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.
