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Experiment 9:

Experiment 9:. Properties of Stereoisomers of Menthols. Objectives. To study physical properties of stereoisomers using TLC and melting point analyses. To study the optical rotation of enantiomers using polarimetry.

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Experiment 9:

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  1. 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

  10. Table 9.2

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

  12. Calculating Specific Rotation (solvent)

  13. Table 9.3

  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.

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