties	In constitutional isomers, molecules’ properties are different from each other.	In stereoisomers, the properties of the molecules are similar.

What are the Constitutional Isomers?

Constitutional isomers are known as structural isomers. It is a compound with a different structure but has the same molecular formula. Different types of constitutional or structural isomers exist, like functional groups, positions, and skeletal.

Skeletal isomers have different physical properties. It has uniform qualitative, quantitative and functional composition rather than a different chain of molecules. This chain can also be different or straight-branched.

The positional isomers have different locations of the chain’s substituent, complex bond, or functional group. The positional isomers have different physical properties, and some of them also are biochemical ones. Metamerism is known as the form of positional isomers. 

The isomers with the same qualitative and quantitative but different functional groups are known as functional isomers. If there is a difference in function groups, this will also lead to their different chemical properties.

Some good examples of functional isomers are fructose and glucose. It is also believed that functional isomers have different biochemical importance.

Another main type of structural isomerism is tautomerism, where under some conditions, some spatial structure can be translated into another alongside dynamic equilibrium between them. This isomer can also be known as a form of functional isomer.

What is Stereoisomerism?

Stereoisomers can also be known as spatial isomers. It has quantitative, qualitative, and functional structures rather than having a different spatial orientation of molecules or their parts.

Spatial isomers are different because they have different spatial symmetries of molecules. Some main elements of symmetry are the axis, plane, and centre.

The stereoisomers can also be geometric, optical, conformational isomers, and configurational isomers. Geometric isomers have a different spatial arrangement of substituents relative to the symmetry plane.

Geometric isometry has the possibility of a substitute group in which one side is a double bond plane, non-aromatic cycle, or Na different side. The isomers with two identical substituents on one side of the plane are known as cis-isomer, and another side of the plane is known as trans-isomer.

It is to be believed that in optical isomers, the molecules have no axis, no centre, and no plane of symmetry. Optical isomers have asymmetric types of molecules. The optical isomers are also known as enantiomers, whose biological meaning is different.

Optical isomers have the same chemical properties but have different optical activities.

Spatial isomerism is an isomer with the same configuration; instead, they have different spatial orientations because it is known as conformational.

The different isomers result in the breakage of the bond and the rotation of different molecule types over the axis of a simple sigma bond, and the displacement of the substituents. If the resulting isomers have different conformations, they are known as conformers.

Main Differences Between Constitutional Isomers and Stereoisomers

1. Constitutional and stereoisomers differ only in the group’s spatial orientation in the molecules.
2. A constitutional isomer is a compound with different structural formulas. On the other hand, stereoisomers are the same molecular functional and formula structure with different molecules’ spatial orientations.
3. The constitutional or structural isomers can be positional, functional, or skeletal group isomers. On the other hand, stereoisomers have been configurational( optical, geometric) and conformational isomers.
4. Constitutional or structural isomers have different properties. On the other hand, stereoisomers have similar properties.
5. Constitutional isomers have different chemical names. On the other hand, stereoisomers have the same chemical name with the same letter or symbol in front of the name to identify the orientation.
6. In constitutional isomers, the arrangements of an atom are the same. On the other hand, stereoisomers have the same arrangement of the atom.
7. In constitutional isomers, chirality is absent. On the other hand, in stereoisomers, chirality is present.

References

1. https://pubs.acs.org/doi/abs/10.1021/ci9502663
2. https://www.chinesechemsoc.org/doi/abs/10.31635/ccschem.020.201900094