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-1-STEREOCHEMISTRY AND STEREOELECTRONICS NOTES
Stereochemistry in Organic Molecules
Conventions used in drawing molecules
Also, Fischer projections can sometimes be useful for acyclic
molecules with chiral centres. The carbon chain is drawn vertical
with the most oxidised atom at the top. By convention the
vertical bonds from the implied central C atom go back, behind
the plane of the paper, while the horizontal bonds come forward.
Clearly, a Fischer formula must not be:
1) rotated through 90o in the plane of the paper, nor
2) flipped over, i.e. rotated through 180o out of the plane of the paper.
A Fischer formula may be rotated through 180o in the plane of the paper.
They can also be used for two or more chiral centres with, conventionally, the longest
carbon chain vertical, the bonds to top and bottom atoms or groups go back, and all
the atoms or groups to one side or the other come forward.
If the Fischer formula has a horizontal plane of symmetry he compound is a meso
isomer and achiral. The number of stereoisomeric compounds is obtained by
enumerating all the distinguishable Fischer formulae, bearing in that:
a) rotation through 180o in the plane of the paper, and
b) cyclic exchange of three ligands on a chiral C atom
are both permitted.
4.33 and 4.34 are each a pair of enantiomers and they cannot be interconverted by
any low energy route. In contrast, 4.36 represents a single achiral compound in
which the two halves of the molecule are related by a mirror plane in the Fischer
formula. 4.36 is called meso-tartaric acid.
Symmetry in molecules
Chiral Point Groups
Symmetry elements in chiral point groups must all be simple rotation axes.
These Notes are copyright Alex Moss 2003. They may be reproduced without need for permission.
www.alchemyst.f2o.org
, -2-
Also, T, O and I.
Achiral Point Groups
Identical with its mirror image. Any point group with one or more of the elements σ, i
and Sn is achiral:
This includes, Cnv, Cnh, Dnd, Dnh, Td, Oh and Ih.
Classification of Isomerism in Compounds
The possible relationships between two molecules with the same molecular formula
are:
Diastereomeric molecules or compounds differ in all properties that depend on
structure. It is only when we define the constitutions of molecules in terms of single,
double bonds etc that we distinguish between diastereoisomers and constitutional
isomers. Thus, always use the term structure to include stereochemistry, otherwise
use the term constitutional isomer.
The relationship between the terms enantiomeric/diastereomeric and
enantiomer/diastereomer
Enantiomeric and diastereomeric are defined as mutually exclusive by the diagram
above. Enantiomer and diastereomer are not necessarily though. For example, there
are three stereoisomers tartaric acid: (+)-tartaric acid is both the enantiomer of (-)-
tartaric acid and a diastereomer of meso-tartaric acid:
Stereogenic Units in Molecules
A molecule has a stereogenic unit (e.g. a chiral centre, axis, or plane, or a double
bond, etc) if exchange of an appropriate pair of ligands generates an stereoisomer.
These Notes are copyright Alex Moss 2003. They may be reproduced without need for permission.
www.alchemyst.f2o.org
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