CHEM 0310 Lecture Notes - Lecture 5: Racemic Mixture, Polarimeter, Racemization

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6 Feb 2017
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Chapter 5 Stereoisomers
Chiral Molecules
-A molecule that is not superimposable on its mirror image
-Superimposable: The ability for an object to be placed over another object, usually in such a way that
both will be visible
-Non-superimposable: molecules cannot be placed on top of one another and give the same molecule
-Each isomer of the image-mirror pair is called an enantiomer
-Chiral means handedness like the mirror images of hands
-Chiral compounds can contain an atom that is connected to four different substituent groups such a
nucleus is called an asymmetric atom or a sterocenter
-Molecules with one sterocenter are always chiral (not all chiral molecules have stereocenters)
Achiral Molecules
-Compounds having structures that are superimposable on their mirror image
-Examples include: balls, water glass, and hammers
Symmetry in Molecules
-The symmetry in molecules helps to distinguish chiral structures from achiral ones
-The only criterion for chirality is the nonsuperomposable nature of object and mirror image
-For most organic molecules, we have to consider only one test for chirality: the presence or absence of
a plane of symmetry
-A plane of symmetry (mirror plane) is one that bisects the molecule so that the part of the structure
lying on one side of the plane mirrors part on the other side
-Chiral molecules cannot have a plane of symmetry
Optical Activity
-Two enantiomers have the same physical properties identical bonds and energy contents
-However, when a special kind of light, called plane-polarized light, is passed through a sample of one of
the enantiomers, the plane of polarization of the incoming light is rotated in one direction
-When the same experiment is repeated with the other enantiomer, the plane of polarized light is
rotated by exactly the same amount but in the opposite direction
-An enantiomer that rotates the plane of light in a clockwise sense of the viewer faces the light is
dextrorotatory and the compound is referred as the (+) enantiomer
-The other enantiomer which will effect counterclockwise rotation, is levorotatory and called the (-)
enantiomer
-This special interaction with light is called optical activity and enantiomers are called optical isomers
Polarimeter
-When ordinary light is passed through a material called a polarized, all but one type of light waves are
filtered away, and the resulting mean oscillates in only one plane: plane-polarized light
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