We are going to continue with isomerism.
Enantiomers: non-superimposable mirror images. They can only occur when chiral centers are present. NOTE: two molecules that are enantiomers will always have opposite absolute configurations (one has R, the other will have S, and vice versa).
Optical Activity: a compound that rotates the plane of polarized light is said to be optically active.
**A pair of enantiomers will rotate plane-polarized light with equal magnitude, but in opposite directions
If it rotates clockwise it is said to be dextrorotarory (d), also denoted (+)
If it rotates counterclockwise it is said to be levorotatory (l), also denoted (-)
Specific rotation: the magnitude of rotation of plane-polarized light for any compound
**Racemic mixtures are not optically active
One last EXTREMELY IMPORTANT NOTE: (+) and (-) say nothing about whether the absolute configuration is R or S. Therefore, there is no correlation between the sign of rotation and the absolute configuration.
Diastereomers: stereoisomers that are not enantiomers; they are non-superimposable, non-mirror images. Take a look at the picture for a better understanding.
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