1. Properties of Stereoisomers of Menthols
Experiment 9:
Properties of Stereoisomers of Menthols

2. Objectives
To study physical properties of stereoisomers using TLC and melting point analyses.
To study the optical rotation of enantiomers using polarimetry.
To practice assigning the absolute configuration of stereoisomers using the R-S convention.
To learn to identify the relationship between stereoisomers.

3. Menthol Stereoisomers
Each stereoisomer has 3 stereogenic centers 5 5 5 5 4 6 4 6 4 6 4 6 1 1 1 1 3 3 3 3 2 2 2 2
Since each menthol isomer has 3 stereocenters, menthol has a total of 8 stereoisomers.
(2n, where “n”= # of stereogenic centers)

4. Fischer Projections

5. Stereochemical Relationships
Diastereomers
Diastereomers
Enantiomers
Enantiomers
Diastereomers
Diastereomers

6. Diastereomers and Meso Forms
Look closely at these two… There is a plane of symmetry.
The top half of the molecules is identical to the bottom half!
The plane of symmetry makes them achiral,
although they do have chiral centers.

7. Identifying Stereochemical Relationship
Identify stereocenters.
Assign absolute configuration to each stereocenter.
Determine stereochemical relationship based on absolute configurations.
ENANTIOMERS
Different absolute configuration at ALL stereocenters
DIASTEREOMERS
Same absolute configuration at SOME stereocenters, different at some.

8. Physical Properties of Stereoisomers
Enantiomers:
SAME: mp, d, solubility.
DIFFERENT: direction of rotation of plane-polarized light.
Diastereomers :
DIFFERENT: mp, d, solubility, AND direction of rotation of plane-polarized light.

9. Table 9.1 Compound TLC Rf values (l)-menthol unitless!
2 decimal places ONLY!
(d)-menthol
(d)-isomenthol
(d)-neomenthol

10. Literature Melting Point (oC) Experimental Melting Point (oC)
Table 9.2
Compound
Literature Melting Point (oC)
Experimental Melting Point (oC)
(l)-menthol
(d)-menthol
(d)-isomenthol
(d)-neomenthol
XXX

11. Optical Activity & Polarimetry
Clockwise rotation = dextrorotatory (d )
Counterclockwise rotation = levorotatory (l )

12. Calculating Specific Rotation
(solvent)

13. Table 9.3 Concentration (c) (g/mL)
This solution is prepared for you by dissolving 3.0 g of menthol in 10 mL of ethanol!
Cell pathlength (l) (dm)
measure the height of the liquid in the polarimeter tube in cm, then convert to dm!
Temperature (t) (oC)
Room temperature = 25 oC
Observed optical rotation (o)
Enter the value for the reading from the polarimeter with the blank correction applied!
Literature value specific rotation (o)
-50o
Calculated specific rotation (o)
This is NOT the same thing as the OBSERVED ANGLE OF ROTATION! You must calculate this value!

14. Safety Concerns
Ethyl acetate, hexane, and ethanol are extremely flammable!
Sulfuric acid is very corrosive! Use extreme caution at all times!

15. WASTE MANAGEMENT
Place used TLC solvent from sample preparation and TLC developing chambers into container labeled, “Organic Waste (Stereochemistry)”.
Place all used TLC capillary tubes in the broken glass container.
Place used TLC plates and filter papers in yellow solid waste can.