Isomers of Organic Compounds (CIE AS Chemistry)

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

Worked example: Isomers of dibromopropane

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

• 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: Deducing isomers of C₂H₂Cl₂

• 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

• Step 3: Determine whether it is optical or geometrical isomerism
  • Optical? No, as there are no chiral centres
  • Geometric? Yes!