AQA A Level Chemistry

Revision Notes

3.1.4 Structural Isomerism

Molecules: Straight, Branched & Cyclic

  • Straight, branched and cyclic organic molecules are also called aliphatic compounds as long as they do not contain a benzene ring

Straight-chain

  • Straight-chain organic molecules are those in which the carbon atoms are connected to each other in one continuous chain

 

An Introduction to AS Level Organic Chemistry Straight-Chained, downloadable AS & A Level Chemistry revision notes

Pentane is a straight-chain organic molecule as the carbon atoms are connected in one continuous chain

Branched

  • Branched organic molecules have side groups attached to the main chain of carbon atoms

 

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

2-methylbutane is a branched organic molecule as the main chain (consisting of 4 carbon atoms) has a methyl branch

Cyclic

  • Cyclic organic molecules are those in which the carbon atoms are connected to each other in a ring shape

 

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

Cyclopentane is a cyclic organic molecule as the carbons are attached to each other in a ring structure

Structural Isomerism

  • Structural isomers are compounds that have the same molecular formula but different structural formulae
    • Eg. 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
    • Eg. 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, butanol and 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

Deducing 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

Worked Example

Deducing isomers of C2H2Cl2

How many isomers are there of the compound with molecular formula C2H2Cl2?

Answer

Step 1: Draw the possible structural formula of the compound

 

An Introduction to AS Level Organic Chemistry Step 1 Deducing isomers of C2H2Cl2, downloadable AS & A Level Chemistry revision notes

Step 2: Determine whether it is a stereo or structural isomer
The compound has to be unsaturated for it to have molecular formula C2H2Cl2 ; Due to the double bond there is restricted rotation about the C-C bond; This compound will therefore display geometrical isomerism

  An Introduction to AS Level Organic Chemistry Answer Deducing isomers of C2H2Cl2, downloadable AS & A Level Chemistry revision notes

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