Isomerism Flashcards
When does isomerism occur?
Isomerism occurs when 2 or more compounds have the same molecular formula but different arrangement of the atoms in a molecule
Define constitutional isomerism and what are the types of constitutional isomerism?
Isomers whose atoms have a different connectivity. Chain isomerism, positional isomerism and functional group isomerism.
How do chain isomers arise? And how do their chemical and physical properties vary?
Arise due to different arrangement of carbon atoms in their carbon skeleton. These molecules differ in their degree of branching.
Similar chemical properties as their functional groups remain unchanged. Physical properties vary.
Define stereoisomerism and what are the types of stereoisomerism?
Isomers who have the same connectivity but different arrangement of atoms in 3D shape. Cis-trans isomerism.
How do positional isomers arise? And how do their chemical and physical properties vary?
Positional isomers belong to same homologous series, and they arise due to functional group located at different positions.
Similar chemical properties as their functional groups remain unchanged. Physical properties vary.
How do functional group isomers arise?
Functional group isomers have different functional groups and so have different chemical and physical properties.
Criteria to exhibit cis-trans isomerism
Restricted rotation about C-C bond due to presence of pie bond.
2 different groups of atoms bonded to each of the C atoms.
*Cis-trans isomerism also possible due to restricted rotation about sigma bonds in ring structure. Must be at least 2 substituent groups bonded to the ring.
Differences between cis-trans isomers. Melting/ Boiling point.
Trans-isomer usually has slightly higher melting point than cis-isomer as packing in crystalline structure is more efficient.
Cis-isomer usually has higher boiling point than trans-isomer as cis-isomer is polar and trans-isomer is non-polar. Hence cis-isomer is held by intermolecular pd-pd interactions which will be stronger than the intermolecular id-id interactions between trans-isomer.
Formula to calculate number of cis-trans isomers
2^n, where n = no. C-C double bonds that can exhibit cis-trans isomerism.
