Isomerism: structural isomerism, stereoisomerism

1. What is Isomerism?

1. Isomerism is the phenomenon in which two or more compounds have the same molecular formula but different structural or spatial arrangements.
2. Isomers have distinct physical and chemical properties despite having the same formula.
3. The main types of isomerism are:
   • Structural Isomerism
   • Stereoisomerism

2. Structural Isomerism

1. Structural isomers differ in the connectivity of their atoms, resulting in different structures.
2. Types of structural isomerism include:
   • Chain Isomerism: Isomers differ in the arrangement of the carbon chain (e.g., n-butane and isobutane).
   • Position Isomerism: Isomers differ in the position of a functional group or substituent (e.g., 1-propanol and 2-propanol).
   • Functional Group Isomerism: Isomers have different functional groups (e.g., ethanol and dimethyl ether).
   • Metamerism: Isomers differ in the distribution of alkyl groups around a functional group (e.g., diethyl ether and methyl propyl ether).
   • Tautomerism: Isomers exist in dynamic equilibrium due to migration of a hydrogen atom (e.g., keto-enol tautomerism).
3. Structural isomerism is common in organic compounds due to the versatility of the carbon atom.

3. Stereoisomerism

1. Stereoisomers have the same structural formula but differ in the spatial arrangement of atoms.
2. Types of stereoisomerism include:
   • Geometrical Isomerism: Isomers differ in the relative positions of groups around a double bond or a ring (e.g., cis- and trans-isomers in alkenes).
   • Optical Isomerism: Isomers differ in the way they rotate plane-polarized light due to the presence of a chiral center (e.g., L- and D-glucose).
3. Geometrical isomerism arises in compounds with restricted rotation (e.g., around a C=C bond).
4. Optical isomers are classified into enantiomers (non-superimposable mirror images) and diastereomers (non-mirror image stereoisomers).
5. Optical isomerism plays a crucial role in fields like biochemistry and pharmacology.

4. Importance of Isomerism

1. Isomerism explains the diversity of organic compounds with the same molecular formula.
2. It is crucial in determining the physical properties (e.g., boiling and melting points) and chemical reactivity.
3. Stereoisomerism is significant in drug design and biological activity.
4. Isomerism helps in understanding reaction mechanisms and synthesis of compounds.

5. Key Points

1. Isomerism is categorized into structural isomerism and stereoisomerism.
2. Structural isomers differ in the connectivity of atoms, while stereoisomers differ in their spatial arrangement.
3. Examples of structural isomerism include chain, position, and functional group isomerism.
4. Examples of stereoisomerism include geometrical and optical isomerism.
5. Compounds with a chiral center exhibit optical isomerism.
6. Isomerism is vital in organic synthesis and biological applications.