3.1.4 Structural Isomerism

• Straight, branched and cyclic organic molecules are also called aliphatic compounds as long as they do not contain a benzene ring

Straight-chain

• Straight-chain organic molecules are those in which the carbon atoms are connected to each other in one continuous chain

Pentane is a straight-chain organic molecule as the carbon atoms are connected in one continuous chain

Branched

• Branched organic molecules have side groups attached to the main chain of carbon atoms

2-methylbutane is a branched organic molecule as the main chain (consisting of 4 carbon atoms) has a methyl branch

Cyclic

• Cyclic organic molecules are those in which the carbon atoms are connected to each other in a ring shape

Cyclopentane is a cyclic organic molecule as the carbons are attached to each other in a ring structure

• 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 three different types of structural isomerism:
  • Chain isomerism
  • Positional isomerism
  • Functional group 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 butanol the functional group is located on the first carbon and in 2-butanol on the second carbon

Functional group isomerism

• When different functional groups result in the same molecular formula, functional group isomers arise
• The isomers have very different chemical properties as they have different functional groups
  • For example, butanol and ethoxyethane

Both compounds have the same molecular formula however butanol contains an alcohol functional group and ethoxyethane an ether functional group

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

Answer

Step 1: Draw the structural formula of the compound

Step 2: Determine whether it is a stereo or structural isomer

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

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

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