Chapter 29 NMR

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

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

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

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

• 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

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

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

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

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

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

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?

Retention time- identify compound Peak integration- relative concentrations

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

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

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

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

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

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

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

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

Chemical shift- so often functional groups Umber of carbon environments

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

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

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