chapter 29 Flashcards
stationary phase
does not move and is normally a solid or liquid suspended on a solid
mobile phase
moves, liquid or gas
chromatography uses
analysis of drugs, plastics, flavourings, air samples & applications in forensic science
Thin Layer chromatography (TLC)
indicates how many components are in a mixture. uses a plastic sheet/glass coated with a thin layer of a solid adsorbent substance- usually silica
-adsorbent= stationary phase, diff components have different affinities for absorbent and bind w/ differing strengths to the surface
adsorption (TLC)
the process by waging solid silica holds the different substances in the mixture to its surface
- separation achieved bu relative adsorptions of substances w/ stationary phase
interpretation of TLC
-calculate retention factor for each component, can be compared w/ known values
Rf= distance moved by component/distance moved by solvent front
gas chromatography-phases
- stationary phase= high bp liquid adsorbed onto a inert solid support
- mobile phase=inert carrier gas eg helium/ Neon
gas chromatography process
- small amount of the volatile mixture is injected into apparatus, mobile carrier gas carries components through capillary column which contains liquid stationary phase absorbed onto solid support
- components slow down as they interact with liquid stationary phase. the more soluble the component the slower it moves through capillary column
- separated depending on solubility- reach detector at different times
- compound retained in column for shortest time has the lowest retention time & is detected first
retention time
time taken for each component to travel through the column
interpretation of a gas chromatogram
- retention times can be used to identify components present by comparing to known retention times
- peak integrations (area under each peak) can be used to determine cons of components in sample
1) prepare standard sol of known conc
2) obtain gas chromatograms for each
3) plot a calibration curve of peak area against concentration= external calibration
4) obtain gas chromatogram of investigated compound
5) use calibration curve to measure conc
qualitative analysis- alkene
add bromine water drowse
decolourise, orange-colourless
qualitative analysis - haloalkane
add silver nitrate and ethanol, warm to 50 degrees in water bath
chloroalkane- white precipitate
bromoalkane- cream
iodoalkane- yellow
qualitative analysis- carbonyl
add 2,4- dinitrophenylhydrazine, orange precipitate
qualitative analysis- aldehyde
add pollens reagent & warm
silver mirror
qualitative analysis- primary & secondary alcohol + aldehyde
add acidified potassium dichromate (VI) & warm in a water bath
orange– green
qualitative analysis- carboxylic acid
add aq sodium carbonate- effervescence
nmr
magnetic field, nuclei of some atoms absorb radiation, energy for absorption measured
nuclear spin
nucleus has nuclear spin if odd number of protons & neutrons- H1 & C13
resonance
electron has 2 diff spin rates, so does nucleus - have different energies
nucleus can absorb energy and rapidly flips between to spin states= resonance
chemical shift & TMS
frequency shift parts per million (ppm)
Tetramethylsilane is used as the standard reference- value of 0ppm
amount of chemical shift is determined by chemical environment
running the spectrum
- sample is dissolved in a solvent & placed in a sample tube w/ small amount of TMS
- spun to even out any imperfections in magnetic field
- spectrometer zeroed against TMS standard and sample given a pulse of radiation, absorptions are detected
- after- sample can be recovered by evaporation of solvent
Deuterated solvents
H1 atoms replaced by H2 (deuterium)
- deuterium produces no NMR signal
- CDCl3 commonly used
carbon-13 NMR spectroscopy
-number of different c environments- from numbers of peaks
-types of carbon environment- from chemical shift
chemical environment of c atom is determined by position of the atom within the molecule
- c atoms bonded to diff atoms or groups have diff environments- diff chemical shifts
-if 2 c atoms are positioned symmetrically within a molecule- same chemical environment - same peak
Proton NMR spectroscopy
- number of proton environments- from number of peaks
- types proton environments- chemical shift
- relative numbers of each type of proton- from integration traces or ratio numbers of peak areas
- the number if non-equivalent protons adjacent to a give proton- from spin-spin splitting pattern
equivalent and non equivalent protons - butanoic acid & butanedioic acid
-equivalent- same chemical shift- increasing size of peak
-non-equivalent- diff shifts
butanoic acid
- the 2 CH2 groups in different environments
-no plane of symmetry
- 4 peaks
butanedioic acid
-2 CH2 groups in same environment
-symmetry
-2 peaks; 1 for 2 equivalent COOH protons, 1 for protons in CH2
relative numbers of each type of proton
- ratio of relative areas under each peak gives the ratio of the number of protons responsible for each peak
- measures area under each peak as an integration trace- number
spin-spin coupling
splitting patterns - caused by proton’s spin interacting w/ nearby protons in diff environments
the n+1 rule
splitting- the number of sub peaks is 1 greater than the number of adjacent protons
- for a proton with n protons attached to an adjacent carbon, number of sub peaks = n+1
more complex splitting
adjacent protons may have different environments eg central CH2 split differently by CH3 and CH2- multiplet
spin-spin coupling occurs in pairs
because each proton splits the signal to another
hydroxyl & amino protons
NH & OH protons may be involved in hydrogen bonding so peaks are often broad and of variable chemical shift
usually not involved in spin-spin coupling
proton exchange- identifying
1) a proton NMR is run normally
2)small vol of deuterium oxide added D2O- shaken and spectrum is run
Deuterium exchanges and replaces OH and NH protons in sample w/ deuterium atoms- OH disappears
combined techniques- structure determination
elemental analysis, use % composition by mass to determine empirical formula——–>
mass spectra, use molecular ion peak to determine molecular mass and fragment ions to determine parts of molecule. molecular can be determined from empirical & mass ———–>
infrared spectra, use absorption peaks to identify bonds and functional groups ——–>
NMR spectra, to determine the number and types of carbon and hydrogen atoms from chemical shifts & order of atoms from splitting patterns
