Properties of Stereoisomers of Menthols

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Presentation transcript:

Properties of Stereoisomers of Menthols 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. To practice assigning the absolute configuration of stereoisomers using the R-S convention. To learn to identify the relationship between stereoisomers.

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)

Fischer Projections

Stereochemical Relationships Diastereomers Diastereomers Enantiomers Enantiomers Diastereomers Diastereomers

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.

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.

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.

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

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

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

Calculating Specific Rotation (solvent)

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!

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

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.