Lesson 4: Polarimetry

Question 1

What is important to note about (R) or (S) configuration

Answer 1

It does not tell you whihc way light will be bent but only the configuration of the molecule

Question 2

How to confirm the identity of an unknown using light

Answer 2

-Specific rotation is an intrinsic property of an optically active molecules
-The published specific rotation values of authentic compounds can be used to confirm the identity of an unknown.
-Also if the specific rotation of one compound is (R) is +5.75 then the enantiomer will have a specific rotation of -5.75

Question 3

Enantiomerically Pure

Answer 3

It is when a sample only contains 1 enantiomer.

Question 4

Enantiomeric Excess

Answer 4

-It allows us to calculate the percentage of each enantiomer in a mixture
-For such a mixture, the observed rotation value is used together with the specific rotation of one of the enantiomers
-It tells you how much in excess 1 enantiomer is

Question 5

What is the ee formula

Answer 5

Observed rotation/specific rotation of pure sample *100

Question 6

what is the observed rotation of a pure enantiomer and ee?

Answer 6

observed= specific
-therefore the ee will be 100%

Question 7

Racemic mixture

Answer 7

-It is a mixture that contains 50% of both enantiomers
-It does not rotate the plane of polarisation because they cancel each other out and so the mixture is optically inactive
-they have the symbol (+-)

Question 8

Why are racemic mitures optically inactive

Answer 8

This is because for every molecule that rotates the POP in one direction, there will be another molecule that roates the POP in the opposite direction which cancels the effect out

Question 9

How to work out a mixure with 75% R and 25% S

Answer 9

+75-25=+50