Download TLC of Analgesics

CHROMATOGRAPHY: the general principle
There are many forms of chromatography used to separate mixtures. All forms involve three
things:
1.
a stationary (or nonmobile) phase
2.
a mobile (or moving) phase
3.
an analyte (the compound or mixture being analyzed)
Chromatography separates the components in a mixture so they can be separately identified.
Separation is achieved by placing a portion of the mixture on the stationary phase and allowing
the mobile phase to pass through. Depending on the properties of the compounds in the mixture,
some will be more attracted to the stationary phase and not move (or move very slowly) others
will be more attracted to the mobile phase and will move very quickly.
THIN LAYER CHROMATOGRAPHY
In thin layer chromatography, or TLC, a small portion of dissolved material is applied as a small
dot near the bottom edge of a plastic or glass plate coated with a thin layer of adsorbent. (The
adsorbent constitutes the stationary phase). The plate is placed in a closed container containing a
small amount of solvent. Capillary action pulls the solvent (the mobile phase) slowly up the
plate like water being soaked up by a sponge.
The compounds in the sample on the TLC plate (the analyte)
can do two things as the solvent moves up the plate:
If a compound is attracted to the
coating it sticks and does not move
up the plate:
If a compound is not attracted to the
coating it will not stick and it does move
up the plate:
Actually, most compounds will be partially attracted to both the coating and the solvent.
Compounds more attracted to the coating move up the plate slowly, while those more attracted to
the solvent travel more quickly and separation is achieved.
What features cause some compounds to prefer the stationary phase to the mobile phase? A
major factor is polarity of the bonds. The coating is composed of aluminum oxide (Al2O3).
The aluminum - oxygen bonds are very polar. The solvent is usually a nonpolar or very
moderately polar organic solvent, in this experiment ethyl acetate is used (formula C4H8O2). In
general, the more polar bonds a compound has then the more attracted it is to the very polar
aluminum oxide and the more slowly it moves up the plate.
The structures of the compounds you will be analyzing are shown below. The bonds possessing
the important polarity are those bonds between the very electro- negative atom O and C or O
and H, and between N and C, or N and H. It is primarily these bonds that determine the overall
polarity of these compounds. Remember that it is the difference in electronegativity of the two
atoms in the bond that makes a bond polar. With this in mind, carbon-carbon bonds are not polar
at all, while carbon-hydrogen bonds are only very slightly polar so both of these types can be
ignored.
O
H3C
O
N
C
C
N
H
CH3
H
N
C
C
N
C
H
H
C
C
H
C
C
C
C
C
N
C
CH3
O
H
ACETAMINOPHEN
CAFFEIN
H
C
C
H
CH3
H
C
H
H
O
C
N
C
H
O
H
SALICYLAMIDE
H
H
H
H
C
C
C
C
H
O
C
C
O
O
H
C
O
CH3
ACETYLSALICYLIC ACID
ANALGESICS
The best known and most widely used analgesic is acetylsalicylic acid (known as aspirin). Other
commonly used analgesics are those shown previously. Caffeine is not an analgesic, but is
sometimes added for its stimulating effects. Caffeine increases the body's sensitivity to carbon
dioxide causing an increase in both the rate and depth of respiration.
The analgesics given above work to inhibit pain by preventing transmission of pain impulses
from the hypothalamus gland (located in the base of the brain) to the cerebral cortex. These
analgesics also reduce fever by the following modes of action:
1.
causing blood vessels to dilate (enlarge) so more blood can be cooled when it
reaches the skin;
2.
stimulating diaphoresis (perspiration) so the skin cools blood even more
efficiently;
3.
by inhibiting the production of certain prostaglandins which normally function to
increase body temperature.
When viruses or bacteria invade the body, prostaglandins are activated to INCREASE body
temperature. Because the invading organisms are more susceptible to high temperatures than
your own uninfected cells, this increase in temperature serves as one of the body's methods of
ridding itself of invading organisms. Thus, while taking an analgesic drug serves to reduce
symptoms associated with infectious disease, it does NOTHING to attack the invading
organisms and may, in fact, be counterproductive!
Aspirin, and to some extent salicylamide, also exhibit an anti-inflammatory action that is
especially useful to people suffering from arthritis. Aspirin is able to reduce swelling and relieve
the pain. Some analgesics tablets include other compounds for therapeutic effects such as
antihistamines, decongestants and sedatives. In addition to the active ingredients, the tablets
must also contain substances such as starch that act as binders to hold the tablets together.
Solvent front
origin
1 2
3
4
5
EXPERIMENTAL
In this experiment you will obtain as an unknown, a well-known analgesic tablet. The object is
to identify the drug by comparison with several known compounds. The tablet will be one of
those listed below.
DRUG
INGREDIENTS
(BRAND NAME)
1) ANACIN
acetylsalicylic acid, caffeine
2) EXCEDRIN
acetylsalicylic acid, caffeine,
acetaminophen, salicylamide
3) TYLENOL
acetaminophen
PROCEDURE
 Capillaries will be provided for you.
 Obtain 1 TLC plate. Don't touch the surface of the plate, or finger prints will appear on the
plate. Lay the TLC plate on a page in your note book. With a pencil mark on the page where
the corners of the plate are. Also make a mark on the page about 1 cm up from the bottom of
the plate. Below the plate make five evenly spaced marks on the page and number them 1-5.
 You will use the materials you synthesized and compare their Rf to the component materials
in the commercial preparations.
 When all five samples have been applied, place the TLC plate spotted end down in a
developing chamber and cover the top of the chamber with a watch glass. Don't disturb the
jar while the TLC plate is developing or the samples may be ruined. About fifteen minutes
are needed for the solvent in the jar to be pulled up the plate. Keep your eye on the TLC
plate, and when the solvent is about 1 cm from the top, pull out the plate and quickly mark
the solvent line on top of the plate. The solvent will evaporate very easily so don't be slow.
Allow the solvent to evaporate in the fume hood.
 What do you see? Probably nothing. You need to visualize the spots on the plate. This is
done in two ways. First, shine an ultraviolet light on the plate and circle any spots you see
with a pencil. Record the colors and any other observations you can make about the spots.
ANALYSIS OF THE PLATE
In TLC, the compounds are compared by their RETENTION FACTORS (or Rf's). These are
determined by measuring the distance each sample moved and dividing by the distance the
solvent moved.
 Start at the origin (the place where the sample was applied) and measure the distance to the
CENTER of a spot. This is the sample distance.
 Now measure the distance from the origin to the line where the solvent reached (called the
solvent front). This is the solvent distance.
 Divide the sample distance by the solvent distance, this is the Rf for that spot.
 Calculate the Rf for each spot on your TLC plate.
 Using the Rf values for each spot and the observations you made when you visualized the
plate, determine which of the known standards are in your unknown analgesic.
 Using the listed ingredients given above, identify the brand name of the analgesic you
analyzed.
www.lcc.ctc.edu/faculty/aherbelin/CH111_Su03/Lab/02_TLC.doc