Chapter 29 Flashcards
What is chromatography used for?
to separate individual components from a mixture of substances
What do all forms of chromatography have?
1) stationary phase
2) mobile phase
What state is the stationary phase?
solid/liquid supported on a solid
What state is the mobile phase?
liquid/gas
What is chromatography used for?
1) analysis of: drugs/plastics/flavouring/air samples/air samples
2) applications in forensic science
What is a TLC plate?
plastic sheet/glass coated with a thin layer of a solid adsorbent substance - usually silica
In TLC, what phase is the adsorbent?
stationary phase
What is adsorption?
the process by which the solid silica holds different substances in the mixture to its surface
How do you carry out TLC?
1) draw a pencil line 1cm from the end of a TLC plate
2) use a capillary tube to spot a small amount of the sample solution on the base line
3) prepare a chromatography tank from a small beaker with a watch glass on top. pour solvent into the beaker to a depth of 0.5cm
4) place the TLC plate in the beaker, making sure the solvent does not cover the spot, and cover with the watch glass
5) allow the solvent to rise until it is 1cm below the top of the plate. remove the plate and mark the solvent front in pencil
6) circle any visible spots/hold a UV lamp over the plate to locate spots/spray with a locating agent (iodine) to locate spots
How are thin layer chromatograms analysed?
by calculating the value for the retention factor, Rf
Rf =
distance moved by the component / distance moved by the solvent front
Why would you run a TLC alongside pure samples of compounds that may be present?
it is easier to identify the amino acids in the unknown sample visually, without needing to calculate any Rf values
What is gas chromatography used for?
separating + identifying volatile organic compounds present in a mixture
What is the stationary phase in gas chromatography?
a high boiling liquid adsorbed onto an inert solid support
What is the mobile phase in gas chromatography?
an inert carrier gas (helium/neon)
How do you carry out gas chromatography?
1) small amount of volatile mixture injected into apparatus - gas chromatograph
2) mobile carrier gas carries components of the sample through capillary column (contains liquid stationary phase)
3) components slow down as they interact with stationary phase inside column
4) components are separated depending on solubility in stationary phase
The more soluble the component is in the liquid stationary phase, the ___ it moves through the capillary column.
slower
What is retention time?
the time taken for each component to travel through the capillary column
What can retention times be used to identify?
the components present in the sample by comparing these to retention times for known components
What is a peak integration?
the area under a peak
What can peak integrations be used to determine?
the concentrations of components in the sample
How do you determine the concentration of a component in a sample?
1) prepare standard solutions of known concentrations of the compound being investigated
2) obtain gas chromatograms for each standard solution
3) plot calibration curve of peak area against concentration
4) obtain gas chromatogram of compound being investigated under same conditions
5) use calibration curve to measure concentration of compound
How do you test for an alkene?
add bromine water drop-wise - bromine water decolourises from orange to colourless
How do you test for a haloalkane?
add silver nitrate + ethanol + warm to 50C in water bath:
1) chloroalkane - white precipitate
2) bromoalkane - cream precipitate
3) iodoalkane - yellow precipitate
How do you test for a carbonyl?
add 2,4-dinitrophenylhydrazine - orange precipitate
How do you test for an aldehyde?
add Tollens’ reagent and warm - silver mirror
How do you test for a primary/secondary alcohol + aldehyde?
add acidified potassium dichromate(VI) + warm in water bath - colour change from orange to green
How do you test for a carboxylic acid?
add aqueous sodium carbonate - effervescence
When is nuclear spin significant?
if there is an odd number of nucleons (protons + neutrons)
What is resonance?
the rapid flipping between two spin states
What does NMR stand for?
nuclear magnetic resonance
What is the frequency required for resonance proportional to?
magnetic field strength
What does chemical shift measure?
the energy needed for nuclear resonance to take place
What units is chemical shift measured in?
parts per million (ppm)
What chemical are all chemical shifts compared to?
tetramethylsilane (TMS), (CH₃)₄Si
What is the chemical shift value of TMS?
0 ppm
What does the amount of chemical shift depend on?
1) chemical environment
2) nearby electronegative atoms
What is a deuterated solvent?
a solvent in which the ¹H atoms have ben replaced by ²H atoms (deuterium, D)
What information does a carbon-13 NMR spectrum provide?
1) number of carbon environments - number of peaks
2) types of carbon environments - chemical shift
Do carbon atoms that are bonded to different atoms have the same or different chemical shifts?
different
Do symmetrical carbon atoms have the same or different chemical shifts?
same
What information does a proton NMR s pectrum provide?
1) number of proton environments - number of peaks
2) types of proton environments - chemical shift
3) number of each type of proton - ratio numbers
4) number of protons adjacent to a given proton - splitting pattern
For proton NMR, what does the ratio of areas under each peak give?
the ration of number of protons responsible for each peak
What is spin-spin coupling/spin-spin splitting?
the splitting of a main peak into sub-peaks
n + 1 =
the number of sub-peaks in a splitting pattern for a proton with n protons attached to an adjacent carbon atom
What is the splitting pattern for a peak area of 1?
singlet
What is the splitting pattern for a peak area of 1:1?
doublet
What is the splitting pattern for a peak area of 1:2:1?
triplet
What is the splitting pattern for a peak area of 1:3:3:1?
quartet
How do you identify -OH + -NH protons?
1) run proton NMR spectrum as normal
2) add small volume of deuterium oxide, D₂O + shake + run second spectrum
3) OH peak will disappear
