Stereochemistry of complex molecules
A single molecule may contain several stereogenic elements. Depending on the configuration of each one, we get different compounds whose chemical, physical and physiological properties can be very different.
Aldohexoses: 4 stereocentres, 16 stereoisomers
An excellent example is that of sugars (carbohydrates) with six carbon atoms, C6H12O6 (HOCH2–CHOH–CHOH–CHOH–CHOH–CHO), known as aldohexoses. They bear four stereocentres and therefore have 24 = 16 stereoisomers. Here are 8 of them. Which isomers are still missing?
- D-(+)-Allose RRRR m.p. 148 °C [α]20D +15°
- D-(+)-Altrose SRRR m.p. 103 °C [α]20D +30°
- D-(+)-Glucose RSRR m.p. 154 °C [α]20D +53°
- D-(+)-Mannose SSRR m.p. 136 °C [α]20D +14.8°
- D-(−)-Gulose RRSR m.p. 131 °C [α]20D −23°
- D-(−)-Idose SRSR [α]20D −32°
- D-(+)-Galactose RSSR m.p. 169 °C [α]20D +80.2°
- D-(+)-Talose SSSR m.p. 134 °C [α]20D +19°
Fischer projections
Emil Hermann Fischer (1852-1919) devised a simple way to represent the stereocentres of such molecules.
In the so-called Fischer projections every tetrahedral carbon is drawn as a cross in which the horizontal lines point towards the viewer and the vertical ones point away.
Enantiomers and diastereomers
Stereoisomers related to each other as object and non-superimposable mirror image are called enantiomers.
Stereoisomers that are not related as object and mirror image are called diastereomers.
Enantiomers always come in pairs. For each enantiomer, every other stereoisomer that is not its pair is a diastereomer.
Meso forms
Two identically substituted stereocentres give rise to only three stereoisomers. One of them is called the meso form and is not chiral.
These are examples of meso forms. They look chiral because they have several stereocentres, but the overall symmetry of the molecule cancels chirality: the mirror image of a meso form is always superimposable.
