Lab D - Identification of an Unknown Using Think layer Chromatography Flashcards
Chromatography, or “Color Writing”, is a simple, economical method for ____________(1). Identification of unknowns is done through ___________(2). Chromatography helps in determining ________________(3), ________________(4), _______________(5). There is _____________(6) on sample type- it can be organic, inorganic, biological, or medical. It is a highly ____________(7), and can detect __________(8).
(1) separating components of a mixture
(2) comparisons with known reference samples
(3) the effectiveness of a purification
(4) the conditions for macroscopic separations(column chromatography)
(5) following the progress of a chemical reaction
(6) no restriction
(7) sensitive
(8) microgram amounts or less
Make a chart for chromatography, comparing the stationary and mobile phase. Include the following rows: what does it do, what is it(physical form), how does it effect sample movement, analogue to, specifically for.
Look at notes
What are the 4 types of chromatographic methods?
TLC: Think layer chromatography
LC: Liquid chromatography(SP - Silica gel or Alumina; MP-a liquid)
HPLC: high performance liquid chromatography(utilizes high pressure exerted by mechanical pumps to force ht Emobile phase through a very small diameter column packing that contains the stationary phase)
GCL Gas chromatography(MP- gas)
Silica gel is an example of an absorbent the is most commonly used as an inexpensive stationary phase. Draw its structure and explain why it is polar.
Diagram in notes
Extended covalent network of tetrahedral SI atoms bridged by O atoms - terminating in very polar Silanol(Si-OH) group, thus creating a very polar surface. The presence of these OH groups renders the surface of silica gel highly polar
Other than silica gel, what is considered another example of a good absorbent?
Aluminum oxide, alumina(Al2O3)
What is the composition of a TLC plate?
250 um silica gel layer impregnated with a fluorescent indicator on a plastic backing
What is added to help the silica gel bind to a plate? What is added to help compounds fluoresce?
CaSO4 used to help silica gel bind to a plate
ZnS used for fluorescence(absorbs at 254 nm). It helps the compound glow under UV light
What are the results of interaction with only stationary phase?
Highly polar molecules will interact strongly with the polar Si-OH bonds in the silica gel absorbent, resulting in slow movement up the TLC plate
Weakly polar molecules are help less tightly than the polar species on the silica gel absorbent - resulting in quicker movement up the TLC plate.
Draw and name the three compounds used in this lab. Rank from most to least polar.
Drawn in ntoes
Most polar is 9-hydroxyfluorene. For 9HF, it is the most polar because the hydroxyl group. Phenyl Benzoate is next most polar because ester group induced polarity. Least polar is fluorene because does not contain polar bonds.
What does Rf stand for? How do you calculate an Rf value?
Retention factor = (distance traveled by compound, or SOLUTE) / (distance traveled by solvent, or ELUENT)
How do Rf values correlate with the polarity of a compound?
More polar compounds = small Rf values = shorter the distance traveled on the TLC plate
Less polar compounds = large Rf values = longer the distance traveled on the TLC plate
What are the four steps of thin layer chromatography?
- Application of sample: use a different TLC capillary tube(open on both ends) to spot the standard solution of each solute
- Development of sample: This is when the separation actually occurs inside the TLC development chamber
- Visualization of sample: the TLC plate will be viewed under UV light
- Interpretation of results: Comparison of RF values
You should always spot on the ________ side of the TLC plate.
ROUGH
Draw the schematic for the developing chamber to run a TLC plate
Drawn in notes
Describe the process for sample application(spotting)
A. Draw the baseline lightly with pencil
1.5 cm from the bottom of the plate.
“baseline”
B. Gently and lightly draw four “x” marks
on the baseline
C. Use capillary tubes that are labelled as
TLC capillary tube (and not MP capillary tubes)
- to draw up the solution from the vial
onto the TLC plate.
(Be sure to spot lightly – to avoid making
a break in the silica gel coating
on the plate- that will hamper the separation.)
What fluent, or eluting solvent was used in this experiment?
9mL hexanes and 1mL ethyl acetate
How are spots visualized on the TLC plate?
Ultraviolet viewing cabinet,
equipped with a short wavelength
(254 nm) UV lamp
Make sure to
A. Mark solvent front. Allow solvent to evaporate from surface of TLC plate.
B. View results under UV light. Use a pencil to
mark the observed bright pink spots
Iodine Visualization
Colorless Organic Compounds (alkanes, alcohols, ethers) can also be visualized by absorption of Iodine
Vapor – Yellowish-brown colored spots form from the reaction of substances with Iodine vapor
Identical Rf values _________(1)
that compounds are the same
(1) suggest, but do not prove
how exactly does chromatography separate organic compounds?
Using a solvent(mobile phase) to move the compounds over a solid(stationary phase). The compounds are separated based on how strongly the compounds bind to the stationary phase(i.e. the silica gel) via non-covalent interactions vs how well the compounds dissolve in the solvent/eluent
Less polar compounds will move _________(1) than more polar compounds
farther up the plate
What time of interactions with SiO2 have with organic molecules
hydrogen bond
How is distance measured for the solvent front vs the spots
Spots: measured from the baseline to the centre of the spot
Solvent front: measure from the baseline to the solvent front
Less polar compounds will have higher or lower Rf values?
Higher
It is not wise to rely only on TLC to establish the identity of a compound. Why?
Two different compounds may have the same Rf value.
What specific chemical property of the three compounds allows them to be seen under UV light?
the three compounds all contain a benzene ring which allows them to be seen under UV radiation as the benzene ring absorbs UV light. Each compound will show up as a pink spot when viewed under the UV lamp/
A good TLC solvent systems should produce distinguishably different Rf for each component. The general rule is that
a more polar solvent will move a compound farther up the TLC plate.
If a solvent is not polar enough or too polar, then the spots
corresponding to the different compounds will run very close together and it will be impossible to distinguish them
In practice, a mixture of
one polar and one non-polar solvent is used
Hexanes is a mixture of
isomers of hexane(same chemical formula but different structural formulas. It is different from n-hexane(pure)
How can TLC tubes be identifies?
open on both ends
Briefly describe the experiment procedure
- Practice spotting
- Prepare the main TLC plate by marking the baseline(1.5 cm from bottom) and the solvent front(1 cm from top). mark 4 “x” and label.
- Spot each compound 2-3 times and ensure it can be seen under UV light
- prepare TLC developing solvent by mixing 9mL of hexanes and 1 mL of ethyl acetate
- Place the TLC plate at an incline inside the beaker.
- Allow the solvent to reach solvent front. mark the solvent front right away once it has reached it
- Viualize the spots inside a UV chamber
- Compute Rf values and determine identity of compound
Why should you spot very lightly?
You dont want to break the silica gel
Why is it important to immediately cover your beaker with the solvent?
Evaporation will start immediately
Why is it important to ensure that the baseline of the TLC plate is above the level of the solvent?
The samples you spotted may dissolve into the solvent
What does waste disposal look like for this lab? For TLC solvent and TLC tubes
- Dispose the TLC solvent into the C,H,O non-halogenated waste container
- TLC tubes in sharps container
What hazards do hexanes and ethyl acetate pose?
Both are flammable and can cause irritation to the skin or eyes
What hazard does UV light pose?
can cause burns to the skin and eyes
Draw ethyl acetate and hexane. Which is more polar and why?
ethyl acetate is more polar than hexane because ethyl acetate contains a polar carbonyl group (C=O) and an oxygen atom, creating a dipole moment within the molecule, while hexane is composed of only carbon and hydrogen atoms with very similar electronegativity, making it essentially nonpolar.
