Isomerism: structural isomerism and stereoisomerism

Full Notes

Structural Isomerism

Structural isomers have the same molecular formula but different arrangements of atoms. There are three types of structural isomerism you should know - chain isomerism, position isomerism and functional group isomerism.

Chain Isomerism

In chain isomers, the carbon chain is arranged differently.

Example C₄H₁₀ (butane, methylpropane)

Butane: Straight chain.
2-Methylpropane: One branch.

Position Isomerism

Positional isomers have the same functional group however its in a different position on the carbon chain.

Example C₅H₁₁Br (Bromopentane isomers)

1-Bromopentane: Br on carbon 1.
2-Bromopentane: Br on carbon 2.
3-Bromopentane: Br on carbon 3.

Functional Group Isomerism

Functional group isomers have different functional groups whilst having the same molecular formula.

Example C₃H₆O (Aldehyde and Ketone Isomers)

Propanal (Aldehyde, –CHO)
Propanone (Ketone, –CO– group)

Stereoisomerism

Stereoisomers have the same structural formula, but atoms are arranged differently in 3D space.

Geometrical Isomerism (Cis/Trans)

Note - Cis and Trans and E and Z isomerism has been covered in more detail here. Below is just what you need to know for CIE A-level chemistry :)

Cis and Trans isomerism occurs in alkenes due to restricted rotation around the C=C double bond, as pi-bonds are unable to rotate without breaking.

To show cis/trans isomerism:

• Each carbon in the double bond must have two different groups
• Cis = the same groups on each carbon are on the same side of the double bond
• Trans = the same groups on each carbon are on opposite sides of the double bond

Example But-2-ene

Cis-but-2-ene: CH₃ groups on same side of C=C
Trans-but-2-ene: CH₃ groups on opposite sides of C=C

This type of isomerism is also common in cyclic compounds when rotation is restricted.

Optical Isomerism

If a carbon atom is bonded to four different atoms or groups it is called a chiral carbon (giving a chiral centre in a molecule)

There are two possible ways the atoms or groups bonded to a chiral carbon can be arranged giving two possible stereoisomers. The isomers are non-superimposable mirror images of one another and are called optical isomers (enantiomers).

These isomers:

• Rotate plane-polarised light in opposite directions
• Have identical physical properties, except how they interact with light and other chiral substances

Some molecules can have more than one chiral centre in them, especially biological molecules such as amino acids.

Example 2-hydroxypropanoic acid (Lactic Acid)

Chiral carbon: CH₃CH(OH)COOH
Two enantiomers exist as mirror images.

Identifying Chiral Centres and Isomer Types

From a structural formula, you should be able to:

• Look for carbon with four different groups.

Identify possible cis/trans pairs:

• Look for a C=C bond where each carbon is bonded to two different groups.

Matt’s exam tip

Remember cyclic compounds can also have cis/trans pairs.

Deduce Possible Isomers

Exam check – Given a molecular formula, you should be able to:

• Count the number of possible structural isomers
• Consider stereoisomers where applicable
• e.g. cis/trans forms for alkenes
• enantiomers for molecules with chiral centres

Note:
You do not need to know meso compounds or diastereoisomers for CIE.

Summary

• Structural isomers: same molecular formula, different structural formula (chain, position, functional group).
• Stereoisomers: same structural formula, different 3D arrangement (geometrical and optical).
• Geometrical isomers (cis/trans) occur due to restricted rotation around C=C.
• Optical isomers occur with a chiral carbon and enantiomers are non-superimposable mirror images.
• You must identify and draw structural and stereoisomers, including for cyclic compounds, and deduce total possible isomers for a given formula.