AQA Chemistry AS/A-level Year 1
E–Z stereoisomerism Because of restricted rotation at the C == C bond, Z or cis and E or trans forms occur when there is suitable substitution: H C
H
R
R
H
C
R
C
Z or cis
Essential Notes E stands for entgegen (German for opposite). Z stands for zusammen (German for together). E–Z stereoisomerism is also known as geometrical or cis–trans isomerism.
H C R E or trans
It is not possible to have E–Z stereoisomerism when there are two identical groups joined to the same carbon atom in a double bond. Restricted rotation about a C == C bond arises due to the interaction between the two adjacent p-orbitals of the carbon atoms, forming a π-bond. Disruption of the π-bond requires significantly more energy than is available at room temperature, so that rotation does not occur readily. E means that two groups, which may be identical, are on opposite sides of the double bond and Z means they are on the same side. For example, but-2-ene exists as two forms that differ only in the arrangement of the bonds in space (see Fig 23).
Fig 23 The two E–Z stereoisomers of but-2-ene
CH3
H3C C
H3C C
C
H
H
H
C
H
CH3
E-but-2-ene (trans)
Z-but-2-ene (cis)
Note that methylpropene is a structural isomer, but not an E–Z stereoisomer of but-2-ene, because it is not possible to have E–Z stereoisomerism when there are two identical groups joined to the same carbon atom in a double bond (see Fig 24). Fig 24 Structure of methylpropene
H
CH3 C
H
C CH3
methylpropene
Methylpropene has two methyl groups on one carbon and two hydrogen atoms on the other. It is a structural isomer of but-2-ene, as also are but-1-ene and the cyclic alkanes cyclobutane and methylcyclopropane (see Fig 25). Fig 25 Other isomers of methylpropene
CH2CH3
H C
C
H
H but-1-ene
H2C H2C
CH2 CH2
cyclobutane
H2C CH
CH3
H2C
methylcyclopropane
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