Chapter 29 NMR Flashcards
What is chromatography?
A method of separating individual components from a mixture of substances, to help identify each component
What are the types of chromatography? What are the phases for each?
TLC Stationary= alumina/silica on inert support material, Mobile= solvent, eluent
Gas chromatography = Stationary= high boiling point liquid on inert solid support, Mobile= inert carrier gas like Ne or He
How does TLC separate components?
The substances will adsorb by different amounts to the surface of the stationary phase
Greater adsorption, harder to move with the solvent, so lower Rf
Separation by relative adsorption
How does gas chromatography separate components?
By relative solubility
The more soluble the substance is in the liquid stationary phase, the longer it will take to travel to the detector with the carrier gas
What is the full method for carrying out TLC?
- Draw the baseline on the TLC plate in pencil, about 1cm from the base
- Use a capillary tube to add a small spot of the sample onto the base line
- Pour the mobile phase into a beaker ensuring the depth is below the pencil line
- Place the TLC upright, without the sides of the plate touching the beaker. And put a lid on
- Allow the solvent to rise until 1cm from the top, and draw the solvent front in pencil
- Allow to dry and use locating agents such as I2 or Uv. Measure the distance travelled from the baseline to the centre of each spot, and the height of the solvent front
Why do you use pencil in TLC? Why do you use a small sample amount only? Why does the solvent amount need to be below the pencil line?
Prevents ink bleeding and contaminating the sample
Small amount to prevent overlap
Below pencil line to stop immediate dissolving of the substance, the solvent needs to flow up rather than being immersed in it for correct Rf
Why can the sides of the TLC not touch the beaker? Why do you use a lid? Why do you draw on the solvent front in pencil?
The solvent needs to travel up the TLC plate evenly to get a horizontal spot
Lid as solvents volatile
Solvent from disappears quickly but needed for calculations
Compare TLC and paper chromatography?
TLC meds more substance, is faster, can sustain heating, has sharper separations, a wider range of useable solvents, and high sensitivity of a spot
What is the formula for Rf values? Why is it useful?
Rf= (distance moved by the component at its centre) / distance moved by the solvent
Can compare to a database, where THE SAME SOLVENT IS USED, to look up what compound it is based on the Rf
What are limitations of TLC?
Difficult to measure the exact centre
Similar Rf values are difficult to tell apart
Needs a solvent that will be able to dissolve all components
If a new compound is synthesised, there will not be a reference Rf
What is the process of carrying out gas chromatography? What is retention time?
The sample is injected, and moves through the stationary phase liquid capillary column with the carrier gas
Components slow down in the stationary phase liquid, more soluble, more slowed down
Components reach detectors at different times based on interactions
Retention time= time take for a component to reach the detector?
What information does a gas chromatogram give you?
Retention time- identify compound
Peak integration- relative concentrations
How can you find out the exact concentration from a gas chromatogram?
Make varying known concentrations of the compound you want to test
Run standard solutions of known concentration of the compound being investigated in the gas chromatogram, record area, use the same conditions (e.g temp) each time
Plot a calibration curve of peak area against concentration
Compare area in the mixture to the graph
What are limitations of gas chromatography? What is GC-MS
Components may have similar retention times
A small amount of one component may hide behind another larger peak
If a new compound, no known retention times
GC-MS= gas chromatography and mass spec in the same thing
How do you test for Alkenes, Haloalkanes, Phenol, and Carboxylic acids?
Bromine water- orange brown to colourless
Add aq AgNO3 and ethanol, heat, precipitate to match colour to halide
Bromine water for decolorisation and white precipitate, weakly acidic but no reaction with carbonates
aq Na2CO3, fizzing
How do you test for carbonyls, aldehydes, and alcohols?
Carbonyls- 2,4 DNP, yellow to orange precipitate
Aldehydes- Tollen’s reagent (Ag+/NH3), heat, silver mirror
Aldehydes and primary/secondary alcohols- Dichromate, orange to dark green
How does NMR work?
Uses a very strong magnet and radio waves which nuclei with an odd number of nucleons can absorb
The nucleus has 2 spin states, and at the right energy, it will rapidly switch between these two states, this is resonance
Depending on the chemical environment, e.w.g and e.d.g, the amount of energy varies, and this correlates to chemical shift, which can help us determine structure
What is TMS and what is it’s purpose? What characteristics does it have that makes it useful?
Tetramethylsilane, acts as a chemical shift standard, a reference points for shifts, 0ppm
Very unreactive, 1 sharp peak so easy to filter
Nuclei highly shielded, so very unlikely there would be a peak below it
Volatile, so easily removed from the sample if it needs to be recovered
What type of solvents are used in NMR and why?
Deuterated solvents, protons replaced by deuterium, an isotope with a neutron
This means no solvent nmr peak
How do you run an NMR sample?
Dissolve in a solvent and placed in NMR sample tube
Run NMR, spin out
0 against TMS, vary the frequency of the radio waves
Absorbance detected and graphed
What information does carbon 13 NMR provide?
Chemical shift- so often functional groups
Umber of carbon environments
What are some general tips for C13 NMR?
Helpful with Benzene substitution, 4 means 1,4 and 6 means 1,3 or 1,2 if two different groups
Final check with n of carbon peaks
Look for symmetry especially with rings look at distance to both sides
What information does proton nmr give?
Chemical shift- so functional groups
N of proton environments
N of protons in each environment (relative)
Splitting, so n of hydrogens on the adjacent carbon
What is the NMR of cyclohexane?
1 singlet peak in the HC-R range, with 12 hydrogens
No splitting as splitting does not occur when the protons are in the same environment
How do you interpret splitting?
N+1= n of peaks in splitting
So 3 peaks, 2 Hs on the neighbouring carbon
Unlike a quartet will be a CH2 and CH adjacent, more likely a CH3
How do you interpret ratio of hydrogens?
If decimals, divide by the smallest number, and scale up the ratio till the number of hydrogens is the same as in the molecular formula
What are some general tips for proton NMR?
Draw the table, and most importantly the section
Assume the data ranges are right at first, unless they don’t match at all, then think about shifting up
Normally alkenes aren’t there so shifted, but sometimes carbonyl can shift up more
Check for degrees of saturation to figure out if Ether or carbonyl
Be careful, some functional groups are still present but there may not be protons nearby, e.g if there are 3 halogens on one carbon in the chain, not protons attached to the same carbon, no peak, but still present
How do you interpret hydroxyl and amino protons? What can you do to help be sure?
Normally a lot wider than the others, shorter, and always a singlet
There will normally be other clues as well, such as in IR or C13 NMR
D2O exchange, mix with the sample, the protons in question become replaced with deuterium, so do not produce a peak
The peaks that disappear will be from hydroxyl/amino protons
What is the general order for analysis of a compound?
Mass spec and molecular formula
IR
13 C NMR
Proton NMR (most useful)
