1.LAB 15

Use Thin Layer Chromatography (TLC) to identify the components of an analgesic drug, which could contain aspirin, acetaminophen, ibuprofen, salicylamide, or caffeine, and identify the tablet using table
Use UV to see spots and Rf to compare distances


Rf =
distance traveled by spot/ distance traveled by solvent

2) Assuming that the drug components whose spots you identified dissolved completely when you stirred the tablet with 1:1 ethanol/dichloromethane, what was the solid that remained behind?
The solid remaining would be starch or any pill coating substances. This is because starch binds the tablet together but is an inactive ingredient that would not dissolve in the solvent.

a) You allowed your chromatogram to develop too long, and you couldn’t find the solvent front
??? If you allowed your chromatogram to develop too long the solutes and solvent would travel too far and possibly run of the plate. Therefore we would not be able to calculate the Rf values without a measured solvent front

b) The lab assistant who prepared the developing solvent mistakenly used aqueous ammonia in place of acetic acid
If aqueous ammonia is used in place of acetic acid, ammonia is basic, so that OH group may react with the TLC plate and the acidic samples, and may even form a solid, this the experiment wouldn’t work.

c) You used an open-ended melting-point capillary rather than a Drummond Microcap to apply the spots
Using an open-ended capillary would cause the spots to be too big that they would run together

d) You marked the starting line with a ballpoint pen
Using pen ink would contaminate the plate because it is pigmented. The solvent would drag the ink up the plate because pen ink is soluble in most solvents.

1. Given aspirin and ibuprofen (shown below) which would you expect to have a lower Rf using a non-polar developing solvent, why?
a. I would expect Aspirin to have a lower Rf because the solvent is nonpolar and aspirin is more polar than ibuprofen, so the less soluble compound travels less with a lower Rf, while the more soluble travels farther giving a higher Rf

2. Explain why the Rf value is more informative than the distance traveld by a spot?
a. The Rf value is more informative than using solely the distance the spot travels because the Rf value allows one to compare different TLC plates, rather than only the compounds measured on one plate. Rf = distance traveled by spot/distance traveled by solvent

3. What is the highest Rf possible, when would this happen?
a. The highest possible Rf value is 1. This occurs when the spot travels the same distance as the dissolving solvent.

4. A pencil should be used to mark the starting line, why is this important?
a. A pencil should be used rather than a pen, because pen ink can run in most solvents, messing up the results. Pencil will not run.

5. Why might it be necessary to look at your TLC plate under UV light to see your spots?
a. It is necessary to look at the TLC plate under UV light because the spots can be colorless, but in UV light the spots can be seen when they quench the fluorescence.

Lower Rf in a non-polar solvent? Aspirin or ibuprofen
Lower Rf = less soluble in non-polar = polar Aspirin
(Like dissolves like)
Polar will not dissolve well in non polar solvent, won’t travel far. Aspirin is more polar with O’s

Why is Rf more informative than just distance traveled by spot?
Rf allows one to compare different plates

What is highest Rf, when?
Highest Rf is 1, when unknown compound travels same length of developing solvent

Why use pencil to mark starting line?
Pen ink can run in many solvents and will mess up results

Why use UV light to see spots on TLC plate
UV allows to see colorless spots when they quench the fluorescence (of the plate)

Stationary Phase
Mobile Phase
Silica gel in TLC plate

More soluble travels greater distance
Less soluble travels shorter distance
Rf = spot distance traveled/solvent distance traveled

Larger Rf
Smaller Rf
More soluble component
Less soluble

TLC idea
To separate components based on solubility

Compounds that quench fluorescence
Fluorescent spots are
Show up as dark spots

Close lid to developing chamber
to prevent evaporation from TLC plate saturated with solvent – chamber must be in saturated condition, or plate will not absorb solvent enough

Capillary Action
Silica gel sucks up liquid

1) Spots on starting line
2) TLC plate into Developing chamber of mobile phase solvent and close lid
3) Mobile phase travels up
4) Remove and pencil finish line

Dissolving Solvent
ethyl acetate/acetic acid (200:1) 1:1 Ethanol/dichloromethane

aspirin acetaminophen

Ethyl acetate – low toxic, doesn’t bioaccumulate, Dichloromethane 5ppb in water

To bind tablet

reduce fever (aspirin, acetaminophen, ibuprofen)

Non steroidal anti inflammatory
reduce swelling (aspirin ibuprofen)

Wonder drug
Aspirin – reduce heart disease, stroke, cancer, cataracts, gallstones

Prostaglandins and aspirin
Aspirin blocks over produce of prostaglandins, by turning off key enzyme that makes them (prostaglandin synthase)

Too many prostaglandins
blood clots, heart attack, stroke

Prostaglandin synthase
assembly line: raw materials in, prostaglandin out

Aspirin molecules sabotage
block channel to prevent raw materials entering

create cancer fighting substances to prevent digestive cancer

too much acetaminophen
liver damage

ibuprofen leads to
ulcers, asthma, high blood pressure, kidney, liver, heart disease