EXPERIMENT #4
Thin-Layer Chromatography
Friday, October 14 – Thursday, October 20, 2011
Thin-layer chromatography is a widely used identification technique. This method can be used to determine the number of compounds in a mixture, identify whether two samples are identical compounds, and monitor the progress of a reaction.
READING ASSIGNMENT:
Technique 17: Thin-Layer Chromatography in Techniques in Organic Chemistry 3 rd
219-235. See Experiments section of course website for video.
Ed. pgs
Safety Information
Ethyl formate is a flammable liquid and decomposes gradually in the presence of water that irritates eyes and respiratory system. Used for the TLC development solvent for the polar compounds. Avoid contact and inhalation.
Formic acid is a colorless liquid with pungent odor and causes eye irritation. Avoid contact and inhalation.
Ethyl acetate is a clear volatile, flammable liquid; characteristic fruity odor. Overexposure causes irritation of eyes, nose, and throat.
Hexane is a flammable, highly volatile liquid immiscible with water. Overexposure causes light headedness, nausea, headache, muscle weakness.
EXPERIMENTAL NOTES:
A. Analysis of Analgesics
In the first part of the experiment, we will be analyzing standard samples of analgesics by thin-layer chromatography (TLC).
What You Need To Do Before Lab:
Ø Complete the reading assignment.
Ø Rewrite the general procedure for TLC from Technique 17.6 in your own words for analyzing standard solutions of analgesics. o The developing (eluting) solvent we will use in a 10:4:1 mixture of toluene, ethyl formate, and formic acid. o Run standard samples and unknown from part B side by side on the same plate. o Use a UV lamp and an iodine chamber for visualization ( Always use the UV lamp first!
)
A part of this experiment is adopted from Elder, J. W.; J. Chem. Educ.
, 1995 , 72 , 1049.
51LA
Rev 12/3/10

Data To Record:
Ø Drawings of your TLC plates after visualizing.
Ø Calculated R f
values for each compound.
Ø Calculated relative R f
values by setting ibuprofen to 1.00
(R f values are varying depending on the polarities of the eluting solvent. However, the relative values are quite consistent when determined using the same eluting solvent.)
R f
Value Relative to Ibuprofen (as 1.00)
Compounds
Ibuprofen
Naproxen
Aspirin
Rf value relative to ibuprofen
1.00
0.83
0.55
Caffeine
Acetaminophen
0.31
0.17
Chemical Structure of Common Analgesics:
NHCOCH
3
COOH
OCOCH
3
CH
3
N
O
CH
3
N
OH
Acetaminophen
4-aceamidophenol
Aspirin acetylsalicylic acid
O N
CH
3
Caffeine
N
CH
3
CH
3
COO - Na +
CH
3
COOH
CH
3
CH
3
O
Ibuprofen
2-(4-isobutylphenyl)propionic acid
Naproxen Sodium
6-methoxy-a-methyl-2-naphthaleneacetic acid sodium salt
B. Identification of Unknown Analgesics
In the second part of the experiment, we will be comparing known samples of analgesics to an unknown analgesic by TLC. To determine the identity of your unknown, you will extract the analgesic from the powder provided and analyze your extract by TLC.
What You Need To Do Before Lab:
Ø Rewrite the procedure below for extraction of unknown analgesics in your own words!
o Place the unknown analgesic (approximately 100 mg) and ~2 mL of 95:5 ethyl formate:formic acid mixture in a small test tube. Gently shake the mixture. Use the solution containing analgesics for spotting without further treatment. After the spotting of unknown extracts onto the TLC plate, check the amount and size of the sample spot under UV-lamp. Apply more if not visible under UV lamp. Keep the size of sample spot as small as possible.
Ø Write a procedure for TLC of unknown using your procedure for the standard solutions as a template.

o Use the co-spotting technique to confirm the identity of your unknown. o Use the same developing solvent as in Part A.
Data To Record:
Ø Drawings of your TLC plates after visualizing.
Ø Don’t forget to mark the spots you see in visualization with a pencil!
Ø Calculated R f
Ø Calculated R f
value for your unknown
of unknown relative to ibuprofen.
Ø Identity of your unknown.
C. TLC Analysis of Extracted Caffeine Sample from Experiment 3
In the final section of this experiment, we will analyze the caffeine isolated in Experiment 3 by
TLC to determine its purity.
What You Need To Do Before Lab:
Ø Rewrite the general procedure for TLC in your own words for analyzing extracted caffeine. o If your sample appears to have sublimed, DO NOT OPEN THE VIAL YET. Cool the sample in an ice bath first to induce deposition. o Include preparation of your sample: add ~0.50 mL of ethyl acetate to your caffeine sample and use this solution for spotting without further treatment. o Run a co-spotting TLC plate with the standard sample of caffeine (from Part A) and your caffeine sample. o The developing (eluting) solvent we will use is ethyl acetate. o Use a UV lamp and an iodine chamber for visualization.
Data To Record:
Ø Drawings of your TLC plate after visualizing.
Ø Don’t forget to mark the spots you see in visualization with a pencil!
Ø Calculated R f
values for any spots present
Ø Identify the spot from your sample that corresponds to caffeine.
Ø Be sure to record the number of spots you see from your sample during visualization.
NOTES FOR WRITING YOUR DISCUSSION:
Your Theory section should include :
Ø An explanation of how TLC works on the molecular level: why do compounds travel different distances on the plate? Which compounds travel faster and why? etc.
Your Results section should include :
Ø A data table with R f
values for all the analgesics from part A, the R f
value of your unknown, and the R f
values for all the sports from your caffeine sample (part C) . Please follow the following format:
Sample aspirin etc…
R f
Value Relative R f
Value I
2
Acivity
###
###
###
###
Yes/No
Yes/No

Your Discussion section should include :
Ø A brief evaluation of the polarity of the different analgesics – relate the relative polarity to the chemical structure of the analgesics.
Ø An explanation of how you deduced the identity of your unknown.
Ø An evaluation of the identity and purity of the product you isolated in Experiment 3
Your Sources of Error/Future Experiments sections should include :
Ø How reliable is TLC? What can easily throw off the results?
Ø Is TLC in general, an experiment that conclusively determines the identity and/or purity of a compound?
Ø What other techniques could one use to confirm the identity/purity of a compound?