Structural Isomerism (OCR A Level Chemistry)

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Types of Structural Isomerism

  • Structural isomers are compounds that have the same molecular formula but different structural formulae
    • E.g. propene and cyclopropane

 

An Introduction to AS Level Organic Chemistry Propene and Cyclopropane, downloadable AS & A Level Chemistry revision notes

Both propene and cyclopropane are made up of 3 carbon and 6 hydrogen atoms but the structure of the two molecules differs

 

  • There are three different types of structural isomerism:
    • Chain isomerism
    • Positional isomerism
    • Functional group isomerism

Chain isomerism

  • Chain isomerism is when compounds have the same molecular formula, but their longest hydrocarbon chain is not the same
  • This is caused by branching
    • E.g. pentane and 2,2-dimethylpropane

An Introduction to AS Level Organic Chemistry Chain Isomerism, downloadable AS & A Level Chemistry revision notes

Both compounds are made up of the same atoms, however the longest carbon chain in pentane is 5 and in 2,2-dimethylpropane it is 3 (with two methyl branches)

Positional isomerism

  • Positional isomers arise from differences in the position of a functional group in each isomer
    • The functional group can be located on different carbons
    • For example, butan-1-ol (or butanol) and butan-2-ol (or 2-butanol)

An Introduction to AS Level Organic Chemistry Positional Isomerism, downloadable AS & A Level Chemistry revision notes

Both compounds have an alcohol group and are made up of 4 carbons, 10 hydrogens and one oxygen, however in butanol the functional group is located on the first carbon and in 2-butanol on the second carbon

Functional group isomerism

  • When different functional groups result in the same molecular formula, functional group isomers arise
  • The isomers have very different chemical properties as they have different functional groups
    • For example, butanol and ethoxyethane

An Introduction to AS Level Organic Chemistry Functional Group Isomerism, downloadable AS & A Level Chemistry revision notes

Both compounds have the same molecular formula however butanol contains an alcohol functional group and ethoxyethane an ether functional group

Determining Isomers

  • You should be able to deduce all possible isomers for organic compounds knowing their molecular formula

Worked example

Isomers of dibromopropane

How many isomers are there of dibromopropane, C3H6Br2?

Answer

Step 1: Draw the structural formula of the compound

 An Introduction to AS Level Organic Chemistry Step 1 Isomers of dibromopropane, downloadable AS & A Level Chemistry revision notes 

Step 2: Determine whether it is a stereo or structural isomer

    • There is no restricted bond rotation around the C-C bond, so it is structural isomerism

Step 3: Determine whether it is a functional group, chain or positional isomerism

    • Functional group? No, as Br is the only functional group possible
    • Chain? No, as the longest chain can only be 3
    • Positional? Yes, as the two bromine atoms can be bonded to different carbon atoms

 An Introduction to AS Level Organic Chemistry Step 3 Isomers of dibromopropane, downloadable AS & A Level Chemistry revision notes

Worked example

Deducing isomers of C4H10

How many isomers are there of the compound with molecular formula C4H10?

Answer

Step 1: Draw the structural formula of the compound

 An Introduction to AS Level Organic Chemistry Step 1 Deducing isomers of C4H10, downloadable AS & A Level Chemistry revision notes

Step 2: Determine whether it is a stereo or structural isomer.

    • There is no restricted bond rotation around the C-C bond so it is structural isomerism

Step 3: Determine whether it is a functional group, chain or positional isomerism

    • Functional group? No, as there are no functional groups
    • Positional? No, as there are no functional groups which can be positioned on different carbon atoms
    • Chain? Yes!

 

An Introduction to AS Level Organic Chemistry Answer Deducing isomers of C4H10, downloadable AS & A Level Chemistry revision notes

Examiner Tip

Use a pencil when deducing isomers as it makes it easier to remove sections of your molecule

Careful: Always check your structures as it can be very easy to draw 2 identical "isomers" without realising that they are simply the same molecule viewed from the other side of your sheet of paper. You can always try naming your isomers to help

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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.