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UNIT 7
Questions 31 - 35
Alkenes are organic compounds, which contain
a carbon - carbon double bond. As a result these types of compounds are able to
exhibit a type of isomerism known as Cis/Trans or geometric isomerism. Perhaps
the clearest example of this is given by the compound but-2-ene below
The compound on the left is
cis-but-2-ene, and that on the left is trans-but-2-ene. The two isomers can be
regarded as non-interconvertible. This nomenclature is clear for compounds of
this type where it is obvious which groups we are comparing the relative
positions of. However for tri and tetra substituted alkenes the situation is
not as clear. In such compounds we need to assign a set of priorities to these
compounds.
In the Cahn-Ingold-Prelog convention we look at the next
atom bonded to each carbon. We then assign the priority of each of these groups
according to the atomic mass of the element concerned. If the two groups of
higher priority are on the same side of the molecule, we give the assignment Z,
and if on opposite the assignment E. An illustration is given below.
Look at the following two compounds
Question 31
The correct
assignment of configuration of these molecules is
Question 32
In molecules
with two or more double bonds the configuration of each is determined
independently and incorporated into the name. If two groups on any one carbon
are the same only one assignment is required. Hence deduce the correct
configurations in the following examples
| A |
2E |
4E |
| B |
2E |
4Z |
| C |
4Z |
2E |
| D |
2Z |
4Z |
Question 33
And here
| A |
2E |
4Z |
| B |
2Z |
4Z |
| C |
3Z |
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| D |
3E |
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This idea can indeed be extended to far more
complex molecules, such as the compound retinal. In the mechanism of vision
this compound is isomerised by the presence of light as indicated below
Question 34
This isomerism
in fact involves a change around only one double bond. Which is this and what
is the correct change.
A bond between C4 and C5 ; from E to Z
B bond between C9 and C10 ; from Z to E
C bond between
C13 and 14; from Z to E
D bond between C15 and C16, from E to Z
Question 35
Carbon - nitrogen double bonds are present
in compounds called imines and certain other derivatives. Some of these can
also exhibit the same type of isomerism as alkenes. The key difference is that
the lone pair on the nitrogen atom takes the place of one of the bonds. Study
the following two molecules and suggest the appropriate configurations.
SECTION
ANSWERS
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