Isomerism SCH4U Spring 2012

Isomerism Isomerism occurs when there are two or more compounds (called isomers) with the same molecular formula but different arrangements of their atoms Isomers have different physical and chemical properties There are two main classes of isomerism: a) structural isomerism b) stereoisomerism

Structural isomerism Structural isomers are molecules with the same molecular formula but with different structural arrangements of the atoms There are three types of structural isomers: a) Chain isomers b) Positional isomers c) Functional group isomers

Chain isomers Isomers have different arrangements of the carbon chain E.g. C4H10

Chain isomers As the number of carbon atoms increases , the number of possible isomers increases How many isomers would C5H12 have? What are they? Use molecular kits, draw them

Positional Isomers Isomers have the same carbon skeleton but the functional group is in different positions in the molecule E.g. C6H4Cl2

Positional isomers What would be a positional isomer of butan-1-ol?

Functional group isomers Isomers have the same molecular formula but different functional groups E.g. C3H8O CH3CH2CH2OH, propan-1-ol H3COCH2CH3 , methoxyethane

Functional group isomers What would be a functional group isomer of cyclohexane?

Stereoisomerism Stereoisomers have identical molecular formula and the atoms are linked together in the same order but have different relative positions in space There are two types of stereoisomers: a) Geometric (or cis-trans ) isomers b) Optical isomers

Geometric isomers Geometric or cis-trans isomers exist because the (pie) π bond of the C=C bond prevents free rotation Cis – same side Trans – opposite side of the double bond E.g. 1,2-dichloroethene two different groups on the left-hand end of the bond and two different groups on the right-hand end.

Optical isomers This type of isomerism occurs when a molecule has one or more asymmetric carbon atoms i.e. a carbon atom linked to four different atoms or groups The optical isomers have a chiral centre and are mirror images of each other E.g. butan-2-ol One of the enantiomers is simply a non-superimposable mirror image of the other one.

Optical isomers Identify all chiral centre(s), if any: