TLC – Flashcards
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Precautions to take when spotting, developing and visualizing
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Use pencil to draw the line, do not flake the absorbant off because then the solvent will not rise up to the plate effectively, don't touch with nails, look for gauges. Oil on fingers will smudge the results.
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Why should bores of capillaries be small?
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So that the spots made are as small as possible (less than .5mm in diameter)
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What are typical characteristics for solvents used as eluents
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In general, the solvents for TLC and Column chromatography are characterized by low boiling points that allow them to be easily evaporated and low viscosities that allow them to migrate rapidly
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Characteristics of solvents used with respect to samples
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Depends on the polarity of the sample, polar solvent will move the samples higher up than non polar solvents
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Characteristics of solvents that allow them to migrate rapidly?
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low viscosities
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Adsorbent materials and specific uses (what kind of adsorbents would be better for what kind of substances)
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A less active stationary phase like silica would be used for the more polar substrates, whereas a more active stationary phase (which will absorb substances strongly) like alumina is the choice for the separation of relatively non-polar substrates
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Uses of TLC and how it is used- all 6
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(1) Determine number of components in a mixture
(2) determine identity of 2 substances
(3) monitor the progress of a reaction
(4) determine effectiveness of purification
(5) determine appropriate conditions for column chromatography separation
(6) monitor column chromatography
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Effects of using too polar or too nonpolar a solvent
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Too polar will have all the spots move to the top of the TLC place and too non-polar will have all the spots stay at the bottom of the plate
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Visualization techniques - why certain kind of visualization technique was used for certain types of compounds
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UV- used for conjugated compounds
I2 vapor- for most compounds
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Why is pencil always used to mark a plate?
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The pencil marks are made of inert carbon that will not move with the eluent.
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Why is ink never used to mark the plate?
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Ink will travel with the eluent bc it is not inert.
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Sources of error for TLC
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Over spotting the plate (makes results blurry), flaking absorbent layer on the plate (skew path of travel), sample too concentrated, spotting capillaries contaminated, solvent vapors escaped from the developing chamber if lid was not closed tightly.
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Sample preparation specifics
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Use pencil for all lines. 10mm from bottom is the spotting line with 3 evenly spaced spots for samples. Don't flake the absorbent. LIGHTLY touch the TLC plate with the capillary to spot. In developing chamber, ethyl acetate must be 4mm high (level has to be below spotting line). Make sure bottom of TLC plate level with the solvent line, make sure solvent doesn't move up past the top line, make sure top is placed back onto the developing chamber, makes sure filter paper is in the chamber.
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What happens if silica is used for nonpolar compounds?
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Nonpolar compounds chromatographed with silica gel will travel with the solvent front (i.e., give high Rf values)
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What happens if alumina is used for very polar compounds?
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Polar substances chromatographed on alumina will not migrate very far from the starting point (i.e., give low Rf values)
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What is capillary action?
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is the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces like gravity.
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Does a single spot on a TLC guarantee a single substance? Why or why not?
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No. If the spot is large, two or more spots of a sample may overlap on the TLC plate
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What is the difference on how column chromatography works vs. thin-layer chromatography?
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Column chromatography involves separation and purification, while TLC involves identification
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What may occur if polarity is changed too suddenly for column chromatography?
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It will cause heat evolution as the alumina or silica gel adsorbs the new solvent. This will vaporize the solvent, causing channels to form in the column that severely reduce its separating power
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What is the problem of collecting very large fractions in column chromotography?
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There may be more than one compound in any particular fraction; the only way to attain the separation is to redo the chromatography
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What is TLC's role in this stage?
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Since most organic molecules are colorless, separation must be monitored by TLC.
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What is TLC's role in this stage (Pt 2)
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The colors of the fractions or the results from analyzing the fractions by TLC will indicate which compound(s) you are interested in isolating
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What will happen if your spot is too large?
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If the spot is large, then two or more spots of a sample may overlap on the TLC plate, thus causing erroneous conclusions about the separation and/or the sample's purity or content.
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After extraction, you realize the ether layer is wet. what observation do you expect to make aboutt the ether layer because of this? What two steps can you take to mediate situation.
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The layer appears cloudy. Add NaCl or add drying agent (CaCl2).
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When carrying out extraction, you realize that you did not mix layers thoroughly. Which source of error did you make worse?Why would not washing the drying agent with additional ether after the dying process is over reduce % recovery?
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Incomeplete transfer. The solute that adsorbs to drying agent is not removed.
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In recrystallization list 3 measure u can take to ensure that you create a saturated solution to eventually start crystallization.
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Dissolve in minimum amount of hot solution
Wait in between drops & give enough time for solvent to dissolve.
Have solid crushed into powder.
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What step is taken to ensure sample is properly packed after taking it into capillary. What is the effect if you did not do this?
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Tap it on surface a few times to ensure propr packing. If not, sample will have a higher range.
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What specific source of error is due to the distillation apparatus. Describe a modification in the expiriment that helps mitigate the situation?
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Loss of heat due to large surface area. Mitigate with foil and pile sand on the flask.
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In TLC if you have 2 substances with 2 different Rfs but only one spot shows up, what conclusion can you draw about that substance that doesn't show? If each had very similar Rfs what would you do?
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That substance is not conjugated.
Re-run the expiriment with different solved or different adsorbent.
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We did not carry out backwashing in our expiriment. What problem did this cause? How did we go about it?
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It created. impurity in organic layer, but we recrystallized benzoic acid to remove impurities
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In MP, your sample is comtaminated with sand, but you do the expiriement anyway. Name one similarity and one difference between pure and what u have.
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Similarity: Will reach the MP
Difference: will have a range.
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Your ice-cold saturated soltion has yielded no crystalls, why? List 2 solutions
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It is supersaturated. Use seed crystals or scratch the bottom of the glass
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In distillation, other than the connectors, what must not be loose? After accounting for loose connectors, list 2 ways to ensure that the liquid is distilled with minimum loss to vaporization
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Make sure thermometer is tight.
Wrap side arm with wet/damp paper towl. End of sidearm should be in the reciever vial.
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Your spotting line is submerged in the eluent in TLC. what might this cause.
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The spots will dissolve in the eluent.
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In extraction, you have a cpd with a low distribution coefficient with a particular solvent. what two specific changes could be made to maximize % recovery.
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Chage solvent to one with better solubility.
Do multiple extractions
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What factor can cause liquid to be forced out around the stoper if you mix your layers by shaking a test tube witha stopper on top. If your extractoin does not produce gas, why do you still have to vent the extracton container.
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High vapor pressure of solvent.
Heat from acid/base rxn.
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Why is it important to slow down heating rate in MP when within 20 degrees of predicted MP.
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To accurately measure the range
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Why is vacuum filtration preferred over gravity filtration in the collecting and washing crystals step in recrystallization?What is the impact on % recovery and MP
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More efficient removal of solvent.
Greater than 100% revovery, depressed mp.
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In distillation, you have covered the flask completely in hot sand. What could result from this action and what effect will this have?
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Distillation will occur too fast. Ascending and descending vapors will not attain temp gradient.
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You wish to take a TLC of a solid sample and therefore have to disolve it. What precaution should you adhere to in this step to ensure a successful TLC.
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Not make it too concentrated.
So spot does not streak.
