Isomerism: Definition, Types, Examples

In the study of organic chemistry, isomerism is one of the most important characteristics of organic compounds. The term isomerism comes from the Greek words isos, meaning “equal,” and meros, meaning “parts.” We can define isomerism as:

Two or more compounds having the same molecular formula but differing in the physical and chemical properties are called isomers, and the phenomenon is known as isomerism.

In other words, isomerism is the phenomenon of the existence of two or more compounds with the same kind and number of atoms, but with different arrangements of atoms. Such compounds are called isomers. As the atoms of isomers have different arrangements, they form distinct organic compounds, which have different properties – physical, chemical or both.

Examples of Isomers

For example, butane is an alkane with four carbon atoms and ten hydrogen atoms. The carbon atoms in butane are linked together in a straight chain. In contrast, isobutane also has four carbon atoms and ten hydrogen atoms, but its carbon atoms are arranged in a branched chain. Therefore, butane and isobutane are isomers of each other and have different chemical and physical properties.

CH3 - CH2 - CH2 - CH3
Butane (MP 135 K and BP 273 K)

CH3 - CH - CH3
      |
      CH3
Isobutane (MP 114K and BP 261 K)

Similarly, 1-propanol and 2-propanol are isomers of each other with the same molecular formula, C₃H₈O.

CH3 - CH2 - CH2 - OH 
1-Propanol

CH3 - CH - CH3
      |
      OH
2-Propanol

Isomers need not necessarily have the same functional groups. For example, ethyl alcohol (ethanol) and dimethyl ether are isomers with the molecular formula C₂H₆O, but they have different functional groups—ethanol is an alcohol (–OH group), while dimethyl ether is an ether (–O– linkage).

CH3 - CH2 - OH (Ethyl alcohol)
CH3 - O - CH3 (Dimethyl ether)

Similarly, propanal and acetone (also known as propanone, a ketone) are isomers with the molecular formula C₃H₆O, but they have different functional groups—propanal is an aldehyde (–CHO group), while acetone is a ketone (C=O group between two carbon atoms).

CH3–CH2–CHO (Propanal)
CH3–CO–CH3 (Acetone)

As the length of the carbon chain increases, the number of possible arrangements of carbon atoms and that of possible isomers—also increases. For example, in the alkane homologous series, only one compound corresponds to each of the molecular formulae CH₄, C₂H₆, and C₃H₈.

Two isomers exist for the molecular formula C₄H₁₀ (butanes), three isomers for C₅H₁₂ (pentanes), five isomers for C₆H₁₄ (hexanes), nine isomers for C₇H₁₆ (heptanes) and so on. When we reach decane (C₁₀H₂₂), there are 75 different isomers, each with different physical properties.

Types of Isomerism

Broadly, there are two main types of isomerism:

• Structural isomerism
• Stereoisomerism

Structural isomerism is further divided into different types:

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

Similarly, stereoisomerism is further divided into different types:

• Configurational isomerism
  • Geometrical isomerism
  • Optical isomerism
• Conformational isomerism

Let’s understand one by one in details.

Structural Isomers and Isomerism

Compounds having the same molecular formula but different structures due to the difference in the arrangement of atoms within the molecule are called structural isomers and the phenomenon is known as structural isomerism. This is also referred to as constitutional isomerism.

Structural isomerism is further subdivided into five types, namely chain, position, functional, metamerism and tautomerism.

Chain or Nuclear Isomerism

Compounds having the same molecular formula but different arrangement of carbon chains or skeletons within the molecule are called chain or nuclear isomers, and the phenomenon is termed chain or nuclear isomerism. In this type of isomerism, the isomers have different carbon chains or skeletons. Minimum four carbon atoms is necessary to show the chain isomerism.

For example, butane (C₄H₁₀) and isobutane (C₄H₁₀) shows two chain isomerism because both differ in the structure of their carbon chains.

CH3 - CH2 - CH2 - CH3
    n-Butane
CH3 - CH - CH3
      |
      CH3
  Isobutane (2-Methylpropane)

Similarly, n-pentane (C₅H₁₂), isopentane, and neopentane are chain isomers.

CH3 - CH2 - CH2 - CH2 - CH3 (Pentane)
CH3 - CH - CH2 - CH3 (Isopentane or 2-Methylbutane))
      |
      CH3
      CH3
      |
CH3 - C - CH3 (Neopentane or 2,2-Dimethylpropane)
      |
      CH3

Position Isomerism

When two or more compounds have the same structure of the carbon atoms but differ only in the position of multiple (double or triple) bond or functional group, they are called position isomers and the phenomenon is termed position isomerism. In this of isomerism, the isomers have different positions of multiple bond or functional group.

For example, but-1-ene and but-2-ene are position isomers.

CH3-CH2-CH=CH2 (But-1-ene)
CH3-CH=CH-CH3 (But-2-ene)

Another example of position isomers is propan-1-ol and propan-2-ol.

CH3-CH2-CH2-OH (1-Propanol)
CH3-CH-CH3 (2-Propanol)
    |
    OH

Functional Isomerism

When two or more compounds have the same molecular formula but different functional groups, they are called functional isomers and the phenomenon is termed functional isomerism. In this type of isomerism, the isomers have different functional groups and thus, belong to different families or groups.

For example, alcohol and ether families show functional isomerism. The molecular formula C₂H₆O represents the following two functional isomers:

CH3-CH2-OH (Ethanol)
CH3-O-CH3 (Methoxymethane)

Another example of functional isomerism is a monocarboxylic acid and an ester with the molecular formula C₃H₆O₂.

CH3-CH2-COOH (Propanoic acid)
CH3–COO–CH3 (Methyl acetate)

Metamerism

When two or more compounds have the same molecular formula but differ in the number or arrangement of carbon atoms (or alkyl groups) on either side of a functional group, they are called metamers, and the phenomenon is known as metamerism. This type of isomerism occurs among members of the same homologous series (such as ethers, esters, and amines). For example,

(1) Ethoxyethane is a metamer of 1-Methoxypropane or 2-Methoxypropane.

CH3CH2-O-CH2CH3 (Ethoxyethane or Diethyl ether)
CH3-O-CH2CH2CH3 (1-Methoxypropane or Methyl propyl ether)
      CH3
      |
CH3-O-CH (2-Methoxypropane or Methyl isopropyl ether)
      |
      CH3

(2) Diethyl thioether is a metamer of Methyl n-propyl thioether or Isopropyl methyl thioether.

CH3CH2-S-CH2CH3 (Diethyl thioether)
CH3-S-CH2CH2CH3 (Methyl n-propyl thioether)
      CH3
      |
CH3-S-CH (Isopropyl methyl thioether)
      |
      CH3

Tautomerism or Enolization

Tautomerism is a special kind of functional isomerism in which isomers exist simultaneously in dynamic equilibrium with each other. This type of isomerism arises due to the migration of a hydrogen atom from one polyvalent atom to another within the same molecule, along with a necessary rearrangement of chemical bonds.

The isomers thus formed are called tautomers and the phenomenon is known as tautomerism or keto-enol tautomerism. It is also referred to as desmotropism (desmos = bond, tropos = turn or change). For example, acetylacetone exists in two forms, as shown below:

At the room temperature, the mixture contains 92.5% keto form and 7.5% enol form. In this example, the hydrogen atom shifts from a carbon atom to the oxygen atom. A double bond forms between two carbon atoms as the carbonyl group (C=O) is converted into a hydroxyl group (C–OH) with an adjacent carbon–carbon double bond. The resulting isomers, which exist in dynamic equilibrium with each other, are called tautomers. They cannot be easily separated, as they rapidly interconvert under normal conditions.

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