chromatography Flashcards
how does chromatography separate compounds?
based on their partitioning between mobile phase and stationary phase
how do the compounds travel on the plate? what travels up the highest?
the mobile phase is varying polarity and nonpolar compounds tend to favor it so they travel up the plate by capillary action
the stationary phase is polar, so polar compounds will stick to the plate and travel more slowly up the plate
what has a higher retention factor? polar compounds or nonpolar?
non polar compounds will have a higher retention factor as they travel up the plate higher
why do we add blotting paper to the jar?
it ensures the atmosphere is saturated with vapors of the developing solvent
if the system is not closed, the solvent could evaporate off and the TLC will stop moving
why do starting spots have to be above the liquid in the jar
the spots will otherwise dissolve off
what happens if we disturb the TLC
the tlc will stop or be disrupted so the retention factors will be wrong
what are the three methods of visualizing a TLC plate
you can visualize colored spots with your eye
non-colored spots can be visualized with an I2 stain which will create brown spots
non-colored compounds can be illuminated by a UV lamp if they are conjugated
why must you mark the dots when visualized?
they will disappear
in general, less polar compounds will have a _____ Rf value compared to more polar compounds
larger
as you increase the solvent polarity, what happens to the Rf for all the components?
the Rf increases for all components as more of the polar compounds will begin to move
as you decrease the solvent polarity, what happens to the Rf for all components
it will decrease
what will elute faster, alkanes or carboxylic acids?
alkanes, they are non polar so they prefer the mobile phase
what has a lower Rf, alkene or amides?
Amides, they are more polar
what happens if starting spots are too big
streaking may occur as the plate is overloaded
what happens if the mobile phase is too polar
the Rf values increase and everything elutes quickly so it is hard to differentiate between them
what happens if mobile phase is too nonpolar
the Rf values decrease and nothing moves
if you have a 1:1 hexane/ethyl acetate and swap it for hexane/dichloromethane
The Rf would increase for all compounds, dichloromethane is more polar than ethyl acetate
how does the mobile phase move down a column
it travels by gravity
how do you increase the rate of elution in column chromatography
use pressured air
Components with a high Rf in TLC will elute early or later?
they will elute first, they are nonpolar
components with a low Rf in TLC will elute early or later?
they will elute slower
what is the benefit of column chromatography separations versus TLC and GC
it allows gram quantity separation
to collect more polar components what do you need to do?
you need to increase the solvent polarity
why do we use a drying tube
to prevent glassware from entering the reaction and creating acetic acid from the carbocation
What would happen if you use a non-symmetrical anhydride, for example methyl
ethyl anhydride, in the mono acylation with toluene
you could get a mixture of products with ethyl acetate and methyl acetate
Explain the relative elution order of the compounds in the product mixture. Draw
the TLC plate to help illustrate your answer
ferrocene had the highest retention factor because it was least polar
acetyl ferrocene had a lower retention factor and diacetyl ferrocene had an even lower retention factor due to its polarity and affinity for the stationary phase
What (if anything) would happen to your separation of compounds in this laboratory
if, by mistake, you eluted the chromatography column with petroleum ether, then
methanol, then ether?
all of the polar compounds would elute at the same time and you would get a mixture of products in the eluted band
What would be the expected elution order if the following compounds were
separated by column chromatography? Toluene, Benzoic Acid, Benzaldehyde, and
Phenol
toluene, benzaldehyde, phenol, benzoic acid
If a polar compound is moving through a gravity chromatography column too slowly,
provide two things you could do to increase its rate of movement.
you could increase the polarity in the solvent or used pressure air to make it move through quickly
what % of analyte should be prepared in developing solvent
1%
