Wrote this up on another forum whilst I was revising, it could be useful to anyone doing/planning on doing A-Level chemistry or equivalent.
A molecule will only exhibit stereoisomers if the following are true:
This level of stereoisomerism only applies where there are only 2 different substituent groups on the double bonded carbon atoms, giving rise to two different formations. 'cis' means that the substituents are on the same side. 'trans' means that they are on opposite sides.
Take 1,2-dichloroethene for example:
There are two stereoisomers as there are two substituent groups on the alkene group: H and Cl.
E-Z
E-Z isomerism applies where the groups bonded to the alkene group are more complex. Cahn-Ingold rules are used to determine which of the two 'branches' are the most significant on each carbon. The 'E-Z' notation is derived from the German descriptors for the spatial arrangement:
Effect of Stereoisomerism
(E)- isomers will have a higher melting point due to the shape of the molecules. They can 'pack together' more easily.
(Z)- isomers will have a higher boiling point as the differences in electronegativities are not cancelled by the effect exhibited by (E) isomers. These isomers are therefore polar, and dipole-dipole interactions require a higher energy to break.
Summary
The cis-/trans- nomenclature is used only in what can be described as 'simple' cases: where there are two different functional groups attached to the double bonded carbons. E-Z nomenclature is used for anything more complex than this.
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Stereoisomers are molecules with a common structural formula, but different spatial arrangement: in contrast to isomers, which share a molecular formula but different structural formula.
A molecule will only exhibit stereoisomers if the following are true:
- It has a rigid double bond.
- It has two different substituent groups on each C atom in the double bond.
This level of stereoisomerism only applies where there are only 2 different substituent groups on the double bonded carbon atoms, giving rise to two different formations. 'cis' means that the substituents are on the same side. 'trans' means that they are on opposite sides.
Take 1,2-dichloroethene for example:
There are two stereoisomers as there are two substituent groups on the alkene group: H and Cl.
E-Z
E-Z isomerism applies where the groups bonded to the alkene group are more complex. Cahn-Ingold rules are used to determine which of the two 'branches' are the most significant on each carbon. The 'E-Z' notation is derived from the German descriptors for the spatial arrangement:
- E - Entgegen - Opposite (so like trans, the major groups are on opposite sides)
- Z - Zusammen - Same (so like cis, the major groups are on the same side)
Effect of Stereoisomerism
(E)- isomers will have a higher melting point due to the shape of the molecules. They can 'pack together' more easily.
(Z)- isomers will have a higher boiling point as the differences in electronegativities are not cancelled by the effect exhibited by (E) isomers. These isomers are therefore polar, and dipole-dipole interactions require a higher energy to break.
Summary
The cis-/trans- nomenclature is used only in what can be described as 'simple' cases: where there are two different functional groups attached to the double bonded carbons. E-Z nomenclature is used for anything more complex than this.
