final exam Flashcards
mobile phase
gas or liquid
in tlc it’s the solvent
in gc it’s the inert gas (helium typically)
stattionary phase
liquid or solid
in TLC it’s moving past silica gel
in GC it’s going through the column
best seperation of TLC
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
what does GC create
molecules come out at different times and then signals are generated by the help of a computer
the injector of GC
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)
The column of GC
stationary phase
where molecules seperate
molecules with strongest attractive forces will move through column the slowest while non polar moves through faster
what happens when column temperature is either too high or too low
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
detector in GC
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
gas chromatogram features
retention time - time needed for compound to pass through the compound
peak area - is proportional to the amount of the compound that is detected
parameters that could affect the outcome of gc
*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
peak area with GC and integrals/ratios
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
response ratios of GC
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
GC theory
Sn2 reaction
CC adsorption/mobile and stationary phase
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
process behind CC
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
seperation of CC
more polar molecules will move slower throughout the column than non polar molecules
slurry packed method vs dry pack method in CC
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
applying compounds to the column (as liquid or solid) in CC
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
detecting separated compounds in CC
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
optimizing seperation in CC
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
partition vs adsorption chromatography model
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
when to use a solid mixture in CC
when the material that is to be purified has a low solubility in eluent
in a seperation of a highly polar from a medium polar compound, which procedure would work
-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
compound B is low polarity, compound a and c are high polarity, a is slightly more polar
- a low polarity solvent
- a solvent of medium polarity
- a highly polar solvent
what are results
- B travels up a little and a + c basically stay right where they are
- A and C move up a little together, B moves up further
- B travels up pretty far, A and C move up about medium distance together
looking at Rf values you can tell
- 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.
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
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.
why is making chalcones a green reaction
uses aldol condensation which uses no solvent
use only ethanol/ water mixture which leads to more uniform product and less waste
chalcones defined as
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
hydrogenation
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.
what are the things hydrogenation can do
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
what is the difference between transfer hydrogenation and a regular hydrogenation
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)
for recrystalization process during the synthesis of a chalcone would you speculate that pure acetone would be a better solvent than 95% ethanol
ethanol has a higher boiling point meaning it will take longer to cool leading to more pure crystals, acetone has lower boiling point
does having a non polar column in GC change much
No, the polar compound will still come out
the mass spectrum basics
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
the fragmentation of the molecular
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
