final exam

Question 1

mobile phase

Answer 1

gas or liquid
in tlc it’s the solvent
in gc it’s the inert gas (helium typically)

Question 2

stattionary phase

Answer 2

liquid or solid
in TLC it’s moving past silica gel
in GC it’s going through the column

Question 3

best seperation of TLC

Answer 3

if polarity of the solvent and the target compounds are similar an optimum seperation can be achieved because even small differences in polarity lead to different degrees of retention by stationary phase

Question 4

what does GC create

Answer 4

molecules come out at different times and then signals are generated by the help of a computer

Question 5

the injector of GC

Answer 5

mobile phase
carrier gas is also within this usually helium or H2 (help push sample through)
injector is kept at a temperature that higher than the boiling point of the sample because to make sure the sample vaporizes instantly (can’t be too low or high though)

Question 6

The column of GC

Answer 6

stationary phase
where molecules seperate
molecules with strongest attractive forces will move through column the slowest while non polar moves through faster

Question 7

what happens when column temperature is either too high or too low

Answer 7

too low - kinetic energy of molecules will be very low, one or more compounds that enter the gc column may be absorbed by the stationary phase, compounds do not move because they do not reach the detector

too high - kinetic energy of molecules is too high, compounds will barely interact in stationary phase, they remain in mobile phase, no seperation

Question 8

detector in GC

Answer 8

different types - TCD, MS, and FID
all have different sensitivies but doesn’t change the result

we use GC-MS - it separates, quantifies, and identifies compounds at the same time

Question 9

gas chromatogram features

Answer 9

retention time - time needed for compound to pass through the compound

peak area - is proportional to the amount of the compound that is detected

Question 10

parameters that could affect the outcome of gc

Answer 10

*most affective one is temperature

longer column - longer retention time
higher column temperature - shorter retention time
higher helium flow - shorter retention time
higher polarity of the stationary phase - longer retention time

Question 11

peak area with GC and integrals/ratios

Answer 11

the area under the peak is called the integral, it’s proportional to the amount of compound that moves through the detector

detectors are sensitive and might not show all the same result

Question 12

response ratios of GC

Answer 12

it’s the response of a given compound relative to the response of another compound that was chosen as a standard
A = 20
B= 30
C = 50

RRa = 20/30 = .67
RRc = 50/30 = 1.67

keep b the same but a and c can be divided further

Question 13

GC theory

Answer 13

Sn2 reaction

Question 14

CC adsorption/mobile and stationary phase

Answer 14

when solvent moves down the stationary in a column filled with silica gel, the strongest intermolecular phases are those between the surface of the silica gel and the solvent or sample molecules because the silica gel is usually the most polar component in the system

while the solvent sample mixture moves down the column all components in the mobile phase are in competition for the silica gel surface

Question 15

process behind CC

Answer 15

a solid adsorbent (silica gel) is used as the stationary phase, it is packed into a column and a solvent (the molbile phase) containing a mixture of compounds is moved past the solid material. The solvent used as a mobile phase is often referred to as an eluent

Question 16

seperation of CC

Answer 16

more polar molecules will move slower throughout the column than non polar molecules

Question 17

slurry packed method vs dry pack method in CC

Answer 17

slurry pack method - formed from a mixture of silica and solvent

after addition of the slurry to the column, the stopcock is opened to allow the eluent to drain as the adsorbent bed packs. the stop cock is closed when the absorbent is settled.

air bubbles mist not be allowed to form in the bed, bubbles cause the bed to be irregular and interfere with the uniform movements of mixture of the compounds, this leads to a decreased efficiency of the seperation

dry pack method - silics is added to an empty column with a cotton plug, the eluent is added to column and pushed (with pressure) through the stationary phase until all silica is wet

Question 18

applying compounds to the column (as liquid or solid) in CC

Answer 18

as a liquid - sample is dissolved in minimal amount of solvent and is taken up by pipette and then put into the top of the column. The solution is allowed to absorb into the silica gel by opening up the stopcock and letting all liquid disappeared into the packing

as a solid - the compounds that have to be separated are dissolved in a solvent, the solution is mixed with a small amount of adsorbent. the solvent is removed by evaporation, the resulting powder has the sample absorbed to the surface of silica gel. The dry mixture of sample and adsorbent is added to the top of the column bed

this application takes more time than as a liquid, so only applies to when material that has to be purified by column chromatography has pnly a low solubility in the preffered eluent

Question 19

detecting separated compounds in CC

Answer 19

when compounds move through silica gel down a column, they form bands (disc shaped areas in which compounds are present)

most of the time collected into test tubes then dotted on TLC plate

Question 20

optimizing seperation in CC

Answer 20

if solvent is too polar, substances are eluted too fast because they hardly interact with the stationary phase

if solvent is not polar enough compounds might stick to the adsorbent too strongly. they will elute very slowly slowly (poor seperation)

best to determine optimization by TLC, the best rF value is .20 and .25 which will show 2 distinct peaks spaced out a little

Question 21

partition vs adsorption chromatography model

Answer 21

adosprtion focuses on seperation on liquids based on their ability to adsorb on the surface of silica gel (based on intermolecular forces). This is a solid liquid extraction

the partition chromotography is based on 2 liquid phases being extracted, the stationary phase will be liquid coated in silica beads, the different compounds seperate based on polarity

Question 22

when to use a solid mixture in CC

Answer 22

when the material that is to be purified has a low solubility in eluent

Question 23

in a seperation of a highly polar from a medium polar compound, which procedure would work

Answer 23

-first use a medium polar solvent then switch high polar solvent

this allows for seperation because the medium solvent will dissolve the medium compound then the high polar solvent can dissolve the high polar compound

if high polar was used first it would dissolve both compounds and there would be no seperation

Question 24

compound B is low polarity, compound a and c are high polarity, a is slightly more polar

1. a low polarity solvent
2. a solvent of medium polarity
3. a highly polar solvent

what are results

Answer 24

1. B travels up a little and a + c basically stay right where they are
2. A and C move up a little together, B moves up further
3. B travels up pretty far, A and C move up about medium distance together

Question 25

looking at Rf values you can tell

Answer 25

- how polar they are, the higher the Rf value means they travel up TLC more and are more non polar trying to isolate a compound use a solvent that gives you an Rf close to .20 or .25 (optimization) trying to asses if one solvent will you a good seperation - look ranges if they are too close it is not good.

Question 26

explain how a change in GC parameters will affect retention time a. detector type b. polarity of the solvent used to dissolve and inject the sample c. polarity of the GC column

Answer 26

a. the retention time will not be affected by the detector, the detector is responsible for identifying compounds. It's not a factor in how long compounds will be retained in column b. the solvent at the beginning is evaporated into the injector, the solvent will go into the column afterward, so the polarity shouldn't be too high or low because that will affect if all the molecules can enter in the column. But in general too small of an effect c. the higher the polarity of two compounds the longer the retention times will have. When compounds have stronger attractive forces they will move slower through column hence leading to longer retention time.

Question 27

why is making chalcones a green reaction

Answer 27

uses aldol condensation which uses no solvent use only ethanol/ water mixture which leads to more uniform product and less waste

Question 28

chalcones defined as

Answer 28

alpha, beta unsaturated ketones with one benzene ring attached to the carbonyl carbon and another benzene ring connected to the beta carbon of the conjugated double bond

Question 29

hydrogenation

Answer 29

used for the addition of a molecule of hydrogen to functional groups in the presence of a catalyst we use transfer hydrogenation, a molecule in the reaction mixture is forced to liberate H2 and another molecule in the same reaction mixture is immediately adding the hydrogen molecule it is common to adsorb palladium on carbon black particles to ensure that a large surface area of the solid Pd interacts with reagents in solution.

Question 30

what are the things hydrogenation can do

Answer 30

can add 2 H's and break double bond make o become oh and take off other functional group replace an x with an h make NO2 become NH2

Question 31

what is the difference between transfer hydrogenation and a regular hydrogenation

Answer 31

hydrogenation is the addition of a hydrogen molecule (gas) to functional groups when there's a catalyst. while transfer hydrogentation is a molecule in the reaction is forced to liberate H2 and another molecule in the mixture is adding the hydrogen molecule. essentially in hydrogenation the molecule uses hydrogen directly to be reduced while in transfer hydrogenation the hydrogen is transferred from one molecule to another (hydrogen donor to substrate)

Question 32

for recrystalization process during the synthesis of a chalcone would you speculate that pure acetone would be a better solvent than 95% ethanol

Answer 32

ethanol has a higher boiling point meaning it will take longer to cool leading to more pure crystals, acetone has lower boiling point

Question 33

does having a non polar column in GC change much

Answer 33

No, the polar compound will still come out

Question 34

the mass spectrum basics

Answer 34

spectrum is usually shown as a bar graph and/or tabke the base peak (=the most intense peak) is assigned intensity 100% the masses are often rounded to nearest whole number the base peak is often not the molecular ion peak (M+*) because the instability of M+* is so unstable that is not detected) M+* often (not always) represents the molar mass of compound the molecular ion is an even number unless an odd number of nitrogen atoms is present nitrogen rule M+* is usually accompanied by small peaks to its right

Question 35

the fragmentation of the molecular

Answer 35

the interpretation of the ions produced by fragmentation gives clues about the structure All reactions of M+* (and of the fragments) are unimolecular because MS works under high vacumn conditions in gas phase