Chapter 29 NMR

Question 1

What is chromatography?

Answer 1

A method of separating individual components from a mixture of substances, to help identify each component

Question 2

What are the types of chromatography? What are the phases for each?

Answer 2

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

Question 3

How does TLC separate components?

Answer 3

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

Question 4

How does gas chromatography separate components?

Answer 4

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

Question 5

What is the full method for carrying out TLC?

Answer 5

1. Draw the baseline on the TLC plate in pencil, about 1cm from the base
2. Use a capillary tube to add a small spot of the sample onto the base line
3. Pour the mobile phase into a beaker ensuring the depth is below the pencil line
4. Place the TLC upright, without the sides of the plate touching the beaker. And put a lid on
5. Allow the solvent to rise until 1cm from the top, and draw the solvent front in pencil
6. 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

Question 6

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?

Answer 6

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

Question 7

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?

Answer 7

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

Question 8

Compare TLC and paper chromatography?

Answer 8

TLC meds more substance, is faster, can sustain heating, has sharper separations, a wider range of useable solvents, and high sensitivity of a spot

Question 9

What is the formula for Rf values? Why is it useful?

Answer 9

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

Question 10

What are limitations of TLC?

Answer 10

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

Question 11

What is the process of carrying out gas chromatography? What is retention time?

Answer 11

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?

Question 12

What information does a gas chromatogram give you?

Answer 12

Retention time- identify compound
Peak integration- relative concentrations

Question 13

How can you find out the exact concentration from a gas chromatogram?

Answer 13

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

Question 14

What are limitations of gas chromatography? What is GC-MS

Answer 14

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

Question 15

How do you test for Alkenes, Haloalkanes, Phenol, and Carboxylic acids?

Answer 15

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

Question 16

How do you test for carbonyls, aldehydes, and alcohols?

Answer 16

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

Question 17

How does NMR work?

Answer 17

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

Question 18

What is TMS and what is it’s purpose? What characteristics does it have that makes it useful?

Answer 18

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

Question 19

What type of solvents are used in NMR and why?

Answer 19

Deuterated solvents, protons replaced by deuterium, an isotope with a neutron
This means no solvent nmr peak

Question 20

How do you run an NMR sample?

Answer 20

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

Question 21

What information does carbon 13 NMR provide?

Answer 21

Chemical shift- so often functional groups
Umber of carbon environments

Question 22

What are some general tips for C13 NMR?

Answer 22

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

Question 23

What information does proton nmr give?

Answer 23

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

Question 24

What is the NMR of cyclohexane?

Answer 24

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

Question 25

How do you interpret splitting?

Answer 25

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

Question 26

How do you interpret ratio of hydrogens?

Answer 26

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

Question 27

What are some general tips for proton NMR?

Answer 27

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

Question 28

How do you interpret hydroxyl and amino protons? What can you do to help be sure?

Answer 28

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

Question 29

What is the general order for analysis of a compound?

Answer 29

Mass spec and molecular formula
IR
13 C NMR
Proton NMR (most useful)