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Stereochemistry, a subdiscipline of chemistry, studies the spatial arrangement of atoms that form the structure of molecules and their manipulation. [1] The study of stereochemistry focuses on the relationships between stereoisomers, which are defined as having the same molecular formula and sequence of bonded atoms (constitution) but differing in the geometric positioning of the atoms in space.
Traditionally, double bond stereochemistry was described as either cis (Latin, on this side) or trans (Latin, across), in reference to the relative position of substituents on either side of a double bond. A simple example of cis–trans isomerism is the 1,2-disubstituted ethenes, like the dichloroethene (C 2 H 2 Cl 2) isomers shown below. [7]
As usual, if the molecule itself is oriented differently, for example, with H towards the viewer, the pattern may be reversed. The L form is the usual one found in natural proteins. For most amino acids, the L form corresponds to an S absolute stereochemistry, but is R instead for certain side-chains.
For example, dibromocarbene and cis-2-butene yield cis-2,3-dimethyl-1,1-dibromocyclopropane, whereas the trans isomer exclusively yields the trans cyclopropane. [ 4 ] This addition remains stereospecific even if the starting alkene is not isomerically pure, as the products' stereochemistry will match the reactants'.
For example, tartaric acid can exist as any of three stereoisomers depicted below in a Fischer projection. Of the four colored pictures at the top of the diagram , the first two represent the meso compound (the 2 R ,3 S and 2 S ,3 R isomers are equivalent), followed by the optically active pair of levotartaric acid (L-( R,R )-(+)-tartaric acid ...
The concepts of syn and anti addition are used to characterize the different reactions of organic chemistry by reflecting the stereochemistry of the products in a reaction. The type of addition that occurs depends on multiple different factors of a reaction, and is defined by the final orientation of the substituents on the parent molecule.
There are many more pairs of diastereomers, because each of these configurations is a diastereomer with respect to every other configuration excluding its own enantiomer (for example, R,R,R is a diastereomer of R,R,S; R,S,R; and R,S,S). For n = 4, there are sixteen stereoisomers, or eight pairs of enantiomers.
Determining stereochemistry in atropisomers using the helicity rule follows the priority: front substituent A > backward substituent A > front substituent B > backward substituent B Determining the axial stereochemistry of biaryl atropisomers can be accomplished through the use of a Newman projection along the axis of hindered rotation.