Isomerism

 

What is Isomerism?

Isomerism is the phenomenon in which more than one compounds have the same chemical formula but different chemical structures.

Chemical compounds that have identical chemical formulae but differ in properties and the arrangement of atoms in the molecule are called isomers. Therefore, the compounds that exhibit isomerism are known as isomers.

The word “isomer” is derived from the Greek words “isos” and “meros”, which mean “equal parts”. This term was coined by the Swedish chemist Jacob Berzelius in the year 1830.

PROPERTIES OF ISOMERS

The properties of isomers are important in fields such as chemistry, biochemistry, and pharmacology, where small structural differences can have significant effects on the properties and functions of molecules.

1. Physical properties: Isomers can have different physical properties such as melting and boiling points, solubility, and density. This is because the different structural arrangements can result in different intermolecular forces between molecules.
2. Chemical properties: Isomers can have different chemical properties such as reactivity and stability. This is because the different structural arrangements can affect the accessibility and reactivity of functional groups.
3. Biological activity: Isomers can have different biological activities. For example, the two isomers of glucose, alpha and beta, have different properties and functions in the body.
4. Spectral properties: Isomers can have different spectral properties such as infrared and nuclear magnetic resonance (NMR) spectra. This is because the different structural arrangements can result in different vibrational frequencies and magnetic environments of atoms.
5. Isomerism type: Isomers can be classified into different types, such as structural isomers, stereoisomers, and tautomers. Each type of isomerism has its unique properties and characteristics.

 

Physical properties of isomers

The physical properties of isomers can vary depending on the structural arrangement of their atoms, and these differences can be important in various fields such as materials science, pharmacology, and environmental science.

1.    Melting and boiling points: Isomers can have different melting and boiling points due to differences in intermolecular forces. For example, branched alkanes have lower boiling points than their corresponding straight-chain isomers because of decreased surface area and weaker van der Waals forces.

2. Solubility: Isomers can have different solubility in different solvents due to differences in polarity and hydrogen bonding capabilities. For example, cis-trans isomers of alkenes have different solubilities in water due to differences in polarity.
3. Density: Isomers can have different densities due to differences in molecular mass and packing efficiency. For example, the isomers of butanol have different densities due to differences in molecular shape and packing.
4. Refractive index: Isomers can have different refractive indices due to differences in molecular symmetry and density. For example, the two isomers of tartaric acid have different refractive indices due to differences in their optical activity.
5. Viscosity: Isomers can have different viscosities due to differences in molecular shape and intermolecular forces. For example, the isomers of pentane have different viscosities due to differences in molecular shape and size.

 

 

CHEMICAL PROPERTIES OF ISOMERS

Chemical properties: Isomers can have different chemical properties such as reactivity and stability. This is because the different structural arrangements can affect the accessibility and reactivity of functional groups.

1. Reactivity: Isomers can have different reactivity due to differences in their functional groups and molecular structures. For example, cis-trans isomers of alkenes have different reactivity in hydrogenation reactions due to differences in their stereochemistry.
2. Stability: Isomers can have different stability due to differences in their molecular structures and bond energies. For example, cyclic isomers of hydrocarbons can have different stability due to differences in ring strain energy.
3. Acidity/basicity: Isomers can have different acidity or basicity due to differences in their functional groups and molecular structures. For example, the two isomers of butanol have different acidity due to differences in their ability to donate a proton.
4. Stereochemistry: Isomers can have different stereochemistry, which can affect their biological activity and reactivity. For example, enantiomers have different optical activity and may have different interactions with biological molecules.
5. Isomerization: Isomers can undergo isomerization, where they can interconvert into different isomers. For example, the conversion of glucose into fructose through isomerization occurs naturally in the body.

Isomerism Types

There are two primary types of isomerism, which can be further categorized into different subtypes. These primary types are Structural Isomerism and Stereoisomerism. The classification of different types of isomers is illustrated below.

Structural Isomerism

Structural isomerism is commonly referred to as constitutional isomerism. The functional groups and the atoms in the molecules of these isomers are linked in different ways. Different structural isomers are assigned different IUPAC names since they may or may not contain the same functional group.

 

The different types of structural isomerism are discussed in this subsection.

Chain Isomerism

·                     It is also known as skeletal isomerism.

·                     The components of these isomers display differently branched structures.

·                     Commonly, chain isomers differ in the branching of carbon

·               An example of chain isomerism can be observed in the compound C₅H₁₂, as illustrated below.

Position Isomerism

• The positions of the functional groups or substituent atoms are different in position isomers.
• Typically, this isomerism involves the attachment of the functional groups to different carbon atoms in the carbon chain.
• An example of this type of isomerism can be observed in the compounds having the formula C₃H₇Cl

 

Functional Isomerism

• It is also known as functional group isomerism.
• As the name suggests, it refers to compounds with the same chemical formula but different functional groups attached.
• An example of functional isomerism can be observed in the compound C₃H₆O.

 

 Metamerism

• This type of isomerism arises due to the presence of different alkyl chains on each side of the functional group.
• It is a rare type of isomerism and is generally limited to molecules that contain a divalent atom (such as sulphur or oxygen), surrounded by alkyl groups.
• Example: C₄H₁₀O can be represented as ethoxyethane (C₂H₅OC₂H₅) and methoxy-propane (CH₃OC₃H₇).

Tautomerism

• A tautomer of a compound refers to the isomer of the compound which only differs in the position of protons and electrons.
• Typically, the tautomers of a compound exist together in equilibrium and easily interchange.
• It occurs via an intramolecular proton transfer.
• An important example of this phenomenon is Keto-enol tautomerism.

Ring-Chain Isomerism

• In ring-chain isomerism, one of the isomers has an open-chain structure whereas the other has a ring structure.
• They generally contain a different number of pi bonds.
• A great example of this type of isomerism can be observed in C₃H₆. Propene and cyclopropane are the resulting isomers, as illustrated below.

Stereoisomerism

This type of isomerism arises in compounds having the same chemical formula but different orientations of the atoms belonging to the molecule in three-dimensional space. The compounds that exhibit stereoisomerism are often referred to as stereoisomers. This phenomenon can be further categorized into two subtypes. Both these subtypes are briefly described in this subsection.

Geometric Isomerism

• It is popularly known as cis-trans isomerism.
• These isomers have different spatial arrangements of atoms in three-dimensional space.
• An illustration describing the geometric isomerism observed in the acyclic But-2-ene molecule is provided below.

    

 

 

 

 

 

 

 

 Optical Isomerism

• Compounds that exhibit optical isomerism feature similar bonds but different spatial arrangements of atoms forming non-superimposable mirror images.
• These optical isomers are also known as enantiomers.
• Enantiomers differ from each other in their optical activities.
• 
  Dextro enantiomers rotate the plane of polarized light to the right whereas laevo enantiomers rotate it to the left, as illustrated below.