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. E/Z system for naming stereoisomers
6. Using Cahn-Ingold-Prelog priority rules to identify E/Z isomers

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 (E/Z) isomers
• Optical isomers

Geometric (E/Z) isomers

Geometric isomers, also known as E/Z isomers, occur due to restricted rotation around carbon-carbon double bonds in alkenes. There are two possible arrangements:

• Z-isomer - Higher priority groups are on the same side (above or below) of the double bond.
• E-isomer - Higher priority groups are across the double bond from each other.

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

Identifying E/Z isomers

1. Use Cahn-Ingold-Prelog priority rules to assign priority to groups bonded to each carbon of the double bond. Higher atomic number means higher priority.
2. Check if the highest priority groups are across (E) or on the same side (Z) of the double bond.

Worked example 4 - Identifying E/Z stereoisomers

Classify the alkene below using E/Z terminology.

Step 1: Examine the molecule

The alkene has a double bond between two carbon atoms. One carbon is attached to an ethyl group and a hydrogen atom, the other to a bromine atom and a fluorine atom.

Step 2: Apply Cahn-Ingold-Prelog priority rules

• Compare directly attached atoms to the double-bonded carbons.
• Higher atomic number means higher priority.

Step 3: Determine priority groups on each carbon

• First carbon: Ethyl group (C) has a higher atomic number than Hydrogen (H).
• Second carbon: Bromine (Br) has a higher atomic number than Fluorine (F).

Step 4: Analyse arrangement of high priority groups

The high priority groups (ethyl and bromine) are positioned on opposite sides of the double bond.

Therefore, the molecule is the E-isomer.

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.