Worked example 1
Draw and name all structural isomers of C₅H₁₂.
Try it first: Start with the straight chain, then shorten it by one carbon and branch.
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Organic ChemistryIsomerismContent level: Core 18 min
Structural Isomers and Stereochemistry
What you'll be able to do: Tell structural isomers from stereoisomers, decide whether an alkene shows cis-trans isomerism, and spot a chiral carbon.
Introduction
Two molecules can share a formula and still be different substances. Sorting out how they differ — connectivity or arrangement in space — is what isomerism is about.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Define isomers and separate structural isomers from stereoisomers
- Draw the chain, position and functional-group isomers of a formula
- Decide whether an alkene shows cis-trans (geometric) isomerism
- Identify a chiral carbon and describe what enantiomers are
Lesson
Same formula, different molecule
Isomers share a molecular formula but are distinct compounds with distinct properties. C₄H₁₀ is either butane (bp −0.5 °C) or 2-methylpropane (bp −12 °C). The formula alone never identifies a compound.
Structural (constitutional) isomers
These differ in which atoms are bonded to which. Chain isomers differ in branching (butane vs 2-methylpropane). Position isomers move a group along the chain (1-propanol vs 2-propanol). Functional-group isomers belong to different families altogether (ethanol vs dimethyl ether, both C₂H₆O).
C₂H₆O → CH₃CH₂OH (alcohol) or CH₃–O–CH₃ (ether).
Stereoisomers: same connectivity, different arrangement
Stereoisomers have identical bonds but differ in how those bonds point in space. The two kinds you need are cis-trans (geometric) isomers around a rigid double bond or ring, and enantiomers, which are non-superimposable mirror images.
Cis-trans isomerism
A C=C cannot rotate, because the pi bond would have to break. If each doubly bonded carbon carries two different groups, the substituents are locked either on the same side (cis) or on opposite sides (trans). If either carbon carries two identical groups, no cis-trans pair exists.
Test both alkene carbons. Two identical groups on either one kills geometric isomerism.
Chirality and enantiomers
A carbon bonded to four different groups is a chiral centre. The molecule and its mirror image cannot be superimposed, giving a pair of enantiomers. Enantiomers share almost every physical property but rotate plane-polarised light in opposite directions and can behave very differently in biology, which is why drug chirality matters.
Two identical substituents on the same carbon means it is not a chiral centre.
Key ideas
Definition
Isomers
Different compounds with the same molecular formula.
Definition
Structural isomers
Isomers whose atoms are connected in a different order.
Definition
Stereoisomers
Isomers with the same connectivity but a different spatial arrangement.
Rule
Cis-trans requirement
Restricted rotation plus two different groups on each doubly bonded carbon.
Rule
Chiral centre
A carbon with four different groups attached.
Equation
Degrees of unsaturation
DoU = (2n + 2 − H) / 2
- n = carbon atoms
- H = hydrogen atoms
Equation
Alkene test for geometric isomerism
each C of C=C must carry two different groups
- C=C = the double bond that cannot rotate
Worked examples
Worked example 2
Does but-2-ene show cis-trans isomerism? Does but-1-ene?
Try it first: Write out the groups on each carbon of the double bond.
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Common mistakes
Counting the same skeleton twice because it was drawn flipped or rotated.
Why it's wrong: Rotating a drawing does not change connectivity, so it is the same isomer.
Check instead: Name each structure with IUPAC rules; identical names mean identical compounds.
Assuming every alkene has cis and trans forms.
Why it's wrong: If either doubly bonded carbon carries two identical groups, only one arrangement exists.
Check instead: Check the substituents on both alkene carbons before claiming geometric isomers.
Calling a carbon chiral without checking all four groups.
Why it's wrong: Two identical branches make the mirror image superimposable, so there is no enantiomer.
Check instead: List the four attached groups explicitly and confirm all four differ.
Practice this skill
No practice questions are available for this topic yet. You can still practice the whole unit.
What you should now know
Isomers share a molecular formula but are different compounds. Structural isomers differ in connectivity and come in chain, position and functional-group varieties. Stereoisomers keep the same connectivity and differ in space: cis-trans isomers arise when a non-rotating C=C or ring carries two different groups on each relevant carbon, and enantiomers arise from a carbon bonded to four different groups. Because properties follow structure, isomers of one formula can have very different boiling points and biological behaviour.
- Same formula plus different structure equals different compound
- Structural isomers: chain, position and functional-group types
- Cis-trans needs restricted rotation and two different groups on each alkene carbon
- A chiral carbon has four different groups; its mirror image is a separate enantiomer
- More branching means weaker dispersion forces and a lower boiling point
Sources and further reading
This lesson is original Chem Help content. No external sources were adapted.