Isomerism

This lesson covers: 

1. Structural isomers and stereoisomers
2. The three types of structural isomer: chain, positional, and functional group
3. The two types of stereoisomer: geometric and optical
4. How to identify structural isomers and stereoisomers
5. cis-trans system for naming stereoisomers

Types of isomerism

Isomers are molecules with the same molecular formula but different arrangements of atoms.

They fall into two main categories:

• Structural isomers - These molecules have the same atoms but different connections.
• Stereoisomers - These molecules are connected in the same way but have different spatial arrangements of atoms.

Structural isomers

Structural isomers are divided into three sub-types:

1. Chain isomers - Differ in the carbon skeleton arrangement (e.g., straight chain vs branched chain).
2. Positional isomers - The functional group is attached at different carbon atoms.
3. Functional group isomers - The atoms form different functional groups.

Worked example 1 - Identifying chain isomers in butane

Determine the possible chain isomers of butane.

Step 1: Identify the molecular formula

Butane has the molecular formula C₄H₁₀.

Step 2: Determine possible structures

• Butane - A straight chain of four carbon atoms.
• Methylpropane - A branched chain with three carbon atoms in the main chain and one methyl group branching off.

So there are two chain isomers of C₄H₁₀: butane and methylpropane.

Worked example 2 - Identifying positional isomers of chloropentane

Determine the positional isomers of chloropentane.

Step 1: Identify the molecular formula

Chloropentane has the molecular formula C₅H₁₁Cl.

Step 2: Determine possible structures

• 1-chloropentane - Chlorine on the first carbon.
• 2-chloropentane - Chlorine on the second carbon.
• 3-chloropentane - Chlorine on the third carbon.

So there are three positional isomers of chloropentane: 1-chloropentane, 2-chloropentane, and 3-chloropentane.

Note: 4-chloropentane and 5-chloropentane mirror 2-chloropentane and 1-chloropentane due to the symmetry of the pentane chain, so they aren't distinct isomers.

Worked example 3 - Identifying functional group isomers of C₃H₆O

Determine the functional group isomers for C₃H₆O.

Step 1: Identify functional groups and carbon skeleton

C₃H₆O can form different functional groups like aldehydes or ketones.

Step 2: Determine possible structures

• Propanal - An aldehyde at the end of a three-carbon chain.
• Propanone - A ketone in the middle of a three-carbon chain.

So C₃H₆O has two functional group isomers: propanal, and propanone.

Stereoisomers

Stereoisomers are categorised into two sub-types:

• Geometric (cis-trans) isomers
• Optical isomers

Geometric (cis-trans) isomers

Cis-trans isomerism can occur in straight-chain alkenes when the carbon-carbon double bond restricts rotation. This leads to two configurations:

• Cis configuration - The substituent groups are positioned on the same side of the carbon-carbon double bond.
• Trans configuration - The substituent groups are positioned across from one another, on opposite sides of the carbon-carbon double bond.

For example, in cis-but-2-ene, the methyl groups are positioned above the same side of the double bond whereas in trans-but-2-ene, the methyl groups are positioned diagonally across the double bond from one another.

Optical isomers

Optical isomers form when a molecule contains a chiral centre - a carbon atom bonded to four different substituents, creating non-superimposable mirror image forms, or enantiomers.

For instance, 2-chlorobutane has a chiral centre on its second carbon, bonded to a chlorine atom, a methyl group, an ethyl group, and a hydrogen atom, resulting in two mirror image forms.

These enantiomers, though structurally identical, show different interactions with plane-polarised light.