Materials Included In Kit

Additional Materials Required

Prelab Preparation

To prepare the 0.5 g/mL solution of sucrose, weigh out 125 g of sucrose, and place it in a 500 mL beaker. Slowly add 250 mL of water, and stir the solution until all the solid has dissolved.

To prepare the 0.5 g/mL solution of levulose, weigh out 125 g of levulose, and place it in a 500 mL beaker. Slowly add 250 mL of water, and stir the solution until all the solid has dissolved.

Safety Precautions

Both sucrose and levulose are considered nonhazardous according to GHS classifications. Although these materials are considered nonhazardous, unpredictable reactions among chemicals are always possible. These products should be treated as laboratory chemicals and are not for consumption. Wear chemical-resistant gloves and chemical splash goggles.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Excess solid sucrose can be handled according to Flinn Suggested Disposal Method #26a, and sucrose solution according to Flinn Suggested Disposal Method #26b. Excess solid levulose can be handled according to Flinn Suggested Disposal Method #26a, and levulose solution can be handled according to Flinn Suggested Disposal Method #26b.

Lab Hints

• Using monochromatic light produces the most consistent results, but the experiment can still be completed with white light.
• If using white light, the different specific rotations for the different wavelengths can be observed by the solution changing color when it is near the angle of extinction.

Teacher Tips

• Compounds with a very high specific rotation can sometimes be mistaken for a compound of lower rotation unless measurements under different conditions are made. You can encourage students to think about this by asking them how they would tell the difference between a +90° and a –270° rotation.

Sample Data

Sucrose

Plotting and analyzing these data gave a specific rotation of 62° dm^–1 mL g^–1

The literature specific rotation of sucrose is 66° dm^–1 mL g^–1

Levulose Plotting and analyzing these data gave a specific rotation of –80° dm^–1 mL g^–1

The literature specific rotation of levulose is –92° dm^–1 mL g^–1

Teacher Handouts

12338_Teacher1.pdf

Introduction

Build your very own polarimeter from a few simple materials. Polarimeters are regularly used by chemists to measure the degree to which polarized light is rotated by optically active compounds. For a certain class of isomers, known as enantiomers, measuring differences in the direction of rotation is often one of the only easy methods for discriminating between the isomers.

Concepts

• Optical activity
• Polarized light
• Electromagnetic waves
• Isomerism

Background

Isomers are molecules that have the same formula but different atomic arrangements. Broadly speaking isomers can be divided into two categories: constitutional isomers and stereoisomers. Constitutional isomers differ in the connectivity of the atoms. For example, both dimethyl ether and ethanol have the molecular formula C₂H₆O. However, the oxygen in dimethyl ether is attached to both carbon atoms whereas the oxygen in ethanol is bound to a single carbon atom and a hydrogen atom (see Figure 1).

Stereoisomers have the same atomic connectivity, but different arrangements in space. You can see an example of this if you look at your hands. Both hands have four fingers and a thumb, with the same order of the fingers; however, your hands are not identical. If you overlap your thumbs and fingers you will see that your palms are now pressed together. If you rotate one of your hands so that the palms now face the same direction, the thumbs will no longer be together. Your hands are mirror images and therefore not superimposable. This makes them stereoisomers. Molecules that are non-superimposable mirror images of each other are called enantiomers.

Enantiomers can be very difficult to distinguish from each other. They have identical colors, melting points and boiling points. One difference that can potentially be used to distinguish between two enantiomers is how they interact with polarized light. Optically active compounds, such as enantiomers, rotate polarized light in either a clockwise or counterclockwise direction. For a pair of enantiomers, the direction of rotation will be reversed for the mirror image.

As shown in Figure 2, polarimeters use two polarizing films to measure the degree of rotation due to the presence of an optically active compound. The sample is placed between the two films, and a detector is used to measure the angle of rotation. For clockwise rotation, the rotation will be reported as positive. Historically this compound is given a D prefix. For counterclockwise rotation, the rotation will be reported as negative. This compound is given an L prefix. In this laboratory, you will build your own polarimeter, and then use it to measure the degree of rotation for a solution of sucrose and levulose (D-fructose). Both sucrose and levulose are naturally occurring and optically active. Each is also one half of an enantiomeric pair. However, the enantiomers are not naturally occurring and are substantially more difficult to obtain and consequently more expensive. As part of your investigation, you will observe how the quantity of sample relates to the degree of rotation and ultimately calculate the specific rotation. Specific rotation is defined as the number of degrees by which a sample with a concentration of 1 g/mL will rotate light traveling through a 1 dm (1 dm = 10 cm) sample cell. Commonly, specific rotation is recorded using light with a wavelength of 589 nm since the degree of rotation is wavelength dependent.

Materials

Safety Precautions

Both sucrose and levulose are considered nonhazardous according to GHS classifications. Although these materials are considered nonhazardous, unpredictable reactions among chemicals are always possible. These products should be treated as laboratory chemicals and are not for consumption. Wear chemical-resistant gloves and chemical splash goggles.

Procedure

Polarimeter Construction

1. Cut out a circular piece of polarizing film the same diameter as the short tube (see Figure 3).
   {12338_Preparation_Figure_3}
2. Affix the piece of polarizing film to one end of the short tube. Note: This is best achieved with hot glue, but tape can be used.
3. Insert the short tube into the narrower of the two cardboard tubes so the polarizing film is down inside the tube. This can be quite a snug fit. If the small tube is only loosely held by the longer tube, glue or tape it in place (see Figure 4).
   {12338_Preparation_Figure_4}
4. Cut out a circular piece of polarizing film the same diameter as the cardboard rings.
5. Place the film between the two cardboard rings and tape them together (see Figure 5).
   {12338_Preparation_Figure_5}
6. Place the slush cup lid over the narrow end of the plastic cup.
7. Mark where the lid touches the cup, and cut along the marked line (see Figure 6).
   {12338_Preparation_Figure_6}
8. Make a 1.4 cm hole in the center of the plastic cup The easiest way to make this hole is to heat a cork borer over a flame and then press it against the center of the cup. This hole is important because plastic will distort the plane polarized light so an opening is needed for it pass through. The hole must be smaller than the base of the vial to stop it from falling against the polarizing film.
9. Cut off the flared section of the plastic cup, leaving only the bottom and a 2–3 mm straight section.
10. Place the section of slush cup lid into the bottom of the plastic cup.
11. Affix the cup-lid assembly to the top of the taped rings with hot glue or tape (see Figure 7).
    {12338_Preparation_Figure_7}
12. Insert the taped rings into the wider tube with the plastic sample holder pointing up (see Figure 8). Note: You might need to put more tape on the outside of the rings to ensure a snug fit.
    {12338_Preparation_Figure_8}
13. Use the narrow tube to push the taped rings to a height of about one third that of the wide tube.
14. Cut out the paper protractor and remove the central unmarked section of paper.
15. Tape the protractor to the top of the wide tube (see Figure 9). This is best done by sticking the tape to the inside of the tube and cutting it in half to form a Y shape that can then be folded over onto the protractor.
    {12338_Preparation_Figure_9}
16. Insert the narrow tube into the wider tube (see Figure 10) to complete your polarimeter.
    {12338_Preparation_Figure_10}

Experiment

Part 1. Finding the zero point of the polarimeter.

1. Take your 30 mL vial and make a mark every 1 cm along the side of the vial with a permanent marker.
2. Fill the vial to the 6 cm mark with distilled water.
3. Remove the inner tube from the polarimeter.
4. Place the vial into the sample holder inside the polarimeter.
5. Reinsert the smaller tube.
6. Looking down through the polarimeter, rotate the inner tube until the light is completely blocked out.
7. Locate the zero on the protractor, and make a mark on the inner tube that lines up with it. This is your zero point.
8. Remove the inner tube.
9. Remove, empty and dry the sample vial.

Part 2. Analyzing the optical activity of sucrose and levulose.

1. Obtain approximately 20 mL of a 0.5 g/mL solution of sucrose.
2. Fill the sample vial to the 1 mL mark.
3. Place the sample vial inside the polarimeter, and find the position at which light is completely blocked out.
4. Record the location of the position in degrees from the zero point.
5. Remove the sample, and fill the vial to the 2 cm line.
6. Record the new position at which light is completely blocked out.
7. Repeat with measurements for the the vial filled to 3 cm, 4 cm, 5 cm, 6 cm, 7 cm and 8 cm lines.
8. Empty, clean and dry the sample vial.
9. Obtain approximately 20 mL of a 0.5 g/mL solution of levulose.
10. Repeat steps 2–8 to record a data set with the levulose.
11. Use your data sets to calculate the specific rotation of sucrose and levulose. This can be done either graphically or by calculating the specific rotation for each data point and taking an average.