Enantiomers Flashcards
What is the observed rotation (alpha)?
The observed rotation is how much we have to rotate the analyzer to allow planes of light that are rotated by enantiomers to pass through. In other words, it is the direction and degree that a compound rotates plane polarized light.
A positive rotation is also known as ______ and rotates light in the ____ direction.
Dextrorotatory (+), clockwise (right)
A negative rotation is also known as ______ and rotates light in the ____ direction.
Levorotatory (-), counterclockwise (left)
Which two factors affect the observed rotation?
The concentration of the compound and the length of the tube.
A compound with a specific rotation of +61 is ____?
Dextrorotatory
How do the specific rotations of enantiomers compare?
Enantiomers have specific rotations that are equal in magnitude but opposite in sign.
The dextrotatory (+) and levorotatory (-) rotations must be determined ______.
Experimentally. We cannot determine whether an enantiomer will rotate light in the positive or negative direction just from the R or S designation.
What is the formula for specific rotation?
Specific rotation = (Observed Rotation)/(Concentration x Length of Tube)
Length of the tube is in decimeters
How does specific rotation relate to observed rotation?
Think about specific rotation as a “standardized” value that normalizes the observed rotation under a standardized wavelength and concentration.
The presence of rotation suggests that the compound is ______.
Optically active
No rotation suggests that the compound is ____, which could be due to either _____ or ______.
Not optically active. This can result if there are no chiral molecules present or if we have a racemic mixture where the opposite rotations of each enantiomer cancel each other out.
A racemic mixture is when we have ____.
50% of both enantiomers.
We have a mixture of a D (+) and L(-) compound with an overall rotation of 5 degrees in the CCW direction. The D form rotates light 15 degrees in the CW direction and the L form rotates light 15 degrees in the CCW direction. What percent of each enantiomer do we have in the mixture?
We have a net CCW direction of 5 degrees, so we much have more of the L form of the compound. Note that the total net rotation of 5 is (1/3) of the “total” rotation of the L form (15 degrees).
This means that the remaining two thirds of the mixture must be a racemic mixture and split evenly between the enantiomers (i.e., 1/3 L and 1/3 D). Therefore, the compound must be 33% D and 66% L.
