In simple terms
A friendly intro before the formal notes — no formulas yet.
Molecular Mix-Ups
Isomers are compounds with the same molecular formula but different structures. This difference can be in how the atoms are connected (structural) or how they are arranged in 3D space (stereo).
Think of isomers like anagrams. The words 'listen' and 'silent' use the exact same letters (atoms) and the same number of each letter (molecular formula), but they are arranged differently to create two distinct words (molecules) with different meanings (properties).
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Structural isomers have the same molecular formula but a different structural formula (connectivity). Use the simulator to build butan-1-ol and 2-methylpropan-2-ol from C₄H₁₀O.
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Functional group isomers are structural isomers with different functional groups. For example, an alcohol (R-OH) and an ether (R-O-R') can both have the formula C₂H₆O.
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E/Z isomerism occurs in alkenes due to restricted rotation around the C=C double bond. For this to happen, each carbon in the double bond must be attached to two different groups.
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Optical isomerism occurs when a molecule has a chiral centre: a carbon atom bonded to four different groups. This results in two non-superimposable mirror image molecules called enantiomers.
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Full topic notes
Formal explanation with the rigour you need for the exam.
1. Structural Isomerism
Structural isomers share the same molecular formula but differ in their structural formula, meaning the atoms are bonded together in a different sequence. This is like having the same set of building blocks but connecting them in different ways. There are three main types of structural isomerism you need to know.
Chain Isomerism: The carbon chain is arranged differently. For example, pentane (a straight chain of 5 carbons) and 2-methylbutane (a branched chain) are chain isomers with the formula C₅H₁₂.
Positional Isomerism: The carbon skeleton is the same, but a functional group or substituent is in a different position. For example, butan-1-ol and butan-2-ol are positional isomers.
Functional Group Isomerism: The isomers contain different functional groups. For example, ethanol (an alcohol, CH₃CH₂OH) and methoxymethane (an ether, CH₃OCH₃) are functional group isomers with the formula C₂H₆O.
2. Stereoisomerism
Stereoisomers have the same molecular formula and the same structural formula (the atoms are connected in the same sequence), but they have a different arrangement of atoms in three-dimensional space. We will look at two types: E/Z isomerism and optical isomerism.
2a. E/Z (Geometric) Isomerism
This type of isomerism occurs in compounds with a C=C double bond. The pi (π) bond prevents free rotation around the C=C axis. For E/Z isomerism to exist, two conditions must be met: there must be restricted rotation about a bond, and each carbon atom of the C=C double bond must be bonded to two different groups. The labels E (entgegen - opposite) and Z (zusammen - together) are assigned based on priority of the groups attached to each carbon, determined by the Cahn-Ingold-Prelog (CIP) rules (higher atomic number = higher priority).
Condition 1: Restricted rotation, typically due to a C=C double bond.
Condition 2: Each carbon in the C=C bond must have two different groups attached.
Z-isomer: The highest priority groups on each carbon are on the same side of the double bond.
E-isomer: The highest priority groups on each carbon are on opposite sides of the double bond.
A common mistake is to assume any molecule with a C=C bond will show E/Z isomerism. Always check the second condition! For example, propene (CH₃CH=CH₂) does not show E/Z isomerism because one of the double-bonded carbons is attached to two identical hydrogen atoms.
2b. Optical Isomerism
Optical isomerism arises when a molecule is 'chiral'. A chiral molecule is one that is non-superimposable on its mirror image, much like your left and right hands. The most common cause of chirality in organic molecules is the presence of a chiral centre, which is a carbon atom bonded to four different atoms or groups. These non-superimposable mirror images are called enantiomers.
Chiral Centre: A carbon atom bonded to four different groups. Also called an asymmetric carbon.
Enantiomers: A pair of molecules that are non-superimposable mirror images of each other. They are a type of optical isomer.
Optical Activity: Enantiomers are optically active. They rotate the plane of plane-polarised light. One enantiomer rotates it clockwise (+), and the other rotates it by the same angle anti-clockwise (-).
Racemic Mixture: A mixture containing equal amounts (50/50) of both enantiomers. It is optically inactive as the two rotations cancel each other out.
To quickly identify a chiral centre, scan the molecule for carbon atoms with four single bonds. Then, for each of these carbons, carefully list the four groups attached. Remember that a 'group' can be a single atom (like -H or -Br) or an entire chain (like -CH₂CH₃). If all four are different, you've found a chiral centre. Star (*) the chiral carbon in your diagrams to make it clear.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
Draw and name the structural isomers of the alkane with the molecular formula C₄H₁₀.
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The molecular formula C₄H₁₀ corresponds to an alkane (, where n=4).
But-2-ene exists as E/Z isomers. Draw the skeletal formula for both E-but-2-ene and Z-but-2-ene.
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The structure is CH₃CH=CHCH₃. Let's analyse the groups on the C=C double bond.
How it all connects
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Glossary
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Revision flashcards
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What are isomers?
Molecules that have the same molecular formula but a different arrangement of atoms in space.
Key takeaways
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Chain Isomerism: The carbon chain is arranged differently. For example, pentane (a straight chain of 5 carbons) and 2-methylbutane (a branched chain) are chain isomers with the formula C₅H₁₂.
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Positional Isomerism: The carbon skeleton is the same, but a functional group or substituent is in a different position. For example, butan-1-ol and butan-2-ol are positional isomers.
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Functional Group Isomerism: The isomers contain different functional groups. For example, ethanol (an alcohol, CH₃CH₂OH) and methoxymethane (an ether, CH₃OCH₃) are functional group isomers with the formula C₂H₆O.
Practice — then mark it
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Test Your Knowledge on Isomerism
Test Your Knowledge on Isomerism
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