Basic Organic Nomenclature
7. Stereochemistry (iii)
|To repeat for emphasis:
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.
Detection of existence of enantiomers for a molecular structure.Chirality is a phenomenon due to the spacial organization of the whole molecule.
For certain molecules, such as hexahelicene above, the recognition that the molecule is chiral and so has an enantiomer is quite difficult, requiring that the three-dimensional structure of the molecule be viewed.
However, for many organic molecules, detection of the chirality is relatively easy. This stems from the fact that for simple organic molecules, the chirality arises from the way that atoms or groups are arranged in a tetrahedral fashion around the sp3 carbon.
In the example of bromochloroiodomethane above, it is the tetrahedral arrangement of the four different groups about the carbon which leads to the chirality. Thus:
for simple organic molecules, the presence or not of chirality can be determed by searching the molecule for an sp3 carbon atom which is bonded to four different atoms, or groups of atoms.
The presence of one (and only one) such carbon in a molecule indicates a chiral molecule.
Such a carbon is a chirality centre. (The term stereocentre is also used.)
A chirality centre is an atom bonded to several other atoms which has the property that interchanging two of the bonded atoms will lead to a different stereoisomer (not necessarily the enantiomer).
For carbon, since it has a tetrahedral geometry for the four groups, this means that only two stereoisomers can be formed by interchanging pairs of atoms.
Note that if more than one chirality centre exists in a molecule, it can no longer be asserted that it is chiral without examining the molecule as a whole.
Examples of molecules which are chiral because of a single chirality centre at a carbon:1.
The chirality centre is carbon number 2, and the four atoms or atom groups attached are:
hydroxy, ethyl, methyl, and hydrogen.
The chirality centre is carbon number 2, and the four different groups or atoms attached are:
chloromethyl, isopropyl, methyl, and hydrogen.
The chirality centre is carbon number 3, and the four different groups or atoms attached are
carboxymethyl, propyl, ethyl, and hydrogen.
Find the single chirality centre at carbon in the following structures.
Review: Chirality I The Phenomenon
Next page: More than one chirality centre
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