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Constitutional And Cis Trans Isomerism

Learn and apply Constitutional And Cis Trans Isomerism in the published Chemistry course sequence.

  • GCE A-Level H1 Chemistry 8873-2027
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H1 Organic Isomerism: Orientation

Isomers share a molecular formula but differ in arrangement. In H1, first decide whether the difference is connectivity or the spatial arrangement about a restricted C=C bond, then justify the classification from the structure.

H1 8873 scope
  • Study constitutional isomerism, including chain, positional and functional-group changes.
  • Study cis–trans isomerism where each alkene carbon has two different groups and a meaningful pair can be compared.
  • E/Z nomenclature is not required in 8873 or 9476. Optical isomerism belongs to the fuller H2 treatment.

Definitions (Must Know)

  • Isomers are different compounds with the same molecular formula.
  • Constitutional isomers have different atom-to-atom connectivity.
  • Chain isomers have different carbon skeletons.
  • Positional isomers have the same skeleton and functional group at different positions.
  • Functional-group isomers have different functional groups.
  • Cis–trans isomers have the same connectivity but differ in the arrangement of groups about a bond with restricted rotation.
  • Restricted rotation about C=C arises because rotation would require breaking the π bond.

Detailed Explanations

A. Constitutional isomerism changes connectivity

Butane and 2-methylpropane both have formula C₄H₁₀, but their carbon skeletons differ. Propan-1-ol and propan-2-ol both have formula C₃H₈O, but the OH group occupies a different position. Propanal and propanone both have formula C₃H₆O, but one is an aldehyde and the other a ketone.

B. A C=C bond locks an arrangement

A carbon–carbon single bond can usually rotate without breaking a bond. A C=C bond contains a σ bond and a π bond; rotation would destroy sideways p-orbital overlap, so the arrangement is locked.

C. Eligibility must be tested on both alkene carbons

Cis–trans isomerism is possible only when each alkene carbon is attached to two different groups. But-2-ene passes: each alkene carbon is bonded to H and CH₃. But-1-ene fails because its terminal alkene carbon is bonded to two H atoms.

D. Use cis and trans only when the comparison is clear

For but-2-ene, cis places the two CH₃ groups on the same side of C=C, while trans places them on opposite sides. Draw the double bond and all four attached groups explicitly before assigning the word.

Worked Examples

Modelled example 1

Enumerate the Isomers of C₄H₁₀

Core

Problem

Draw and name all constitutional isomers with molecular formula C₄H₁₀.
Study the worked solution

  1. Draw the unbranched skeleton

    Method

    Connect all four carbons in one continuous chain.

    Reason

    This accounts for the straight-chain connectivity before branching is considered.

    Working

    CH₃CH₂CH₂CH₃: butane.

  2. Change the carbon skeleton

    Method

    Use a three-carbon parent with one methyl branch on carbon 2.

    Reason

    This is the only distinct branched connectivity for four carbons.

    Working

    (CH₃)₃CH: 2-methylpropane.

  3. Check completeness

    Method

    Confirm that further redrawings duplicate one of these two connectivities.

    Reason

    Reversing or bending a chain does not change which atoms are bonded.

    Working

    Total constitutional isomers: 2.

Common misconception 2

Not Every Alkene Shows Cis–Trans Isomerism

Find and correct the mistake

Learner claim

A learner says but-1-ene and but-2-ene both show cis–trans isomerism because each contains C=C. Explain why but-1-ene fails the test but but-2-ene passes.

Try this before viewing the solution

But-1-ene
But-2-ene

View solution step by step

  1. Test but-1-ene

    Method

    List H and H on its terminal alkene carbon.

    Reason

    One carbon of C=C fails the requirement for two different attached groups.

    Working

    But-1-ene: no cis–trans isomerism.

  2. Test but-2-ene

    Method

    List H and CH₃ on each alkene carbon.

    Reason

    Restricted rotation preserves two arrangements because both carbons have two different groups.

    Working

    But-2-ene: cis and trans forms.

Challenge 3

Classify Two Constitutional-Isomer Pairs

Minimal support

Classification transfer

Classify the relationship in each pair and state the structural evidence: (a) propan-1-ol and propan-2-ol; (b) propanal and propanone.

Try this before viewing the solution

Alcohol pair
Carbonyl pair

Hints

Hint 1: same formula first
Confirm that each pair shares a molecular formula before naming the relationship.
Hint 2: locate the change
Ask whether the skeleton, position or functional-group class changes.
View solution step by step

  1. Classify the alcohols

    Method

    Identify propan-1-ol and propan-2-ol as positional isomers.

    Reason

    They share C₃H₈O and the same carbon skeleton and functional group, but OH occupies a different position.

    Working

    Same group, different locant → positional isomerism.

  2. Classify the carbonyl compounds

    Method

    Identify propanal and propanone as functional-group isomers.

    Reason

    They share C₃H₆O, but one has a terminal aldehyde group and the other an internal ketone group.

    Working

    Different functional group → functional-group isomerism.

Mind Stretchers

Attempt each task before opening the hint.

Mind stretcher 1: Classifying a formula familyExtension

Question. Propanal and propanone both have formula C₃H₆O. Explain precisely how they are related and why they are not cis–trans isomers.

Show Hint
Compare which atoms are bonded to the carbonyl carbon.
Show Answer
They are functional-group constitutional isomers: propanal has a terminal aldehyde carbonyl bonded to H, whereas propanone has an internal ketone carbonyl bonded to two carbon groups. Their connectivity differs, so the relationship is constitutional rather than cis–trans.

Mind stretcher 2: Predicting before drawingExtension

Question. Decide whether pent-2-ene shows cis–trans isomerism. Then describe the two arrangements without relying on the shape of a carbon chain drawn on paper.

Show Hint
List the groups attached to carbon 2 and carbon 3 separately.
Show Answer
Carbon 2 is bonded to H and CH₃; carbon 3 is bonded to H and CH₂CH₃. Both alkene carbons therefore have two different groups. In the cis arrangement the two H atoms are on the same side of C=C; in the trans arrangement they are on opposite sides.