6.3 Analysis

6.3 Analysis

1. Chromatography

   • Rf value- comparison between how far a component has moved compared to the solvent in thin layer chrom.
     • Rf= distance moved by component/distance moved by solvent
   • Retention time- time taken for a component to travel from the inlet to the detector in a GC
   • Separation technique to identify chemicals in a mixture
     • Phase one- stationary-This could be a liquid in GC where liquid components have different attractions to each phase. Or a solid in TLC where the components of a mixture are absorbed by different amounts.
       • Phase two- mobile- moves in a definite direction. This could be an inert gas as in GC or a liquid solvent in TLC.
     • The separation occurs as each chemical in a mixture has a different attraction to each phase. Eg if attracted to mobile phase then will move far up TLC.
   • One-way thin layer TLC. check purity or extent of chemical reaction. Stationary phase of thin inert material e.g. glass, covered in absorbent chemical e.g. aluminium oxide. Mobile phase is organic solvent which will move vertically up the plate.
   • GC. used to separate volatile components in a mixture. connected to mass spec. stationary phase is solid of liquid coating on coiled tube (usually hydrocarbon w/high boiling point. mobile is unreactive carrier gas e.g. helium
     • Produce chart of absorption against time. x-axis is retention time & can be compared to known values. Greater peak=greater conc.

2. Test for organic functional groups.

   • Alkenes
     • Bromine water
     • Add few drops and shake well. decolourises
   • Haloalkanes
     • Silver nitrate(aq) & ethanol
     • White ppt=Cl. Cream ppt=Br. Yellow ppt=I
   • Carbonyl
     • Bradys reagent
     • Yellow/orange ppt
   • Aldehydes
     • Fehlings solution. Tollens reagent. Acidified potassium dicromate
     • FS, blue solution forms dark red ppt. TR, silver mirror. APD, orange solution turns green.
   • Aliphatic carboxylic acids
     • Universal indicator(UI). Reactive metal eg magnesium. Metal carbonate eg calcium carbonate
     • Check pH for weak acid. Will cause effervescence with other two.
   • Phenols
     • UI. no reaction with carbonate
     • pH for weak acid
   • Alcohols
     • Acidified potassium dicromate
     • Orange to green as first aldehyde is formed then carboxylic acid. Sec. alcohols form ketone. Teritary no observation.

3. Intro to NMR

   • TMS-internal standard for NMR.
     • Chemical shift- scale that compares frequency of NMR absorption with freq of the reference peak of TMS
     • Deuterium- isotope of hydrogen & does not produce single proton on NMR
   • Non-distructive analytical technique. Confirms formula mass of organic molecule and clue of structure.
   • The direction of spin of nucleons aligns with direction of magnetic field. Energy (radio waves) is used to for the spins to flip in other direction. Energy required depends on environment that the atom is in & compared to standard.
   • Internal standard- reference chemical is added to NMR spectrometer to compare values. Tetramethylsilane(TMS) is used. TMS=0
     • TMS is good as- contains both H and C. produces one sharp signal as H atoms are in single environment. As theres only one carbon atom, this was also produce one signal in C13 NMR. non-toxic. low bp & volatile so easily removed from sample. inert so unlikely to react w/chemical being tested.
   • Chemical shift. x-axis on NMR spectrum.
     • Everything can be evaporated off to get back original sample.
   • Solvents. First dissolve molecules in organic solvent e.g. benzene. However, this contains C & H so make spec hard to read. H-1 is only H isotope to have spin property so need to use deuterium as not active in NMR. Has greater atomic number as is described as 'heavy'.

4. Carbon-13 NMR

   • 3 isotopes but only C-13 has spin. NMR gives number of carbon environments but not ratio of atoms in each environment. Equivalents are C atoms in same environment, bonded to the same atoms and feel the same magnetic field.

5. Proton NMR

   • TMS has 12 equivalent protons. Much smaller sample size can be used as there will be a signal for each environment that a carbon atom is found in.
   • Spin-spin coupling causes a splitting pattern of neighbouring protons. Protons on neighbouring carbon atoms affect spin as well as magnets. N+1.

6. NMR spectre of -OH and -NH protons

   • peaks can appear over wide range of different chemical shift values
   • Signals are often broad and usually no spinning pattern.
   • Way of filtering out absorptions is use of D2O
   • D2O
     • Heavy Water & immiscible
     • React identically but Deuterium nucleus does not produce signal in proton NMR spectrum
     • Exchanges with the H present in -OH and -NH
   • -OH and -NH usually show as singlet (aren't split)
     • If sample aren't dry & have water in them then the H bonds to make the peak broad.
     • Protons on adjacent atoms are not split, nor on the singular atoms themselves.

7. Combined techniques

   • Mass Spec.
     • Chemical analysis provided by % mass of element.
     • Empircal formula determined
     • Gives relative Mr
     • Fragmentation values.
   • Infrared Spec.
     • Identify functional group.
     • Bonds present in a molecule & non-destructive technique.
   • NMR
     • No. and types of carbon environments in molecule.
     • Proton- info about number of each type os proton environment & no. of protons on adjacent C atoms.
     • Gives most info.