Structural Isomerism (OCR AS Chemistry A)
Revision Note
Types of Structural Isomerism
Structural isomers are compounds that have the same molecular formula but different structural formulae
E.g. propene and cyclopropane
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
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)
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
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 ofdibromopropane, C3H6Br2?
Answer
Step 1: Draw the structural formula of the compound
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
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
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!
Examiner Tips and Tricks
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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