This document discusses chiral and achiral molecules, specifically enantiomers. It defines enantiomers as chiral molecules that are non-superimposable mirror images of each other. While enantiomers have identical chemical and physical properties, they differ in their ability to rotate plane-polarized light in opposite directions by equal amounts. This optical activity property is known as specific rotation. Racemic mixtures contain equal amounts of both enantiomers and are optically inactive, while enantiomeric excess describes mixtures with unequal amounts, neither being pure nor racemic. The document provides examples of calculating enantiomeric excess and optical purity percentages in chiral mixtures.

1. Specific Rotation

2. Chiral and Achiral Molecules
•Although everything has a mirror image, mirror
images may or may not be superimposable.

3. Not superimposed
(enantiomers)
(Chiral)
superimposed
(identical)
(Achiral)

4. Enantiomers
• Enantiomers are chiral molecules that are mirror
images of one another. Furthermore, the molecules
are non-superimposable on one another. This means
that the molecules cannot be placed on top of one
another and give the same molecule.
• Enantiomers have identical chemical and physical
properties (M.p & B.P) except for their ability to
rotate plane-polarized light (+/−) by equal amounts
but in opposite directions, this property of
enantiomer is called (Optical Activity).

5. Achiral compound with
plane polarized light

6. Chiral compound with
plane polarized light

8. (+) charge meaning:
(right )
(clockwise)
(dextro-rotatory)
(-) charge meaning:
(left)
(counter-clockwise)
(Levo-rotatory)
0 ° +  0 °
- 
CH3 CH3
C
H
HO2C
HO
0 ° +  0 °
- 
CH3
C
H
HO
HO2C

9. Enantiomeric state
(inside polarimeter tube)
1. Pure enantiomer
2. Mixtures of enantiomers
3. Enantiomeric Excess & Optical purity

10. 1. Pure Enantiomers
• It is solution containing a single
enantiomer and the other
enantiomer is entirely absent.
• It may also called:
– Optically pure (or)
– enantiomerically pure.
• Example: (R)-2-bromobutane is
an optically active compound
with a specific rotation of -23.1o.
100 %
R
100 %
S

12. Quiz
+
1.99
0.03 1
+
66.3

13. 2. Mixtures of enantiomers
It is solution containing equal
amounts of both enantiomers.
It is called a racemic mixture.
Racemic mixtures are not chiral as
they are not optically inactive. This is
a result of rotating the plane of the
light by the two enantiomers to the
same extent but opposite directions.
The net rotation is simply canceled
out.

14. 3. Enantiomeric Excess &Optical purity
• It is solution containing un-
equal amounts of both
enantiomers.
• It is mixture contains for
example:
– 80% (R)-2-bromobutane &
– 20% (S)-2-bromobutane.
• The sample is neither
optically pure, nor is it a
racemic mixture.

15. Enantiomeric Excess and Optical purity
• There is more of one
enantiomer than the other, or,
in other words one
enantiomer (in this case the
(R)-2-bromobutane) is said to
be in excess.
• In order to describe and
quantify the sample, the term
enantiomeric excess (ee) is
used.
• Enantiomeric excess tells
us how much more of one
enantiomer is present in the
mixture.

16. Enantiomeric Excess and Optical purity
• In this example,
the ee is determined by
the difference of
percentages of the two
enantiomers:

17. Enantiomeric Excess
can be measured by
Of
total un-equal
solution

18. Quiz
-
45
-
53
%
85
(
excess (S
The reminder = 15 %
where
7.5 % for (R)
7.5 % for (S)
So, The Total
where
7.5 % for (R)
92.5 % for (S)