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Basic Organic
Nomenclature
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Stereochemistry in Cyclic Alkanes
Structural or constitutional isomers are compounds with the same number
and types of atoms, but bonded together in different arrangements. These
types of compounds will have different names as the chains of carbon atoms
will differ. In cyclic structures it is possible to have another type of
isomer, a stereo isomer (specifically a geometric isomer). This occurs
because it is now possible to have atoms bonded together in the same arrangement,
but in different locations in space. This is due to the fact that there
can be no rotation about the carbon-carbon bonds in the ring, at least
not without breaking a bond, a feat that is possible in non-cyclic alkanes.
(Rotation about a C-C single bond (a sigma bond) is possible at room temperatures,
but these different conformations of the molecule are not isomers, but
rather are referred to as conformers).
For example compare the two isomers of 1,2-dimethylcyclopropane:
You should note:
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that the connectivity of the atoms is the same in both molecules, they
are both 1,2-dimethylcyclopropane
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that the methyl groups are in different relative locations in the two isomers,
on opposite sides in Isomer 1, and both on the same side of the ring in
Isomer 2.
The two compounds can be inter-converted only by breaking a C-C bond in
the ring. This will not occur normally so the two molecules are different
and their names needs to reflect this difference.
These geometric isomers are named with an additional prefix to indicate
the relative location of the two substituents. Isomer 1, with the two methyl
groups on opposite sides of the ring is the trans- isomer (note,
it is usually in italics). Trans indicates the substituents are
on opposite sides of the ring to one another. Isomer 2 is the cis-
isomer, cis indicates that the substituents are on the same side
of the ring.
Thus the correct name for the isomers are trans-1,2-dimethylcyclopropane
(Isomer 1) and, cis-1,2-dimethylcyclopropane (Isomer 2).
The following examples will help illustrate this idea further. You
should note there are several conventions in representing three dimensional
information in line diagrams. Solid lines are assumed to be in the plane
of the page, solid wedges come out of the page towards you, and dashed
wedges go back into, or behind, the plane of the pages. There are some
exceptions to these rules for specific molecules or types of drawings,
these will be pointed out as they arrive.
Note: the last two line diagrams are a standard way of drawing a cyclohexane
ring. This view is intended to represent the ring as viewed on edge. The axial
groups are then displayed as vertical lines up or down from the ring carbons,
and the equatorial groups are approximately horizontal lines. Compare it to
the 3 dimensional model and it will become apparent.
Tutorial Quiz 3
Next Page: Polycyclic Alkanes
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