Isomers are molecules that share the same chemical formula but have different arrangements of atoms. In organic chemistry, isomerism is divided into two primary branches: structural isomerism and stereoisomerism. Recognizing and drawing isomers is a core skill heavily assessed in AQA examinations.
🔑 Key Principle
Structural isomers have different bonding arrangements, whereas stereoisomers have the same bonding connectivity but their atoms are arranged differently in 3D space.
What is Isomerism?
Isomerism occurs because carbon can bond in many different ways. If two structures have identical counts of each element (same molecular formula) but differ in structure, they are isomers. The two main branches are:
- Structural Isomerism: Different connections between atoms.
- Stereoisomerism: Same connections, but different spatial orientation.
Structural Isomerism
There are three distinct types of structural isomerism required by the AQA specification:
1. Chain Isomerism
Chain isomers occur when there are variations in the arrangement of the carbon skeleton (e.g. straight-chain versus branched-chain). They have similar chemical properties but differ in physical properties like boiling point due to differences in surface contact area.
Example: Butane (\( \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3 \)) and Methylpropane (\( \text{CH}_3\text{CH(CH}_3\text{)CH}_3 \)) both have the molecular formula \( \text{C}_4\text{H}_{10} \).
2. Position Isomerism
Position isomers have the same carbon skeleton and functional groups, but the functional group is attached to a different carbon atom in the chain.
Example: Propan-1-ol (\( \text{CH}_3\text{CH}_2\text{CH}_2\text{OH} \)) and Propan-2-ol (\( \text{CH}_3\text{CH(OH)CH}_3 \)) both have the molecular formula \( \text{C}_3\text{H}_8\text{O} \).
3. Functional Group Isomerism
Functional group isomers contain different functional groups and therefore belong to different homologous series. They have completely different chemical and physical properties.
Example: Aldehydes and Ketones with the same number of carbons. Propanal (\( \text{CH}_3\text{CH}_2\text{CHO} \)) and Propanone (\( \text{CH}_3\text{COCH}_3 \)) both have the molecular formula \( \text{C}_3\text{H}_6\text{O} \). Similarly, Alcohols and Ethers (e.g., Ethanol \( \text{CH}_3\text{CH}_2\text{OH} \) and Methoxymethane \( \text{CH}_3\text{OCH}_3 \)) share the molecular formula \( \text{C}_2\text{H}_6\text{O} \).
Molecules with the same molecular formula but different structural formulae.
When asked to find or draw all structural isomers, proceed systematically. Draw the longest straight chain first. Then, shorten the main chain by one carbon and place it as a methyl branch in all possible positions. Repeat this, then consider moving the functional group, or changing the functional group entirely (functional group isomerism).
Stereoisomerism: E/Z Isomerism
Stereoisomers have the same structural connectivity but different arrangements in space. The first type of stereoisomerism you study is E/Z isomerism (also historically called cis-trans isomerism), which is found in alkenes.
E/Z isomerism arises due to two key requirements:
- Restricted rotation about the C=C double bond. This is because the pi (\( \pi \)) bond prevents the carbons from rotating relative to each other without breaking the bond.
- Two different groups attached to each of the carbon atoms of the C=C double bond.
Molecules with the same structural formula but different arrangements of atoms in space.
A type of stereoisomerism caused by restricted rotation about a C=C double bond, where groups are prioritized using Cahn-Ingold-Prelog (CIP) rules.
Cahn-Ingold-Prelog (CIP) Priority Rules
To assign E or Z notation to an isomer:
- Look at the two atoms directly bonded to the left-hand carbon of the double bond. Assign priority based on atomic number (higher atomic number = higher priority).
- Do the same for the right-hand carbon of the double bond.
- If the two higher-priority groups are on the opposite sides (diagonally) of the double bond axis, it is the E-isomer (from German entgegen, opposite).
- If the two higher-priority groups are on the same side (above/below the C=C axis), it is the Z-isomer (from German zusammen, together).
- If the atoms directly bonded to the carbon are the same (e.g. two carbon atoms), compare the atoms attached to them, moving along the chain until a point of difference is reached.
Worked Examples
Solution:
- Start with a straight chain of 3 carbon atoms: \( \text{C-C-C} \).
- Place the \( \text{-OH} \) group on Carbon 1. This gives: \( \text{CH}_3\text{CH}_2\text{CH}_2\text{OH} \), which is named propan-1-ol.
- Move the \( \text{-OH} \) group to Carbon 2. This gives: \( \text{CH}_3\text{CH(OH)CH}_3 \), which is named propan-2-ol.
- If we put the \( \text{-OH} \) group on Carbon 3, it is equivalent to Carbon 1 (just numbered from the other direction).
- Can we branch the carbon chain? A 3-carbon chain cannot be branched further (a methyl group on carbon 2 of a 2-carbon chain is just a 3-carbon chain).
- Therefore, there are exactly two alcohol isomers: propan-1-ol and propan-2-ol. (Note: methoxyethane is also an isomer, but it is an ether, not an alcohol).
Solution:
- Write out the structure: \( \text{CH}_3\text{CH=C(Cl)(Br)} \).
- Check Carbon 1 (attached to double bond): It has a chlorine (\( \text{Cl} \)) and a bromine (\( \text{Br} \)) atom attached. Since bromine and chlorine are different, this carbon satisfies the requirement.
- Check Carbon 2 (attached to double bond): It has a hydrogen (\( \text{H} \)) atom and a methyl (\( \text{CH}_3 \)) group attached. Since they are different, this carbon also satisfies the requirement. Therefore, 1-bromo-1-chloropropene does show E/Z isomerism.
- Determine priorities:
- On the left carbon: Bromine (atomic number 35) has a higher priority than Chlorine (atomic number 17).
- On the right carbon: Carbon (atomic number 6 in the methyl group) has a higher priority than Hydrogen (atomic number 1).
- To draw the E-isomer, place the higher priority groups (Bromine and the Methyl group) on opposite sides of the C=C double bond axis.
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