Isomerism: Structural Isomerism and Stereoisomerism

What Are Isomers?

Isomers are molecules that have the same molecular formula but different structures or different arrangements of atoms in space.

This means they contain exactly the same number and type of atoms, but these atoms are connected differently or arranged differently in 3D space.

Example: Both propene (CH₂=CH-CH₃) and cyclopropane (a triangle of three carbon atoms) have the molecular formula C₃H₆, but their structures are completely different.

Isomers have different physical properties (like boiling point or melting point) and often different chemical properties too.

There are two main types of isomerism:

1. Structural isomerism - atoms are connected in different ways
2. Stereoisomerism - atoms are connected in the same way, but arranged differently in space

1. Structural Isomerism

A. Chain Isomerism

Chain isomerism occurs when compounds have the same molecular formula but different arrangements of the carbon chain. This happens because of branching.

In chain isomers:

• The longest carbon chain is different in length
• Some isomers are straight chains, others have branches

Example: Pentane (C₅H₁₂)

• Pentane: A straight chain of 5 carbon atoms (CH₃-CH₂-CH₂-CH₂-CH₃)
• 2,2-Dimethylpropane: A central carbon with four CH₃ groups attached (the longest chain is only 3 carbons, with two methyl branches)

Both have the formula C₅H₁₂, but the carbon chains are arranged differently.

Why does branching matter?

• Branched molecules are more compact and spherical
• This reduces the surface area in contact between molecules
• Less surface area means weaker intermolecular forces
• Therefore, branched isomers have lower boiling points than straight-chain isomers

B. Positional Isomerism

Positional isomerism occurs when the functional group is attached to different positions on the carbon chain.

The carbon skeleton stays the same, but the functional group moves to a different carbon atom.

Example: Butanol (C₄H₁₀O)

• Butan-1-ol: The -OH group is on the first carbon (CH₃-CH₂-CH₂-CH₂OH)
• Butan-2-ol: The -OH group is on the second carbon (CH₃-CH(OH)-CH₂-CH₃)

Both have the same molecular formula and the same carbon chain, but the -OH group is in different positions.

Another example from halogenation: When propane reacts with chlorine, two positional isomers form:

• 1-chloropropane (CH₃-CH₂-CH₂Cl) - chlorine on the end carbon
• 2-chloropropane (CH₃-CHCl-CH₃) - chlorine on the middle carbon

C. Functional Group Isomerism

Functional group isomerism occurs when compounds have the same molecular formula but different functional groups.

These isomers belong to different chemical families and have very different chemical properties.

Example: C₄H₁₀O

• Butan-1-ol: Contains an alcohol group (-OH), so it's an alcohol (CH₃-CH₂-CH₂-CH₂OH)
• Ethoxyethane: Contains an ether group (C-O-C), so it's an ether (CH₃-CH₂-O-CH₂-CH₃)

Both have C₄H₁₀O, but one is an alcohol and one is an ether - completely different types of compounds with different reactions.