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Isomers
Compounds that have the same molecular formula
but are not identical are called isomers. Isomers fall into two main
classes: constitutional isomers and stereoisomers.
Constitutional isomers
Constitutional isomers differ in the way
their atoms are connected.
Stereoisomers
Stereoisomers
(also called configurational isomers) differ in the way their atoms are arranged
in space. There are three kinds of stereoisomers: cis-trans isomers,
enantiomers and diastereomers.
Chirality Centers
A tetrahedral carbon atom that bears four different
substituents is variously referred to as a chiral center, a chiral
carbon atom, an asymmetric center, or an asymmetric carbon atom. A
compound with one asymmetric carbon, can exist as two different
stereoisomers are called enantiomers.
Optical Activity
Optical activity is the ability of a chiral substance to rotate the plane of
plane-polarized light and is measured using an instrument called a polarimeter.
If an optically active compound rotates the
plane of polarization clockwise, it is called dextrorotatory, indicated by (+).
If an optically active compound rotates the plane of polarization
counterclockwise, it is called levorotatory, indicated by (-)
Stereochemistry of
reactions
A stereospecific reaction is one which,
when carried out with stereoisomeric starting materials, gives a product from
one reactant that is a stereoisomer of the product from the other. A
stereoselective reaction is one in which a single starting material gives a
predominance of a single stereoisomer when two or more are possible.
Resolution of enantiomers
The separation of a
racemic mixture into its enantiomeric components is termed resolution. The first
resolution, that of tartaric acid, was carried out by Louis Pasteur in 1848
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