Basic Organic Nomenclature
7. Stereochemistry (iii)
Stereoisomers are compounds with the same molecular formula and the
same bond connectivity which differ from each other in the way that
the atoms are distributed in space.
Stereoisomers were met in this series on nomenclature in the discussions to name cycloalkenes (stereochemistry (i)) and alkenes (stereochemistry (ii)). In these cases, the nature of the ring (which prevents rotation about the C,C bonds in the ring) and of the C,C double bond (planar and, again, no free rotation at the C=C due to the nature of the bonding) lead to the possibility of two spacial arrangements around the double bond. These were identified as either cis or trans (in the case of cycloalkanes) or E or Z for alkenes.
For the alkenes:
In addition to stereoisomerism due to hindered rotation about bonds, a second form of stereoisomerism occurs in compounds which have a 3-dimensional structure. In this case stereoisomers occur because the molecule is such a shape that it and its mirror image are not superimposable, that is, they are different molecules.
Any object that has this property is termed a chiral object. We are used to seeing such objects, for example hands, and screws (the mirror image of a left-hand thread screw is a right-hand thread screw!).
Because molecules are three dimensional, chiral molecules exist: some molecules cannot be superimposed on their mirror image.
Two chiral molecules and their mirror images are given below, each as two separate images which can be individually rotated. With the pair, rotate the molecules to see if they are superimposable (that is imagine sliding one across on top of the other). In the first example, the two isomers are also shown in one frame as if a mirror were placed between them to further show the phenomenon.
Example 1. Bromochloroiodomethane.
In this case, rotate the models such that two of the halogens coincide directionally, then look at the other two and you will see that they are pointing in opposite directions: the two molecules cannot be superimposed, they are different molecules. In this case, to convert one isomer into the other would require that at least one bond in the molecule is broken and remade in a new direction. This is often difficult or impossible to do without putting in enough energy to destroy the molecule.
Example 2. Hexahelicene.
In this case the molecule has the shape of a screw thread, and so its mirror image is the opposite screw thread, and the two are not superimposable. Again, the two are different molecules: to convert one into the other would require distorting the molecule so that the two ends could pass. At room temperature, this much energy is not available, and the two isomers exist.
Enantiomers.When a molecule is chiral, the two forms (the molecule and its mirror image) are termed enantiomers.
enantiomers are mirror images of each other.
there are only two enantiomers for a chiral molecule.
Next page: Detecting Chirality in Molecules
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