Isomers - Structural (Edexcel International A Level (IAL) Chemistry): Revision Note
Exam code: YCH11
Isomers - Structural
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 two different types of structural isomerism you need to be aware of:
Chain isomerism
Positional 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 and butan-2-ol
Both compounds have an alcohol group and are made up of 4 carbons, 10 hydrogens and one oxygen, however in butan-1-ol the functional group is located on the first carbon and in butan-2-ol on the second carbon
Worked Example
Isomers of dibromopropane How many isomers are there of dibromopropane, 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
Answer: there are four structural isomers of C3H6Br2: 1,1-dibromopropane, 2,2-dibromopropane, 1,2-dibromopropane and 1,3-dibromopropane
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!
Answer: there are two structural isomers of C4H10: butane, CH3CH2CH2CH3, and 2-methylpropane, CH3CH(CH3)CH3
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