Isomers: The Chemistry of Twins
Have you ever heard of isomers? The term could be quite alien to most of us, but isomers play a significant role in the study of organic chemistry.
Isomers are molecules that have the same chemical formula but different physical and chemical properties. This article will provide a comprehensive understanding of isomers, focusing on their types, specifically structural isomers.
Types of Isomers
Structural Isomers
Structural isomers, also known as constitutional isomers, are molecules that have the same chemical formula but differ in their spatial arrangement of atoms. They differ in their functional groups, molecule chain, and connectivities.
The simplest example is the pentane molecule. It has three different isomers, namely n-pentane, isopentane (2-methylbutane), and neopentane (2,2-dimethylpropane), which have very different structures.
Positional Isomers
Positional isomers, as the name suggests, vary in the position of the functional group on a hydrocarbon chain. For instance, a four-carbon hydrocarbon named butane can have two isomers: n-butane (normal butane), and isobutane, differing only in the position of a methyl group.
Functional Isomers
Functional isomers, also known as functional group isomers, have the same molecular formula, but the functional groups attached to the hydrocarbon chain differ from one another. An example of functional isomers is propanal and propanone.
They both have the same formula (C3H6O) but different functional groups, making them entirely separate compounds.
Stereoisomers
Stereoisomers are molecules with the same molecular formula, same skeletal structure, and same connectivity, but different spatial orientation. This type of isomerism has two primary subtypes: geometric isomerism and optical isomerism.
Geometric Isomers
Geometric isomers arise when two molecules differ only in their stereochemistry around a carbon-carbon double bond or a ring system. This subtype of stereoisomerism is also named cis-trans isomerism, since this kind of isomerism is due to two types: cis and trans.
Cis-isomers have functional groups attached to the same side of the molecule, while trans-isomers have functional groups on opposite sides of the molecule. The best example is the molecule 2-butene, which has two isomers, as cis-2-butene and trans-2-butene.
Optical Isomers
Optical isomerism occurs when a molecule possesses chiral centers. Molecules with chiral centers are not superimposable on their mirror image and in consequence, are non-superimposable mirror images.
To differentiate them from one another, they are usually labeled as R or S. The prefix R implies clockwise, while S implies counterclockwise.
To explain better, the simplest example is the common molecule lactic acid. It has one chiral center, and its mirror image would be D-lactic acid.
Conformational Isomers
The last subtype of isomerism is conformational isomerism, which arises due to rotation around a single bond. Like skeletal isomers, this type of isomerism does not require a change in molecular formula.
The simplest example is the molecule ethane, in which the molecule can have Newman projections, either eclipsed or staggered.
In a Nutshell
In conclusion, isomerism is one of the most important aspects of organic chemistry, and it is a concept that every chemistry student must understand. Structural isomers have three primary forms, namely skeletal isomers, positional isomers, and functional isomers.
Stereoisomers have two primary subtypes, geometric isomers, and optical isomers. Remember, the study of isomers is crucial to understand the properties of molecules in organic chemistry.
Stereoisomers
Isomers are vital to the study of organic chemistry, and one type of isomer is stereoismers, which have identical chemical formulas and connectivity, but they differ in their spatial arrangement in three dimensions. They arise due to the presence of a chiral center or a ring structure, or where two atoms, groups of atoms, or substituent in a molecule have a different spatial position relative to each other.
Geometric Isomers
Geometric isomers, also known as configurational isomers, are a type of stereoismers that arise due to restricted rotation in molecules with double bonds, rings, or other rigid structures. The restricted rotation in these molecules makes them adopt different physical and chemical properties.
Geometric isomerism is also called cis-trans isomerism or E-Z isomerism. In cis-trans isomers, molecules have different configurations when functional groups are in cis (on the same side) or trans (on opposite sides) of the molecule.
In E-Z isomerism, two molecules differ in their priority groups, which are assigned based on the atomic number of atoms.
The best explanation of this type of isomerism is the molecule 2-butene.
It is an unsymmetrical alkene C4H8 with a double bond in the middle. 2-butene molecule can have two stereoisomers: cis-2-butene and trans-2-butene.
Suppose both double bonds of an alkene have substituents differing in their sizes or properties. In that case, it can exist in cis and trans geometrical isomers.
Cis-2-butene has the two larger groups on the same side, while trans-2-butene groups are on different sides, as shown in the figure below. 
Physical and chemical properties of geometric isomers differ due to their shape.
Density, boiling point, melting point, and solubility are examples of physical properties affected. For instance, cis-isomers have more interactions between the larger groups, making them more polar and with higher boiling points.
In contrast, trans-isomers are less polar and have lower boiling points.
Optical Isomers
Optical isomerism is another subtype of stereoismers that arises due to a molecule having a chiral center, also known as an asymmetric carbon. Chiral centers have four different substituent groups arranged around them, making the molecule non-superimposable on its mirror image.
Thus, optical isomers are stereoisomers that have similar chemical and physical properties but differ in the direction of rotation of plane-polarized light. The figure shows the difference between optical isomers.
Enantiomers are mirror images with equal but opposite rotational direction, which means that they are not identical or superimposable. They are like left and right hands or shoes which are different and cannot fit into the other’s space.
Hence enantiomers have identical properties except for how they interact with light and biological molecules. Diastereomers are stereoisomers that are not mirror images and include all isomers which are not enantiomers.
They have different physical and chemical properties from one another and can rotate the plane-polarized light but to a different extent. Epimerism is a subtype of diastereomerism that refers to the stereoisomers that differ in only one substituent group’s spatial arrangement.
A common example of optical isomers is lactic acid, which has one chiral center and exists in two forms: L-lactic acid and D-lactic acid.
Conformational Isomers
Conformational isomers arise when a molecule can rotate around a sigma bond’s axis to assume different conformations while keeping its chemical formula and connectivity. The freedom of rotation may come from the presence of single bonds in a molecular chain.
The simplest example is ethane, which is C2H6. In ethane, the two CH3 groups can rotate around the carbon-carbon single bond to form different structures like staggered ethane and eclipsed ethane.
Staggered ethane refers to the type of ethane where the CH3 groups (green balls) are staggered from each other. They are aligned in a way that the maximum distance between them can be achieved.
In contrast, eclipsed ethane refers to CH3 groups aligned on the opposite side of the molecule. This arrangement of atoms is less stable because the carbon-hydrogen bond of one CH3 is close to a carbon-hydrogen bond of another CH3, leading to collisions.
Therefore, the molecule is more likely to exist as the staggered conformation than the eclipsed conformation. 
Conclusion
In conclusion, isomerism is a fundamental concept in organic chemistry that is essential to understand for students and professionals alike.
Stereoisomers, specifically geometric and optical isomers, arise due to differences in their spatial configuration. Geometric isomers result from restricted rotation of molecules with double bonds, pivots, or rings, while optical isomers arise from a molecule having a chiral center.
Conformational isomers results from rotation around single bonds in a molecule, an example of which is ethane. Understanding these isomers and their properties is essential to understanding organic chemistry fully.
In summary, isomerism is a crucial concept in organic chemistry that plays a significant role in the identification and study of compounds.
Stereoisomers, including geometric and optical isomers, are isomers that vary in their spatial arrangement in three dimensions. Geometric isomers arise due to restricted rotation in molecules, while optical isomers arise due to a molecule having a chiral center.
Conformational isomers results from rotation around single bonds in a molecule. Understanding these isomers is essential to fully comprehend the properties and behavior of compounds in organic chemistry.
FAQs:
Q: What are isomers? A: Isomers are molecules that have the same molecular formula but different physical and chemical properties.
Q: What are the subtypes of isomers? A: The subtypes of isomers are structural isomers, stereoisomers, and conformational isomers.
Q: What is the difference between geometric and optical isomers? A: Geometric isomers differ in their spatial configuration due to restricted rotation in molecules, while optical isomers differ in their spatial configuration due to a molecule having a chiral center.
Q: What is a chiral center? A: A chiral center is a carbon atom in a molecule that has four different substituent groups arranged around it.
Q: What are the different types of stereoisomers? A: The different types of stereoisomers are enantiomers, diastereomers, and epimers.
Q: What are conformational isomers? A: Conformational isomers arise when a molecule can rotate around a sigma bond’s axis to assume different conformations while keeping its chemical formula and connectivity.
Q: Why is understanding isomerism essential? A: Understanding isomerism is crucial to fully comprehend the properties and behavior of compounds in organic chemistry.