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This lesson provides a comprehensive treatment of isomerism in organic chemistry, covering structural isomerism (chain, position, and functional group isomerism), E/Z (geometric) isomerism with the Cahn-Ingold-Prelog priority rules, and optical isomerism (chiral centres, enantiomers, racemic mixtures, and polarimetry). Isomerism is a unifying theme that connects nomenclature, bonding, reaction mechanisms, and biological activity. A secure understanding of all types of isomerism is essential for A-Level Chemistry and is tested explicitly in both short-answer and extended-response questions.
Key Definition: Isomers are molecules that have the same molecular formula but a different arrangement of atoms. This different arrangement can affect their physical properties, chemical reactivity, and biological activity.
Isomerism is divided into two broad categories:
Diagram description: Imagine a branching tree. At the top is 'Isomerism'. This branches into 'Structural isomerism' (left) and 'Stereoisomerism' (right). Structural isomerism then branches into 'Chain', 'Position', and 'Functional group'. Stereoisomerism branches into 'E/Z (geometric)' and 'Optical'.
graph TD
A["Isomerism<br/>(same molecular formula,<br/>different arrangement)"] --> B["Structural Isomerism<br/>(atoms bonded in<br/>different order)"]
A --> C["Stereoisomerism<br/>(same bonds, different<br/>spatial arrangement)"]
B --> D["Chain<br/>(different carbon<br/>skeleton)"]
B --> E["Position<br/>(different position<br/>of functional group)"]
B --> F["Functional Group<br/>(different functional<br/>group entirely)"]
C --> G["E/Z Geometric<br/>(restricted rotation<br/>about C=C)"]
C --> H["Optical<br/>(chiral centre with<br/>4 different groups)"]
Structural isomers have the same molecular formula but different structural formulae — the atoms are connected in a different order or arrangement.
Key Definition: Chain isomers differ in the arrangement of the carbon skeleton — one may have a straight (unbranched) chain while others have branched chains.
Chain isomers have the same functional group but different carbon chain arrangements. For example, C₅H₁₂ has three chain isomers:
Chain isomers have slightly different physical properties. Branched isomers have lower boiling points than their straight-chain counterparts because they are more spherical, reducing the surface area available for London force interactions.
Key Definition: Position isomers have the same carbon skeleton and the same functional group, but the functional group is attached at a different position on the chain.
For example, C₃H₇OH has two position isomers:
Another example: C₄H₈ (butene) has two position isomers:
Position isomers share the same functional group chemistry but may differ in reactivity. For instance, propan-1-ol is a primary alcohol (oxidised to an aldehyde then carboxylic acid), whilst propan-2-ol is a secondary alcohol (oxidised to a ketone only).
Key Definition: Functional group isomers have the same molecular formula but contain different functional groups, and therefore belong to different homologous series.
For example, C₃H₆O can be:
Another common pair: C₂H₆O can be:
And C₃H₆O₂ can be:
Functional group isomers have very different chemical properties because they belong to different homologous series. For example, ethanol is a liquid that is miscible with water and undergoes oxidation, whereas methoxymethane is a gas at room temperature and is chemically unreactive towards mild oxidising agents.
Question: Draw and name all the structural isomers of C₄H₉Cl and identify the type of isomerism between each pair.
Solution:
There are four structural isomers:
Isomers 1 and 2 are position isomers (same chain, different Cl position). Isomers 1 and 3 are chain isomers (different carbon skeleton arrangement). Isomers 3 and 4 are position isomers on the branched chain.
Note that 1-chlorobutane and 2-chlorobutane are both primary and secondary halogenoalkanes respectively, which affects their reactivity and mechanism of nucleophilic substitution.
Exam Tip: When asked to draw structural isomers, work systematically. First draw all the chain isomers (vary the carbon skeleton), then for each chain consider all the positions where the functional group can be placed (position isomers). Finally, check whether any functional group isomers are possible. Always name each isomer using IUPAC rules.
Stereoisomers have the same structural formula (atoms bonded in the same order) but differ in the spatial arrangement of atoms. The first type of stereoisomerism is E/Z isomerism.
Key Definition: E/Z isomers arise when there is restricted rotation about a double bond (C=C) and each carbon of the double bond has two different groups attached to it. The two isomers have different spatial arrangements of groups around the rigid double bond.
Two conditions must both be met:
The E/Z system uses the CIP rules to assign priorities to the groups on each carbon of the double bond:
Rule 1: Look at the atom directly bonded to the C=C carbon. The atom with the higher atomic number gets higher priority.
Rule 2: If the directly bonded atoms are the same, move outward along each chain and compare the next set of atoms until a point of difference is found. At the first point of difference, the atom with the higher atomic number wins.
Rule 3: A double bond to an atom counts as two single bonds to that atom. For example, a C=O is treated as if carbon is bonded to two oxygen atoms (and oxygen is bonded to two carbon atoms).
Once priorities are assigned on both carbons of the C=C:
flowchart TD
A["Molecule with C=C<br/>double bond"] --> B{"Does each C of the C=C<br/>have 2 DIFFERENT groups?"}
B -->|"No"| C["No E/Z isomerism<br/>possible"]
B -->|"Yes"| D["Apply CIP priority rules:<br/>Higher atomic number<br/>= higher priority"]
D --> E{"Are the two higher-priority<br/>groups on the SAME side?"}
E -->|"Yes — same side"| F["Z isomer<br/>(zusammen = together)"]
E -->|"No — opposite sides"| G["E isomer<br/>(entgegen = opposite)"]
Question: Assign E or Z to the following molecule: 1-bromo-1-chloropropene, CHBr=CClCH₃.
Solution:
On C-1: the attached groups are Br (atomic number 35) and H (atomic number 1). Br has higher priority.
On C-2: the attached groups are Cl (atomic number 17) and CH₃ (C, atomic number 6). Cl has higher priority.
Diagram description: Draw the C=C horizontally. On the left carbon, Br is above and H is below. On the right carbon, Cl is above and CH₃ is below. The higher-priority groups (Br and Cl) are both above the double bond — they are on the same side.
Therefore this is the Z isomer: Z-1-bromo-1-chloropropene.
If Br and Cl were on opposite sides, it would be the E isomer.
E and Z isomers have different physical properties:
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