Stereochemistry and polarimetry PDF

Preview

Summary

Lab 8: Stereochemistry and polarimetry...

Description

Will Davidson Chem 253-TA: Erome Hankore Lab 8: Sterochemistry and Polarimetry 2/10/15

Purpose: The purpose of this lab is to determine the difference in two stereoisomers and explains the polarimetry and stereochemistry of certain molecules. This experiment will also show which techniques are capable of identifying enantiomers and which ones are not.

Theory:

Stereoisomers can look very similar to each other but have entirely different properties. Stereochemistry is the study of the 3 dimensional make up of molecules and how it effects their properties. Constitutional isomers have the same molecular formula but different bonding patterns. Stereoisomers also have the same molecular formula but only differ in how they take up space 3 dimensionally. This lab will focus mainly on enantiomers, a type of stereoisomer which is a non-superimposable mirror image of an original molecule. Another type of stereoisomer, which are not enantiomers, are diastereomers which are non-mirror images that have different physical properties and different chemical properties. With enantiomers there is a positive version which rotates light to the right, and a negative version which rotates light to the left. This experiment will focus on (-) carvone and (+) carvone. Because of the different 3 dimensional make up of stereoisomers light interacts with them differently. Plane-polarized light can be used to measure enantiomers because it is light which has been passed through such a filter that only allows the waves to oscillate in one plane. When plane-polarized light is passed through certain substances, the plane of its vibration is rotated through a certain angle, and the angle of rotation is different for different substances and molecules that are optically active. A polarimeter is used to create this type of light and measure

its effect on molecules. A 50:50 mixture of a pair of enantiomers is known as a racemic mixture. In these mixtures the rotation of the two enantiomers cancel each other out and the overall optical rotation is zero. The observed rotation of a molecule is the angle that an optically activated substance rotates the plane of light around it, and the specific rotation uses length and concentration to make the observed rotation a standard value which can be compared to substances and used in experiments. Calculating the enantiomer excess of an enantiomer can show what proportion two enantiomers are present in a mixture. This technique will be used in this lab, the specific rotation being calculated by dividing the observed rotation by the path length times the sample concentration ( αs=αobs/lc). TLC will also be run with the (+) and (-) enantiomers but this will not give us any information because the molecules have the same polarity. Infrared spectrometry will also be used in this lab, but it will not be able to give us information about the two different enantiomers because the light is still vibrating on every plane, and has not been isolated to one plane like in polarimetry.

Reaction: Isomer

(+) Carvone specific rotation 128° - 118.25° = 9.75 9.75/(0.2*0.9600) = 50.78° (-) Carvone specific 108.5° - 118.25° = -9.75 -9.75/(0.2*0.9600) = -50.78°

(+)

Observed Degree of Rotation 128°

(-)

108.5°

Blank

118.25°

Unknown (Y)

121.75°

Unknown (Y) specific rotation 121.75°- 118.25° = 3.5 3.5/(0.2*0.9600) = 18.23° Enantiomer Excess (18.23°/61°)* 100% = 29.89 enantiomer excess 29.89° = (50.7) Xa + (-50.7) (1-Xa) Xa=(+ carvone) = 0.7943

(+) 79.43%

(-) 20.57%

Methods/Procedures This experiment was followed verbatim from the Organic Chemistry lab manual pages 70 through 72. No exceptions were made in these instructions and everything went as expected. One mistake was made when the TLC plate was discarded before measurements were made so no Rf values were recorded. The compounds in this experiment can be harmful to skin and eyes upon contact, so wear proper safety equipment. Also, both forms of limonene are flammable, so they should be kept away from flames and hot surfaces.

Observations/Results A mistake was made during this lab and measurements were not made in order to obtain data necessary for calculating Rf values of the substances. However this information would have been irrelevant because these results show that TLC is not able to give information about two different enantiomers. Because the molecules have such similar structure their polarities are identical and TLC is not useful. The different enantiomers smell differently because the body uses a “lock and key” mechanism for smell, meaning the shape of a molecule will determine how our brain processes

TLC RESULTS

the scent. According to our results our unknown had (+) carvone in excess, making up 79.43 percent of the mixture.

Discussion/Conclusion Everything in the experiment went as planned. After viewing all the results it is apparent that TLC and infrared spectrometry are not capable of showing information about different stereoisomers. The smell test and polarimetry were the only tests that could discern the different enantiomers. The results from polarimetry gave us enough information to calculate the amount of each enantiomer in the unknown mixture.

Exercises 1) The four methods used to examine the different enantiomers of carvone were TLC, IR, polarimetry, and a smell test. TLC and IR were unable to detect the difference in the molecules because they test chemical properties that are too similar between the compounds to tell apart like polarity. A smell test is effective because the different shaped molecules have a different effect on our sense of smell, and polarimetry works because it uses light which has had the waves isolated to one plane. 4) Racemic mixtures have a 50:50 mixture of enantiomers which cancel each other out to make the mixture optically inactive. This means the mold must have interacted with only one of the enantiomers and either converted it or removed it from the mixture, leaving only one of the enantiomers. After this happened the mixture would have stopped canceling each other out and become optically active.

5)...