Structural isomers are molecules with the same molecular formula but different arrangements of atoms in space. There are three main types of structural isomerism: 1. Chain isomerism occurs when carbon chains branch. Butane has two chain isomers with straight and branched arrangements. 2. Position isomerism keeps the carbon skeleton unchanged but moves important groups around it. C3H7Br has isomers where the bromine is on the end or middle. 3. Functional group isomerism contains different functional groups, belonging to different families, like propanal and propanone or propanoic acid and methyl ethanoate.

1. WHAT ARE THEY?
THESE ARE MOLECULES THAT HAVE THE SAME
MOLECULAR FORMULA BUT HAVE A DIFFERENT
ARRANGEMENT OF THE ATOMS IN SPACE.
THAT EXCLUDES ANY DIFFERENT ARRANGEMENT ARRANGEMENTS
WHICH ARE SIMPLY DUE TO THE MOLECULE ROTATING AS A WHOLE OR
ROTATING ABOUT PARTICULAR BONDS.
TYPES OF STRUCTURAL ISOMERISM
1. CHAIN ISOMERISM
2. POSITION ISOMERISM
3. FUNCTIONAL GROUP ISOMERISM

2. 1. CHAIN ISOMERISM
THESE ISOMERS ARISE BECAUSE OF THE POSSIBILITY OF BRANCHING IN
CARBON CHAINS.
FOR EXAMPLE THERE ARE TWO ISOMERS OF BUTANE.
Q: BY THE WAY, WHAT IS ITS MOLECULAR FORMULAE?
IN THE FIRST ISOMER, THE CARBON ATOMS LIE IN A “STRAIGHT CHAIN”
WHEREAS IN THE OTHER, THE CHAIN IS BRANCHED.
LEARNER ACTIVITY: DRAW THE TWO POSSIBLE STRUCTURES.
N.B. BE CAREFUL NOT TO DRAW “FALSE” ISOMERS WHICH ARE TWISTED
VERSIONS OF THE ORIGINAL MOLECULE.

4. 2. POSITION ISOMERISM
THE BASIC CARBON SKELETON REMAINS UNCHANGED, BUT IMPORTANT
GROUPS ARE MOVED AROUND ON THAT SKELETON.
e.g. THERE ARE TWO STRUCTURAL ISOMERS WITH THE MOLECULAR
FORMULA C3H7Br. IN ONE OF THEM, THE BROMINE ATOM IS ON THE END
OF THE CHAIN, WHEREAS IN THE OTHER, ITS ATTACHED IN THE MIDDLE.
ANOTHER SIMILAR EXAMPLE OCCURS IN ALCOHOLS SUCH AS C4H9OH OR

5. 3. FUNCTIONAL GROUP ISOMERISM
THE ISOMERS CONTAIN DIFFERENT FUNCTIONAL GROUPS –
THAT IS THEY BELONG TO DIFFERENT FAMILIES.
EXAMPLES INCLUDE PROPANAL & PROPANONE.
PROPANOIC ACID & METHYL ETHANOATE.
EXTENSION.
Q: DRAW THREE ISOMERS THAT EXHIBIT FUNCTIONAL GROUP
ISOMERISM.