Stereochemistry
- Stereoisomerism
| Optical Isomers | Stereoisomers
| Cis-Trans Isomers | Tautomers
|
Optical
Isomers
Stereochemistry defines the position of atoms in space. A compund
is considered to be symmetrical if a line or a plane through
the molecule bisects it in two halves which are mirror images
of each other. Thus if one were to fold the bisected molecule,
the two halves would overlap each other. The following examples
(from MIT)
are illustrations of symmetry, and then of asymmetry.
Stereoisomers
The following figure on the left has two stuctures of a molecule
which are asymmetrical (like a carbon atom with four bonds substituted
by nonidentical groups). Both these molecules are optically
active (i.e. rotate the plane of polarized light) but the right
side structure is exact mirror image of the left side molecule,
so the two molecules will rotate the plane of polarized light
in opposite directions. These two are called enantiomers.
The image on the right adds another molecule which is an isomer
but not a stereoisomer. All three are optically active. All
figures are from
MIT.
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Cis-Trans
Isomers
Isomerization can
also arise from consideration of geometry of substituents. For
example, in unsaturated compounds like unsaturated fatty acids,
the position of substituents around C=C produce different geometric
isomers. In R-CH=CH-R1, the spatial position of H atoms determines
the geometry. If H atoms are in the same plane as the double
bond, the isomer is called Trans
but if these atoms are ina different
plane than the double bond, then the structure is called cis.
These compounds are not optically
active.
Tautomers
When a structure can exist in equilibrium with another structure,
the compounds are known as tautomers. For example: RCHOH-CHO
<----> RCO-CH2OH exist in equilibrium with each other and
are called tautomers. An example one comes across early in biochemistry
course is glyceraldehyde and dihydroxyacetone, which are tautomers.
