ISOMERISM

Definition

Organic compounds having the same molecular formula but different physical or chemical properties are called isomers. The phenomenon is called isomerism.

Classification of Isomerism

• Structural Isomerism
• Stereoisomerism (Space Isomerism)

A) Structural Isomerism

Arises due to difference in the manner of linkage of atoms.

1) Chain Isomerism (Skeleton Isomerism)

• Difference in length or branching of carbon chain
• Example: n-butane and iso-butane

2) Positional Isomerism

• Difference in position of:
• Double bond
• Triple bond
• Functional group
• Example: But-1-ene and But-2-ene
• Example: 1-propanol and 2-propanol

3) Functional Isomerism

• Different functional groups but same molecular formula
• Examples:
• Alcohol ↔ Ether (C_nH_2n+2O)
• Aldehyde ↔ Ketone (C_nH_2nO)
• Acid ↔ Ester (C_nH_2nO₂)
• Cyanide ↔ Isocyanide

4) Metamerism

• Different distribution of carbon atoms on either side of the same functional group
• Observed in ethers, ketones, esters
• Example: 2-pentanone and 3-pentanone

5) Ring–Chain Isomerism

• Occurs between open-chain and cyclic compounds
• Example: But-1-ene and cyclobutane
• It is a type of functional isomerism

B) Stereoisomerism

Arises due to different spatial arrangement of atoms.

1) Geometrical Isomerism (Cis–Trans / E–Z)

• Occurs due to restricted rotation around C=C bond
• Condition: Each carbon must have two different substituents
• Cis: similar groups on same side
• Trans: similar groups on opposite sides

2) Optical Isomerism

• Occurs due to chirality
• Presence of asymmetric carbon (chiral centre)
• Optical isomers rotate plane-polarised light
• Enantiomers: non-superimposable mirror images

C) Tautomerism

• Special type of functional isomerism
• Involves migration of acidic hydrogen and shift of double bond
• Dynamic equilibrium between two forms

Keto–Enol Tautomerism

• Aldehydes and ketones containing –CO–CH₂– group show tautomerism
• Keto form is generally more stable
• β-diketones show higher enol content due to chelation

Other Types

• Nitro–aci tautomerism
• Nitroso–oxime tautomerism