Difference Between Constitutional Isomers and Stereoisomers

Main Difference – Constitutional Isomers vs Stereoisomers

Isomerism is an important phenomenon in organic chemistry that explains why there are compounds having the same molecular formula with different properties. Isomerism can be defined as the relation between two or more molecules having the same molecular formula but different structures or spatial arrangements. The main categories of isomers are structural isomers or constitutional isomers and stereoisomers. The main difference between constitutional isomers and stereoisomers is that constitutional isomers are molecules having the same molecular formula but different atomic arrangements whereas stereoisomers are molecules having the same molecular formula and atomic arrangement but different spatial arrangements.

Key Areas Covered

1. What are Constitutional Isomers
      – Definition, Types, Properties, Examples
2. What are Stereoisomers
      – Definition, Types, Properties, Examples
3. What is the difference Between Constitutional Isomers and Stereoisomers
      – Comparison of Key Differences

Key Terms: Chain Isomerism, Chirality, Cis-trans Isomers, Constitutional Isomers, Functional Group Isomerism, Geometric Isomers, Isomerism, Optical Isomers, Position Isomerism, Stereoisomers, Structural Isomers

What are Constitutional Isomers

Constitutional isomers are also called structural isomers. These are molecules having the same molecular formula but different atomic arrangements. Atomic arrangement refers to the way or order that the atoms of the molecule are bound to each other. This results in the constitutional isomers having functional groups and side groups that are different from each other.

According to the location of isomerism in a molecule, the constitutional isomers are classified into few groups.

• Chain isomerism
• Position isomerism
• Functional group isomerism
• Metamerism
• Tautomerism

Few of these groups are discussed below.

Chain Isomerism

Chain isomerism is the different arrangements of the carbon chain of a particular compound. For example, C₅H₁₂ compound can have different structures as below.   

Figure 01: Linear and Branched Structures of Pentane are Constitutional Isomers

Above two images show two different atomic arrangements of pentane. Due to these different arrangements, two molecules have different chemical and physical properties. But the molar mass of the two molecules would be the same because the atomic compositions are the same.

Position Isomerism

Position isomerism occurs when the same functional groups are attached to different carbon atoms in a carbon chain.  In other words, the position of the functional group is different from one molecule to the other, but the carbon chain is the same.

Figure 02: Position Isomerism of Propanol

Functional Group Isomerism

In functional group isomerism, the molecular formula is the same, but the isomers bear different functional groups.

What are Stereoisomers

Stereoisomers are molecules having the same molecular formula and the atomic arrangement, but different spatial arrangements. The two main groups of stereoisomers are;

• Geometric isomers
• Optical isomers

Geometric Isomers

Geometric isomers are also called cis-trans isomers. These isomers always occur in pairs. The two isomers are the cis-isomer and the trans-isomer. These isomers occur in molecules having double bonds. The difference between these two isomers is the attachment of a functional group to the vinylic carbon atom. (Vinylic carbon is the carbon atom having a double bond with another carbon atom.)

Figure 03: Cis-trans isomers of C₄H₈

Optical Isomers

Optical isomers occur in molecules with a chiral carbon. A chiral carbon is a carbon atom having four different groups attached to it. This chiral carbon causes the occurrence of a stereoisomer, which is the non-superimposable mirror image of that molecule.

Figure 04: Optical isomers of C₃H8O₃

Difference Between Constitutional Isomers and Stereoisomers

Definition

Constitutional Isomers: Constitutional Isomers are molecules having same molecular formula but different atomic arrangements.

Stereoisomers: Stereoisomers are molecules having the same molecular formula and the atomic arrangement, but different spatial arrangements.

Arrangement of Atoms

Constitutional Isomers: The arrangement of atoms in constitutional isomers is different from one isomer to the other.

Stereoisomers: The arrangement of atoms in stereoisomers is the same.

Chirality

Constitutional Isomers: Chirality is absent in constitutional isomers.

Stereoisomers: Chirality is present in stereoisomers.

Chemical and Physical Properties

Constitutional Isomers: Constitutional isomers have very different chemical and physical properties.

Stereoisomers: Stereoisomers have relatively close chemical and physical properties.

Conclusion

Constitutional isomers and stereoisomers are the main classifications of isomers. The main difference between constitutional isomers and stereoisomers is that constitutional isomers are molecules having same molecular formula but different atomic arrangements whereas stereoisomers are molecules having the same molecular formula and atomic arrangement but different spatial arrangements.

References:

1. “Stereoisomers” Study.com. Study.com, n.d. Web. Available here. 30 June 2017. 
2. “Basic principles in organic chemistry: Structural isomerism.” Open Teaching Project. N.p., n.d. Web. Available here. 30 June 2017. 

Image Courtesy:

1. “Cis-trans”By D.328 2008/03/10 19:38 (UTC) – drawn by D.328 (CC BY-SA 3.0) via Commons Wikimedia
2. “Structural isomers” By V8rik – (CC BY-SA 3.0) via Commons Wikimedia
3. “Milchsäure Enantiomerenpaar” By NEUROtiker (talk) – Own work (Public Domain) via Commons Wikimedia