1. Optical Isomers

2. Constitutional Isomers
Have same molecular form but different structural formula
Both isomers to the right are C4H10
These have different chemical and physical properties
B.P = -0.5C
B.P = C

3. Stereoisomers Appear to have same structure in 2D
In 3D have different structures
Are non-superimposable mirror images of each other
With modeling kit build a model: carbon atom with four different colors attached (use same size bonds!)
Look at your model in a mirror, build what you see in the mirror.
Are the two superimposable? Animation (watch Chirality)

4. How to draw 3D: Dash-Wedge Formula
Draw two straight lines about 110  from each other
Bond angle is 109.5
These represent the bonds in the plane of the page
Draw dashed line for the bond that extends behind the plane of the page
Draw wedge for bond that extends in front of the plane of the page

5. Drawing Enantiomers
Draw the molecule you built, using the dash-wedge formula and colored pens
Use a mirror to see the enantiomer
Sketch the mirror image, using the dash-wedge formula
Draw the two stereoisomers of 1-chloro-1-bromo-ethane.
There is no convention for which atom is attached to a wedge.

6. Chiral centers Carbon attached to 4 different substituent groups
Even if two substituent groups start with C, they could still be different
C=O, never chiral (C only attached to 3 things)
CH3, CH2 groups, never chiral because they are symmetric:

7. Vocabulary Stereoisomers have chiral centers. Carbon atoms can be
chiral or achiral
Asymmetric or symmetric
If two molecules are stereoisomers, they are also called enantiomers
Chiral molecules are optically active
Enantiomers are optical isomers

8. Properties of Optical Isomers
same physical and chemical properties
rotate plane polarized light
Each isomer rotates it in a different direction
What is plane polarized light?
Animation (watch Optical Activity)
Try polarizers

9. Optical Activity
Stereoisomers are said to be optically active if the rotate plane polarized light
Each type of enantiomer rotates light the same amount, but in different directions.
Amount and direction of rotation must be experimentally determined using a polarimeter

10. Optical Activity The amount of rotation depends on
Length of sample tube
Concentration of enantiomers
Both isomers present, rotation cancels out called: racemic mixture