Optical Isomers Terms and Concepts
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Chiral – asymmetrical
Achiral – symmetrical
Chiral carbon – asymmetrical carbon by being bonded to 4 different groups
Optically active – ability to rotate plane polarized light – all chiral molecules are
optically active however you need a polarimeter in order to determine the direction
and angle the isomer rotates light
a. dextrorotary – rotates plane polarized light clockwise [ά] = (+)
b. levarotary – rotates plane polarized light counterclockwise [ά] = (-)
Enantiomers – non super imposable mirror images – only chiral molecules can have
enantiomers
a. opposite configuration (R vs. S) at all chiral centers
b. identical physical properties (ie. Boiling point, density etc) – difficult to
separate – the separation process is called resolution
c. rotate light in equal but opposite directions
Racemic mixture – an optically inactive solution that has 1:1 ratio of enantiomers
Enantiomeric excess – an optically active solution that contains an uneven ratio of
enantiomers
Diastereomers – stereoisomers that are not enantiomers – not limited to chiral
molecules like enantiomers
a. cis/trans or E/Z
b. partially opposite configuration – minimum of one chiral center for each isomer
must have identical configuration (if applicable)
c. different physical properties – can be separated physically for example via
distillation
d. there is no relationship as to how they rotate light with respect to each other
Meso molecule – achiral molecules that contain chiral centers
a. optically inactive
b. has diastereomers but don’t have an enantiomer
How to assign R vs. S configuration
a. Prioritize the 4 groups bonded to the chiral center
b. If group #4 is not on a wedge or dash rotate and redraw the molecule until group
#4 is on a wedge or dash
c. Draw an arrow from #1 to #2 to #3 – arrow may pass through #4
d. Assign configuration based on direction of arrow and orientation of #4
#4 on a dash
#4 on a wedge
Clockwise
Counterclockwise
R
S
S
R