CHEM 2152 Lecture Notes - Lecture 3: Azobenzene, Silica Gel, Tanning Bed

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30 Apr 2018

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6.3 Thin-Layer Chromatography

Obtain a 10-ctn x 3-cm strip of silica gel chromatogram sheet without a fluores

cent indicator. Handle the strip only by the sides in order to avoid contaminat

ing the plate with oils from your hands. Place one pencil dot about 1 cm from

the left side and about 1 cm from one end of the sheet and another about

1 cm from the right side the same distance from the bottom as the first. Using a

micro or capillary pipet, carefully apply a spot of a 10% solution of commercial

azobenzene in toluene, which you should obtain from your instructor, over one

of the pencil dots. Do not allow the spot to diffuse to a diameter of more than

1-2 mm during application of the sample. Allow the spot to dry and then

expose the plate to sunlight for one to two hours (or a sun lamp for about 20

min).

While irradiating the TLC plate, prepare a developing chamber as described

in Part A using a 9:1 mixture (by volume) of hexane and chloroform as the

developing solvent; there should be no flames in the vicinity.

When the irradiation is complete, apply another spot of the original solu

tion on the plate over the second pencil dot in the same manner as just

described and allow the strip to dry. Place the strip in the developing chamber,

being careful not to splash solvent onto the plate. Both spots must be above the

solvent level. Allow the solvent to move to within approximately 2-3 mm of

the top of the strip and then remove the strip. Mark the position of the solvent

front with a pencil, and allow the plate to air-dry. Note the number of spots

arising from each of the two original spots. Pay particular attention to the rela

tive intensities of the two spots nearest the starting point in each of the samples;

these are jyn-azobenzene.

Calculate the Revalues of each of the spots on your developed plate. In your

notebook include a picture of the developed plate drawn to scale as a permanent

record.

MsCoOiiDg ‘ii’®(sisllo80 Pour the unused mixture o f hexane and chloroform

used as eluent into the container for halogenated solvents, and put the dry

chromatographic plates in the hazardous solid waste container, since they

contain small amounts of azobenzene.

iKgROSB

1. Which of the two diastereomers of azobenzene would you expect to be

more thermodynamically stable? Why?

2. From the results of the TLC experiment with the azobenzenes, describe the

role of sunlight.

3. In a TLC experiment why should the spot not be immersed in the solvent in

the developing chamber? If you have time, see what happens when this is

done.

4. Explain why the solvent must not be allowed to evaporate from the plate

during development.

5. Explain why the spot should be no larger than 2 mm.

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Document Summary

Obtain a 10-ctn x 3-cm strip of silica gel chromatogram sheet without a fluores cent indicator. Handle the strip only by the sides in order to avoid contaminat ing the plate with oils from your hands. Place one pencil dot about 1 cm from the left side and about 1 cm from one end of the sheet and another about. 1 cm from the right side the same distance from the bottom as the first. Using a micro or capillary pipet, carefully apply a spot of a 10% solution of commercial azobenzene in toluene, which you should obtain from your instructor, over one of the pencil dots. Do not allow the spot to diffuse to a diameter of more than. Allow the spot to dry and then expose the plate to sunlight for one to two hours (or a sun lamp for about 20 min).

Related Questions

Gather 2 pieces of square filter paper, the scissors, a pencil, and a ruler. DO NOT use a pen

or any form of ink. Create a strip of filter paper that measures 14 cm by 7 cm. See Figure 3.

Use the ruler to measure 14 cm across the edge of the paper. Use the pencil to draw a small

âxâ at 14 cm. On an adjacent edge of the paper, use the ruler to measure 7 cm up the edge

of the paper. Use the pencil to draw a small âxâ at 7 cm. Using the ruler as a straightedge,

lightly trace a rectangle that is approximately 14 cm by 7 cm. Use the scissors to cut out the

rectangle. Create two additional strips of paper.

Note:There should be enough filter paper to create a fourth 14 x 7 cm strip of paper if needed.

2.Gather the 25 mL graduated cylinder, 100 mL beaker, distilled water, and salt. Use the

graduated cylinder to add a total of 50 mL of distilled water to the beaker. Add a pinch of salt

to the beaker. Use the stirring rod to stir the solution until the salt has completely dissolved,

resulting in a salt solution of approximately 0.1% NaCl.

3.Gather 2 sheets of the filter paper â 14 x 7 cm from step 1.

a.Create a base line by drawing a thin horizontal line with a pencil (NOT a pen!)

across the paper, 10 mm (1 cm) from the bottom. The line should barely be visible.

b.With a pencil, draw 9 small cross lines along the horizontal line, 1.5 cm apart,

Experiment

Chromatography of Food Dyes

4.Repeat step 3 for the second piece of filter paper.

5.On Sheet 1, from left to right, use a pencil to lightly label each section between the cross lines

with the abbreviation for the six FD& C food colors from your experiment bag that will be

tested: B1, B2, R3, R40, Y5, and Y6.

6.Also on Sheet 1, after Y6, place the labels KG and KS to represent the Kool-

Grape and Kool-Aid

Â®Strawberry drink mixes to be tested.

7.On Sheet 2 from left to right, use a pencil to lightly label each section between the cross lines

with the abbreviation for the grocery store food coloring that will be tested: SR, SY, SG, and SB.

8.Also on Sheet 2, after SB, place the label âUKâ for unknown and then âMâ plus a letter for the

color of 4 different candies from your bag of M&Ms: such as; M-R (for M&MÂ® red), M-Y, M-B,M-G.

9.Set Sheet 2 aside and perform the following for Sheet 1. After you have completed the

experiment for Sheet 1, repeat for Sheet 2.

10.Put on your safety gloves and goggles.

11.For each dye on the sheet to be tested:

a.Use the scissors to cut open the pipets and place them in the 24-well plate, bulb

side down, using the well plate as a pipet holder.

b.Place a few drops of the dye in a well of the 24-well plate, as shown in Figure 4.

Dye drops and pipets in well

12.Once the paper is spotted with all the dyes, allow the spots to dry for a few minutes.

13.Form the spotted filter paper into a cylinder (colored spots on the inside) with the edges

touching, but NOT overlapped, and staple at the top and bottom as shown in Figure 7. (You

may want to use small pieces of tape on the outside to lightly hold the cylinder together

while you securely staple it. If so, remove the tape after stapling.) Set the cylinder aside for a

moment.

14.The salt water (~0.1% NaCl) you previously prepared will be your eluting solvent and a Petri

dish will be your elution chamber. Pour salt water into the clean and empty Petri dish to a

depth of about 1/4 cm. This is just enough to cover the bottom of the elution chamber.

15.Set your paper cylinder next to (not in) the Petri dish, with the spots at the bottom, and look

to make sure the solvent level is below the base line. If it is not, pour out a little solvent until

it is.The base line (and spots at the bottom of the chamber) must be above the solvent level

for this experiment to work.

16.Carefully place the paper cylinder into the eluting chamber, making sure not to touch the Petri

dish sides. See Figure 8. The solvent-front will travel up the paper rapidly at first and then will

slow down. Let the solvent-front rise, monitoring every few minutes. Immediately remove the

cylinder if any dye or the solvent front moves up higher than 2 cm from the top of the paper.

If all the solvent is soaked up before the front has time to move toward the top of the paper

carefully add a little more solvent to the Petri dish.

17.When the solvent-front has traveled up the filter paper, 2 cm from the top, remove the filter

paper cylinder from the chamber and immediately mark the top the solvent-front, on the dry

part of the paper, with a pencil. See Figure 9.

18.Allow the paper to dry, carefully remove the staples,

and draw an outline around each spot

using a pencil.

19.Take a photograph of your chromatogram after you have circled the spots.

Resize and insert the photograph of your chromatogram in Data Table 1of your

Lab Report Assistant

20.Use the ruler to measure the distance of the solvent front, in millimeters. Use Figure 1 in the

Background section as needed. Record the distance of the solvent front in Data Table 2 of your

Lab Report Assistant

For each of the dye spots on the filter paper chromatogram, measure and record to the

nearest millimeter the distance each of the spots has traveled up the solvent front. Start from

the original horizontal pencil line (base line) and measure to the top center of where the dye

stops in each column on the paper. See Figure 2 in the Background section as needed.

22.Calculate the Rf value for each spot and record in Data Table 2

23.Repeat steps 10 through 22 for the second sheet of filter paper. Record all data for this

chromatogram in Data Table 3 of your Lab Report Assistant

24. Compare the Rf values for the spots of the Kool-Aid Â®, unknown sample, store food colorings,and M&Ms Â®

, to those of the known FD&C food dyes, to determine which FD&C dyes are used in the Kool-AidÂ®, the grocery store food colorings, and in the M&Ms Â®.

25.Record the FD&C dyes present in each of the samples in Data Table 4 of your Lab ReportAssistant

26.Clean up all equipment and return to the lab kit for future use.

27.When you are finished uploading photos and data into your Lab Report Assistant, save your

file correctly and zip the file you can send it to your instructor as a smaller file.

Enter the results from chromatography filter paper Sheet 1 of your experiment in the following tables. If more than one colour is present in a sample, please indicate colour when referring to distance and Rf measurement.

Sheet 1: FD&C Food Colours

Colour	Blue 1	Blue 2	Red 3	Red 40	Yellow 5	Yellow 6	Solvent
Distance (mm)
Rf							--

Sheet 1: Drink Mixes

Substance	Kool-AidÂ® Grape	Kool-AidÂ® Strawberry	Solvent
Distance (mm)
Distance (mm)			--
Rf			--
Rf			--

Enter the results from chromatography filter paper Sheet 2 of your experiment in the following tables. Indicate the colours of M&Ms used in the experiment.

Sheet 2: Store Food Colours

Substance	Store Red	Store Yellow	Store Green	Store Blue	Solvent
Distance (mm)
Distance (mm)					--
Rf					--
Rf					--

Sheet 2: Candy Colours and Unknown

Substance	M&M	M&M	M&M	Unknown	Solvent
	Colour:	Colour:	Colour:
Distance (mm)
Distance (mm)					--
Distance (mm)					--
Rf					--
Rf					--
Rf					--

Answer the following questions based on your experimental results.

Show the calculation for the Rf value for Red40.

What FD&C colour(s) make up the unknown sample?

What FD&C colours make up the food colourings? State the brand used.
What FD&C colours make up the Kool-AidÂ® drinks and M&MsÂ®?

Procedure Part A:

Oxone reaction: Dissolve 0.40 g Oxone Â® in 2.0 mL of water by

shaking vigorously, but carefully, in a sealed vial. (SAFETY: Be sure that the flask is

sealed well. Oxone Â® solution is strongly oxidizing and should not come into contact

with skin.) Dissolve 50 ?L of cyclohexene in 2.0 mL of acetone in a small Erlenmeyer

flask. Cool the cyclohexene solution in an icebath, then add the oxone solution dropwise

over 5 minutes with swirling in the icebath. Remove the Erlenmeyer flask from the

icebath and allow the reaction to sit for at least 15 minutes. (This is a good point to begin

the permanganate reaction.) After 15-30 minutes, add about 0.1 mL of conc. HCl to the

reaction dropwise with swirling. Allow the reaction to sit for about 10 minutes, swirling

occasionally.

Procedure Part B:

Potassium permanganate reaction: Dissolve 100 mg of KMnO4 in

4.0 mL of 0.1 M NaOH solution in a small Erlenmeyer flask. Cool the KMnO4 solution

in an icebath. Dissolve 50 ?L of cyclohexene in 2.0 mL of t-butanol in a separate

container. Quickly add the cyclohexene solution into the potassium permanganate. Swirl

the flask in the icebath for 3-5 minutes. Remove the Erlenmeyer flask from the icebath

and allow to sit for 10 minutes.

Procedure Part C:

Thin Layer Chromatography: Obtain a TLC plate. Mark the plate

with four marks, two for each reaction mixture. Spot the crude reaction mixtures lightly.

It may be hard to spot the KMnO4 slurry, but keep trying, you may find it easier if you

use a slightly wider capillary for this one. Allow the plate to dry for a couple of minutes,

or use a heat gun to gently dry the aqueous spots. Develop the plate in a chamber with

ethyl acetate as the eluent. Remove it from the chamber and mark the solvent front. Stain

the plate using anisaldehyde stain, and heat the plate gently to develop the spots. The cis

isomer should stain red, and have an Rf value of ~0.37, the trans isomer should stain blue,

and have an Rf value of ~0.31.

Questions

1. Draw a sketch of the TLC analysis for both reactions.

2. Calculate the Rf of both products formed.

3. Using you text and any other information you can find predict the products of the

4. reactions you performed.

5. Are these reactions stereoselective, stereopsecific or do they give mixtures?

give chemical equations and synthesis reactions for the TiO2 solar cell

dye + light â dye*

dye* + TiO2 â e- (TiO2) + oxidized dye

oxidized dye + 3/2 I- â dye + Â½ I3 -

Â½ I3 - + e- (counter electrode) â 3/2 I

PROCEDURE

Preparation of TiO2 paste Grind about 0.5 gram of nanocrystalline titanium dioxide (TiO2) in a mortar and pestle with a few drops of very dilute acetic acid. Alternate grinding and addition of a few drops of very dilute acetic acid until you obtain a colloidal suspension with a smooth consistency, somewhat like latex paint. A toothpaste-like consistency is too thick. Also mix in a drop of clear dishwashing detergent as a surfactant. This quantity of TiO2 is enough for several solar cells. For easier distribution, transfer the TiO2 paste to a syringe. Wrap the end of the syringe with parafilm to keep the paste from drying out when not in use. (If the paste dries out, the titanium dioxide will need more water. See previous step.) Preparing the electrode 1. Preheat your hotplate. 2. Obtain two pieces of conducting glass (fluorine doped tin oxide). One will be your anode with the titanium dioxide film and dye, the other the cathode. 3. Identify the conducting side of a tin oxide-coated piece of glass by using a multimeter to measure resistance. The conducting side will have a resistance of 20-30 ohms. Switch the multimeter to the resistance setting (â¦) and test the piece of glass for conductivity. Only one side is conductive and this is the side you must use as a substrate for the titanium dioxide film and dye. 4. With the conducting side up, tape the glass on three sides to the center of a spill tray using one thickness of tape. Wipe off any fingerprints or oils using a tissue wet with ethanol. Opposite sides of tape will serve as a spacer (see below) so the tape should be flat and not wrinkled. The third side of tape gives an uncoated portion where an alligator clip will be connected. Applying the film 5. Add a small amount of titanium dioxide paste and quickly spread by pushing down and across with a microscope slide before the paste dries. The tape serves as a 40-50 micrometer spacer to control the thickness of the titanium dioxide layer if you push down. 6. Wait approximately two minutes for the film to dry. Carefully remove the Scotch tape border, paying attention to not splatter any of the wet paste or scratch the surface. 7. Carefully place the piece of glass on top of your preheated hotplate. Keep it there for 20 minutes. The surface turns brown as the organic solvent and surfactant dries and burns off to produce a white or green sintered titanium dioxide coating. (Note: this requires a plate that gets quite hot.) Allow the glass to slowly cool by turning off the hotplate. The sample will look quite similar before and after heating; you only know it is done if you have observed the darkening stage along the way. Preparing the dye 8. While the film is heating/cooling, begin to prepare the dye. Obtain one (1) raspberry or blackberry and crush it in a beaker. Add 3-4 drops of deionized water and continue to crush the berries, obtaining red juice.

9. Remove the cooled piece of glass (your anode) and place enough berry dye on the titanium dioxide film to completely cover it. Keep it there for 10 minutes. The white TiO2 will change color as the dye is absorbed and complexed to the Ti(IV). 10. Rinse gently with water to remove any berry solids and then with ethanol to remove water from the porous TiO2. The ethanol should have evaporated before the cell is assembled. Preparing the Counter electrode 11. Using a pencil, âcolor inâ the conductive side of the second piece of glass. It will take on a slight gray color. The graphite will function as a catalyst for the redox electrolyte. Make sure that this is done on the conductive side! Wipe off any carbon along the perimeter of three sides of the carbon-coated glass plate using a dry cotton swab. Assembling the cell 12. Place the titanium dioxide/dye electrode face up on the lab bench. Place the counter electrode on top of it, offsetting it so that the film is still covered by the counter electrode, but there is about â â of the glass exposed for connections on both electrodes. Do not rub or slide the plates.

13. Clamp the two electrodes together using two binder clips. They should be placed on the sides adjacent to the electrodes. 14. Add the redox electrolyte by placing 1 drop of a triiodide solution to opposite edges of the plate. Capillary action will cause the KI3 solution to travel between the two plates. (The KI3 electrolyte solution consists of 0.5 M KI and 0.05 M I2 in anhydrous ethylene glycol.) The solution can corrode the alligator clips in the next step so wipe off an excess. Properties of the cell Connect a multimeter using an alligator clip to each plate (the negative electrode is the TiO2 coated glass and the positive electrode is the carbon coated glass). â¢ Test the current and voltage produced by solar illumination, or... â¢ Test the current and voltage produced by illumination from an overhead projector..

CHEM 2152 Lecture Notes - Lecture 3: Azobenzene, Silica Gel, Tanning Bed

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