Isomers of Organic Compounds (CIE AS Chemistry)

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

 

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!

 

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!