Is the stereochemistry in organic molecules purely theoretical
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Molecules with more than one stereocenter
Molecules can have more than one stereogenic center. In the case of compounds with n stereocenters there is the possibility of stereoisomers. A molecule with two stereocenters has up to = 4 stereoisomers, a compound with three stereocenters has up to 8 stereoisomers.
Even without referring to special structural formulas, one can consider which stereoisomers are possible, e.g. with 3 centers of chirality:
- Tab. 1
- Possible combinations with three stereocenters
|(RRS) (RSR) (SRR)||two (R), a (S)|
|(SSR) (SRS) (RSS)||a (R), two (S)|
The four stereoisomers of 2,3-dibromopentane
A simple example is 2,3-dibromopentane; this compound has two asymmetric carbon atoms. There are therefore the possible combinations of for the two asymmetric carbon atoms (S, R), (R, S), (R, R) and (S, S). These four structures are all different from one another (see below) and cannot be converted into a congruent configuration by rotating around a single bond. They are therefore stereoisomers. All four molecules are optically active compounds. However, since there can only ever be one pair of image and mirror image, we are dealing here with two pairs of enantiomers. Molecules I and II are one pair and molecules III and IV are the other pair of enantiomers.
What is the relationship between the enantiomers of enantiomeric pair I and those of pair II? They are not enantiomeric to one another because they do not relate to one another like an image and a mirror image. But they have the same connectivity, i.e. the links between the atoms are identical, i.e. they are stereoisomers. This corresponds exactly to the definition for diastereomers.
Definitions: Isomerism forms
Molecule III, for example, is enantiomeric to molecule IV, but diastereomeric to molecules II and I. The relationship shown below results for the four stereoisomers of 2,3-dibromopentane. You can use a decision tree to clarify which isomeric forms Two molecules with the same molecular formula are related to each other.
Align the molecules with the mouse for comparison.
Are the two connections
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Diastereomers have different physical properties, such as melting points, boiling points, rotation values, etc., while enantiomers differ only in an opposite rotation of polarized light and in the interaction with other enantiomers.
- A structure with several stereocenters does not have to have the maximum number of possible stereoisomers (); in the case of meso compounds, the number of possible stereoisomers is smaller.
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