Structure and Stereoisomerism 

1.0Introduction 

Isomerism is a fundamental concept in chemistry where compounds with the same molecular formula have different arrangements of atoms or different spatial orientations. These variations give rise to distinct chemical and physical properties.

The two major categories of isomerism are:

• Structural Isomerism – Difference in the connectivity of atoms.
• Stereoisomerism – Same connectivity, but different spatial arrangement of atoms.

Understanding both forms is crucial to mastering organic chemistry and coordination compounds.

2.0Classification of Isomerism

Isomerism can be broadly divided into:

• Structural Isomerism (constitutional isomerism): Same molecular formula, different connectivity of atoms.
• Stereoisomerism: Same molecular formula and connectivity, but different spatial arrangements.

3.0Structural Isomerism

Structural isomers differ in how atoms are linked within the molecule.

1. Chain Isomerism: Occurs when compounds have the same molecular formula but differ in the arrangement of the carbon chain.

• Example: C₄H₁₀ → n-butane (straight chain), isobutane (branched chain).

2. Position Isomerism: Arises when the functional group or substituent is attached to different positions in the carbon chain.

• Example:: C₃H₇OH → 1-propanol (–OH on C-1) and 2-propanol (–OH on C-2).

3. Functional Group Isomerism: Occurs when compounds have the same molecular formula but different functional groups.

• Example: C₂H₆O → Ethanol (alcohol) and dimethyl ether (ether).

4. Metamerism: Observed in compounds with polyvalent functional groups (like ethers, esters, amines) due to the different distribution of alkyl groups on either side of the functional group.

• Example: C₄H₁₀O → Ethoxyethane (C₂H₅–O–C₂H₅) and methoxypropane (CH₃–O–C₃H₇).

5. Tautomerism: A special case where compounds exist in dynamic equilibrium between two structural isomers differing in the position of a proton and a double bond.

• Example: Keto–enol tautomerism: CH₃COCH₃ ⇌ CH₂=C(OH)CH₃

Read More: Structural Isomerism

4.0Stereoisomerism

Stereoisomerism arises when compounds have the same structural formula but differ in the three-dimensional arrangement of atoms in space.

Geometrical (Cis–Trans / E–Z) Isomerism

• Occurs due to restricted rotation around the C=C double bond or in cyclic compounds.
• Cis isomer: Same groups on the same side.
• Trans isomer: Same groups on opposite sides.

Example:

• 2-butene → cis-2-butene (both CH₃ on same side), trans-2-butene (CH₃ on opposite sides).
• For complex cases, the E–Z system (based on Cahn-Ingold-Prelog rules) is used.

Optical Isomerism

• The optical isomerism arises due to the presence of chiral centers (asymmetric carbon atoms with four different substituents).
• Such compounds can rotate plane-polarized light.

Types:

• Enantiomers: Non-superimposable mirror images.
• Diastereomers: Stereoisomers that are not mirror images.
• Meso compounds: Achiral molecules despite having stereocenters (due to internal symmetry).

Chirality and Enantiomers

• A carbon atom is chiral if it is bonded to four different substituents.
• Enantiomers have identical physical properties (boiling point, melting point, density) but differ in optical activity and some biological interactions.
• Example: Lactic acid exists as two enantiomers: L-(+)-lactic acid and D-(–)-lactic acid.

Diastereomers

• These are stereoisomers that are not related as mirror images.
• They usually differ in physical and chemical properties.
• Example: 2,3-dichlorobutane has both enantiomers and diastereomers.

Meso Compounds

• Molecules with multiple stereocenters but are achiral due to a plane of symmetry.
• They do not exhibit optical activity.
• Example: Meso-tartaric acid.

5.0Differences Between Structural and Stereoisomerism

6.0Importance of Structure and Stereoisomerism in Organic Chemistry

• Explains diversity of organic compounds: Same formula can produce many distinct compounds.
• Predicts physical properties: Boiling point, melting point, solubility, and color often differ in isomers.
• Determines chemical reactivity: The position and type of a functional group influence reactions.
• Pharmaceutical significance: Enantiomers of drugs can have dramatically different biological effects.
• Industrial applications: Many dyes, polymers, and agrochemicals rely on specific stereoisomeric forms.