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From the molecular pointed view, an achiral (NOT chiral) molecule will exhibit an internal symmetry plane, the basis for the mirror image (above); by contrast, a chiral molecule (below) will exhibit an asymmetric center (carbon) and therefore will not exhibit internal symmetry planes
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The molecule pictured above is called 2-propanol can we can find one internal plane of symmetry-- note the line passing through H-C-OH and the methyl groups (CH3) symmetrically present both above and below the symmetry line. Each half of the molecule will be an exact mirror image of the other half and will be superimposible.plane of symmetry
By contrast, the molecule pictured below, named 2-butanol exhibits no internal plane of symmetry. In the absence of a plane of symmetry, mirror images will NOT be superimposible and therefore constitute enantiomers of each other.
Furthermore, chiral molecules will have a carbon atom with four non-equivalent groups attached to it. This carbon is therefore asymmetric and is designated a stereocenter.
Examining 2-propanol above we note that the center carbon has two equivalent groups attached to it, the two methyl groups 2 x CH3.
By contrast, in the 2-butanol structure below, we can identify a carbon which has in fact four different groups attached to it: CH3, H, OH, CH2-CH3.
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We can identify common objects whose mirror images are superimposible.
For example, a baseball bat has a symmetry plane running through from top to bottom, but not so the baseball glove.
A glass of water has a symmetry plane, again from top to bottom of the glass, but not so with a pair of shoes.
Figures above and below from: "Enantiomers and Chirality" by R. H. Logan, Instructor of Chemistry, Dallas County Community College District, North Lake College. (©) 1997
Formal Definitions:
Chirality refers to a molecular property indicating "handedness". A chiral molecule is not superimposable on its mirror image, has no plane of symmetry, and rotates plane-polarized light. All molecules are chiral when they have one stereogenic (asymmetric) carbon.
Stereoisomer refers to isomers that have the substituent bonding pattern but have different 3D arrangement of the atoms.
Stereoisomers may appear as enantiomers or diastereomers.
Enantiomer refers to a type of stereoisomer. Recall that enantioners are nonsuperimposible mirror-images of each other.
Enantiomers have identical physical properties other than the direction that a solution of each enantiomer rotates plane-polarized light.
The asymmetric center of one enantiomer exhibits an opposite configuration, meaning R or S designation, compared to the other enantiomer.
The relationship between the absolute configuration (R or S) and the direction of rotation of plane-polarized light (dextrorotatory or levorotatory, i.e. d- or l- form) is not simple.
Diastereomers are stereoisomers that are NOT mirror images of each other, by contrast to entantiomers which have stereoisomers that ARE mirror images of one another.
Diastereomers have differing arrangement of atoms in space. One example would be Cis-Trans isomers. The comparison between cis- and trans-2-butene is an example of a pair of diastereomers that do not have asymmetric carbon atoms.
cis- and trans-2-butene
Drugs with two asymmetric centers have four diasteriomers (e.g., labetalol (Trandate, Normodyne): an alpha (α) and beta (β) -receptor antagonist)
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above to
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Many drugs, over
50% of all drugs actually, are chiral (existing as
enantiomeric pairs)
