3.1.3 isomerism

isomerism

same molecular formula, different arrangement of atoms within the molecule and different properties

structural isomerism

same molecular formula, different structural formula:

• chain

• functional

• positional

structure is different, displayed formula is different

chain isomers

isomers that occur due to branching in the carbon chain

functional isomers

different functional groups, so belong to different homologous series

examples:

• alkenes and cycloalkanes.

• carboxylic acids and esters.

• alcohols and ethers.

positional isomers

same functional group and same carbon backbone, but the functional group is positioned on a different C atom on the C chain.

stereoisomerism

same structural formula, different spatial arrangement of atoms

• E-Z isomerism occurs due to restricted rotation around a C=C bond where the two double bonded C atoms each have two different atoms or functional groups attached to them

E isomer

highest priority groups are on opposite sides of the C=C bond (opposite sides of the molecule)

Z isomer

highest priority groups are on the same side of the C=C bond (same side of the molecule)

rules for cis-trans isomerism

• each atom in the C=C bond must have 2 different groups attached but must have one of these groups in common.

• cis isomer = Z isomer = the group in common is on the same side.

• trans isomer = E isomer = the group in common is on opposite sides.

CIP rules for assigning priority of groups attached to C=C

• higher atomic number (proton number) = higher priority.

• atomic number is used because it is unique to each element.

• if atomic number is the same, compare the next atom in line with the highest atomic number, and assign priority.