Basic Organic Nomenclature
So far we have met the possibility of isomers occuring because the
carbon atoms may be bonded together in different ways. These are
constitutional or positional or regioisomers. Recall that naming
these requires the identification of both the longest chain, and the
position of the branches on that chain.
Another way in which atoms can be arranged differently from one molecule to another is spatially. That is, if you examine the order in which the atoms are bonded (the connectivity), it is the same for all isomers. What differs, making the compounds isomers, is the way the atoms are organized spatially. Compounds which have the same atoms and bond connectivity but which differ in the spatial arrangement of the atoms such that the two arrangements cannot be interconverted at room temperature are called stereoisomers.
Note, it is assumed that single (2-electron) bonds in open chain compounds can rotate about the bond axis at room temperature and so conformations, which differ by rotation around single bonds, are not considered to be stereoisomers.
The rigid structure of a ring, even though it is made up of single bonds, prohibits free rotation of the atoms in the ring and so leads to the possibility of stereoisomers.
For example, look at the two arrangements of the methyl branches on the two isomers of 1,2-dimethylcyclopropane.
Compound A :
Compound B :
Use the molecular models for the following:
2. Note that for one isomer the two methyl groups appear on the same side of the plane of the ring; for the other, they appear on different sides of the plane of the ring.
3. Note which compound, A or B, has the two methyls on the same side of the plane of the ring, and which one, B or A, has them on opposite sides.
Because the carbons in the ring cannot rotate around the connecting bonds, the two compounds are not interconvertable without breaking the ring open (which won't occur at room temperature!). These two compounds are stereoisomers of each other.
To name stereoisomers in which a ring is the cause of the stereoisomerism, the prefixes cis- and trans- are often used. In printed material the cis and trans are often italicised, as here
trans means 'across' and so is used to name isomers in which the two groups are positioned across the ring, i.e. on opposite sides of the plane of the ring. Thus compound A, with the two methyls on opposite sides of the plane of the ring, is named trans-1,2-dimethylcyclopropane.
cis is used to designate groups on the same side of the plane of the ring. Thus compound B, above, is named cis-1,2- dimethylcyclopropane.
For the following compounds, look at the way the line structures are drawn to indicate the stereoisomers and then look at the molecules with RasMol. For large rings, the actual orientation of the atoms (as in the models) may have to be studied quite carefully to determine whether they are the cis- or trans-forms.
Here are a few more examples, named.
3. cis- and trans-1,2-dimethylcyclopentane:
3. For cyclohexane isomers, the puckered ring structure leads to some problems in seeing the trans isomer. The three models below show one form for the cis (first) and two forms for the trans isomer.
cis- and trans-1,2-dimethylcyclohexane:
Review Cycloalkanes (i)
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