Further organic chemistry: Spectroscopy & Chromatography -4

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  • Spectroscopy and Chromatography
    • Different types of radiation on molecules
      • UV
        • Used to initiate reactions
        • Causes covalent bonds to break homolytically, resulting in free radicals
      • Radio waves
        • Used in nmr
        • Affects the spin of hydrogen nuclei in a strong magnetic field
      • Microwaves
        • Used for heating
        • Cause molecules with polar bonds to rotate faster
      • Infrared
        • Used in analysis
        • Causes the bending and stretching of bonds
    • NMR spectroscopy
      • Identifies the structure of molecules using radio waves
        • How many hydrogens there are and how they're arranged
      • Hydrogen nuclei (protons) are spinning, this means that in the nuclei there is a weak magnetic field
        • NMR looks at how this magnetic field reacts when it is placed in a larger external magnetic field
        • Spin states are degenerate
      • When in an external magnetic field, protons align themselves either with the direction of the fields or opposing it
        • Aligned protons are at lower energy than the opposing protons
          • If they absorb radio waves they can be promoted to the higher energy level
            • Energy emitted when returning to group state
        • Opposing protons, at higher energy, can emit radio waves and 'flip' to the lower energy level
        • Spin states non-degenerate
      • Protons in different chemical environments absorb different amounts of energy
        • The environment is dependent on the groups it is attached to
          • If protons are in the same chemical environments, they are referred to as being 'equivalent'
        • Signals on a NMR spectrum are due to one or more protons in a particular proton environment
          • Signals can be split, these are dependent on the number of protons on the neighbouring carbon atom
            • Add 1 to the number of Hs on adjacent atoms
            • Splitting only seen on a high resolution nmr
        • Electrons provide shielding
          • Deshielded end of spectrum (left) - stripped electrons
          • Shielded end of spectrum (right) - electron density not stripped
    • Infrared
      • Infrared spectroscopy can be used to follow a reaction
        • Changing functional groups
    • Chromatography
      • Gas chromatography
        • Mobile phase = a gas (normally inert)
        • Stationary phase = solid - silicia on outside of column
        • Low affinity for stationary phase are eluted first
      • HPLC
        • Mobile phase = a solvent
        • Stationary phase = solid - silicia on outside of column
        • Reverse phase - uses polar solvents and a non-polar stationary phases
        • Normal phase - uses non-polar solvent and a polar stationary phase
      • The separation of mixtures
    • Mass spec
      • Has characteristic fragment patterns, e.g CH3 = peak @15
      • Molecular ion peak = Mr

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