Isomers of Organic Compounds (Cambridge (CIE) A Level Chemistry)
Revision Note
Deducing Isomers of a Compound
You should be able to deduce all possible isomers for organic compounds knowing their molecular formula
Worked Example
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
Worked Example
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 and there is no chiral centre 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!
Worked Example
How many stereoisomers are there of the compound with molecular formula C2H2Cl2?
Answer:
Step 1: Draw the possible structural formula of the compound
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
1,1-dichloroethene is a structural isomer of 1,2 dichloroethene and does not exhibit stereoisomerism
Step 3: Determine whether it is optical or geometrical isomerism
Optical? No, as there are no chiral centres
Geometric? Yes!
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