chromatography

Question 1

how does chromatography separate compounds?

Answer 1

based on their partitioning between mobile phase and stationary phase

Question 2

how do the compounds travel on the plate? what travels up the highest?

Answer 2

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

Question 3

what has a higher retention factor? polar compounds or nonpolar?

Answer 3

non polar compounds will have a higher retention factor as they travel up the plate higher

Question 4

why do we add blotting paper to the jar?

Answer 4

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

Question 5

why do starting spots have to be above the liquid in the jar

Answer 5

the spots will otherwise dissolve off

Question 6

what happens if we disturb the TLC

Answer 6

the tlc will stop or be disrupted so the retention factors will be wrong

Question 7

what are the three methods of visualizing a TLC plate

Answer 7

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

Question 8

why must you mark the dots when visualized?

Answer 8

they will disappear

Question 9

in general, less polar compounds will have a _____ Rf value compared to more polar compounds

Answer 9

larger

Question 10

as you increase the solvent polarity, what happens to the Rf for all the components?

Answer 10

the Rf increases for all components as more of the polar compounds will begin to move

Question 11

as you decrease the solvent polarity, what happens to the Rf for all components

Answer 11

it will decrease

Question 12

what will elute faster, alkanes or carboxylic acids?

Answer 12

alkanes, they are non polar so they prefer the mobile phase

Question 13

what has a lower Rf, alkene or amides?

Answer 13

Amides, they are more polar

Question 14

what happens if starting spots are too big

Answer 14

streaking may occur as the plate is overloaded

Question 15

what happens if the mobile phase is too polar

Answer 15

the Rf values increase and everything elutes quickly so it is hard to differentiate between them

Question 16

what happens if mobile phase is too nonpolar

Answer 16

the Rf values decrease and nothing moves

Question 17

if you have a 1:1 hexane/ethyl acetate and swap it for hexane/dichloromethane

Answer 17

The Rf would increase for all compounds, dichloromethane is more polar than ethyl acetate

Question 18

how does the mobile phase move down a column

Answer 18

it travels by gravity

Question 19

how do you increase the rate of elution in column chromatography

Answer 19

use pressured air

Question 20

Components with a high Rf in TLC will elute early or later?

Answer 20

they will elute first, they are nonpolar

Question 21

components with a low Rf in TLC will elute early or later?

Answer 21

they will elute slower

Question 22

what is the benefit of column chromatography separations versus TLC and GC

Answer 22

it allows gram quantity separation

Question 23

to collect more polar components what do you need to do?

Answer 23

you need to increase the solvent polarity

Question 24

why do we use a drying tube

Answer 24

to prevent glassware from entering the reaction and creating acetic acid from the carbocation

Question 25

What would happen if you use a non-symmetrical anhydride, for example methyl
ethyl anhydride, in the mono acylation with toluene

Answer 25

you could get a mixture of products with ethyl acetate and methyl acetate

Question 26

Explain the relative elution order of the compounds in the product mixture. Draw
the TLC plate to help illustrate your answer

Answer 26

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

Question 27

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?

Answer 27

all of the polar compounds would elute at the same time and you would get a mixture of products in the eluted band

Question 28

What would be the expected elution order if the following compounds were
separated by column chromatography? Toluene, Benzoic Acid, Benzaldehyde, and
Phenol

Answer 28

toluene, benzaldehyde, phenol, benzoic acid

Question 29

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.

Answer 29

you could increase the polarity in the solvent or used pressure air to make it move through quickly

Question 30

what % of analyte should be prepared in developing solvent

Answer 30

1%