Isomers of Organic Compounds (Cambridge (CIE) AS Chemistry): Revision Note

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 there is no geometrical isomerism. Next, check each structural isomer for a chiral centre: in 1,2-dibromopropane, CH₃CHBrCH₂Br, carbon 2 is bonded to four different groups (H, Br, CH₃ and CH₂Br), so it exists as two optical isomers. C₃H₆Br₂ therefore has 5 isomers in total: the 4 structural isomers shown below, one of which has two optical isomers

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!