Chapter 29 - Chromatography and Spectroscopy Flashcards
What are the stationary and mobile phases of chromatography?
Stationary - no movement, normally a solid or liquid supported on a solid
Mobile phase - does move, normally a liquid or gas
What is TLC?
Thin layer chromatography - DISSOLVED SUBSTANCES
Uses a TLC plate (plastic sheet or glass) coated with a thin layer of a solid adsorbent substance (usually silica) - this is the stationary phase.
Separation is achieved by the relative adsorptions of substances with the stationary phase
What is adsorption?
The process by which the solid silicas holds the different substances in the mixture to it’s surface
What process separates components in a mixture in both gas and thin layer chromatography?
Adsorption
What is Rf, how is it calculated?
Retention factor
Rf = distance moved by component/distance moved by solvent front
What are the stationary and mobile phases of gas chromatography?
Stationary = high boiling liquid adsorbed onto an inert solid support in column
Mobile = inert carrier gas like helium or neon
How are mixtures separated in gas chromatography?
Dependent on their relative solubility in the liquid stationary phase, meaning they reach the detector at different times - retention time is the time taken for each component to travel through the column
What 2 key pieces of information can be gained for gas chromatograms?
- retention time, which can be compared to known component retention times
- peak integrations (area under each peak) used to determine concentrations of components in the sample
What is retention time?
The time between injection and emergence/detection of a component
How can you determine the concentration of a component in a sample (procedure)?
- prepare standard solutions of known conc of the compound being investigated
- obtain gas chromatograms for each standard solution
- plot a calibration curve of peak area against concentration
- obtain a gas chromatogram of the compound being investigated under same conditions
- use calibration curve to measure concentration of the compound
How can you test for an alkene functional group?
Add bromine water drop wise, bromine water is decolourised from orange to colourless
How can you test for a haloalkane?
Add silver nitrate and ethanol and warm to 50 degrees in a water bath
Chloro = white precip
Bromo = cream precip
Iodo = yellow precip
How can you test for a carbonyl functional group?
Add 2,4 - DNP and an orange precipitate will form
How can you test for an aldehyde functional group?
Add Tollen’s reagent and warm, silver mirror will form
How can you test for a primary and secondary alcohol and an aldehyde?
Add acidified potassium dichromate (VI) and warm in a water bath. There will be a colour change from orange to green
How can you test for a carboxylic acid functional group?
Add aqueous sodium carbonate, observe effervescence
What is NMR spectroscopy?
Nuclear magnetic resonance spectroscopy
What is resonance?
Property of nucleus to absorb energy when in the right combination of a strong magnetic field and radio frequency radiation and flip rapidly between electron’s 2 spin states
What is the frequency shift in NMR measured in?
Chemical shift 𝛿 in units of parts per million
What is TMS used for in NMR?
TMS = tetramethylsilane, used as a standard reference chemical against which all chemical shifts are measured
How is an NMR spectrum produced?
Sample dissolved in solvent and placed in a narrow NMR sample tube with a small amount of TMS. Placed inside NMR spectrometer where it is spun and the sample is given a pulse of radiation, any absorptions of energy are detected and displayed on a computer screen
What are deuterated solvents and why are they used?
Solvent where 1H atoms have been replaced by 2H atoms (deuterium), which produces no NMR signal in frequency ranges
What does the number of peaks in carbon NMR tell you?
The number of different carbon environments within the molecule (each peak = different environment)
What does the shift of peaks in carbon NMR tell you?
The types of carbon environments present within the molecule
When do carbon atoms have the same environment in C NMR and why?
If they are positioned symmetrically within a molecule they have the same chemical environment. They will then absorb radiation at same chemical shift and contribute to same peak
What does the number of peaks in proton NMR tell you?
The number of different proton (H) environments present
What does the chemical shift of peaks in proton NMR tell you?
The types of different proton (H) environments present
What does the relative peak areas/integration trace in proton NMR tell you?
The relative numbers of each type of proton (H)
What does the spin splitting pattern in proton NMR tell you?
The number of non equivalent protons adjacent to a given proton
What is the n+1 rule?
For a proton (H) with n protons attached to an adjacent carbon atom, the number of sub peaks in a splitting pattern = n +1
What are the common splitting patterns/what they show?
No H on adjacent atoms = singlet
1 H on adjacent C atom = doublet (CH)
2 H on adjacent C atom = triplet (CH2)
3 H on adjacent C atom = quartet (methyl)
6 H on adjacent C atom = heptet (dimethyl)
What do multiplets in proton NMR often suggest?
Aromatic protons
What type of peaks do N-H and O-H groups create in proton NMR?
Broad peaks
How does proton exchange identify -OH and -NH protons?
- normal proton NMR spectrum run
- small volume of D2O added, mixture shaken and a second spectrum run
Deuterium exchanges and replaces OH and NH protons with deuterium atoms, which are not absorbed on chemical shift frequency, so peaks disappear
What is a typical sequence for identification?
Elemental analysis (% comp by mass)
Mass spectra
Infrared spectra
NMR spectra
