che 321: advanced synthesis

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che 321 advanced synthesis
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Mass spectrometry

• Mass spectrometer
• Subjects molecules to bombardment by a stream of
  high-energy electrons
• Accelerated ions are separated according to their
  mass-to-charge ratios in a magnetic or electric
  field
• Ions having a particular mass-to-charge ratio are
  detected by a device that can count the number of
  ions striking it
• Detectors output is amplified and fed to
  recorder/computer
• Results in mass spectrumgraph of number of
  particles detected as a function of
  mass-to-charge (m/z) ratio

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Mass spectrometer
http//upload.wikimedia.org/wikipedia/commons/b/b8
/Mass_spectrometer_schematics.png
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GC-MS
http//www.chemsoc.org/ExemplarChem/entries/2004/w
estengland_smith/ExempWeb/images/GC-MS.2.gif
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Mass spectrum
Base peak m/z 82
Fragment ions
Molecular ion M 155
Isotope peak
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Interpreting mass spectra

• Determining molecular mass
• Observing true mass (not average)
• Molecules may fragmenthow do you know which is
  M peak? M must
• Have highest mass in spectrum (excluding isotope
  peaks)
• Have odd of electrons (radical cation)
• Must be capable of forming important fragment
  ions in the spectrum

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Interpreting mass spectra

• Observed abundance of suspected M must
  correspond to expectations based on molecular
  structure
• Lifetimes of molecular ions
• Aromatics gt conjugated alkenes gt alicyclic
  compounds gt organic sulfides gt unbranched
  hydrocarbons gt mercaptans gt ketones gt amines gt
  estersgt ethers gt carboxylic acids gt branched
  hydrocarbons gt alcohols

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Interpreting mass spectra

• Nitrogen rule
• If a compound has no N atoms or an even number of
  N atoms, it will have an even mass number
• If the compound contains odd of N atoms, the
  mass number will be odd

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Isotope ratios

• Can use abundance data to predict height of
  isotope peaks
• Bromine 11 peaks at M and M2
• Chlorine 31 peaks at M and M2
• For other elements, use isotopic abundance and
  molecular formula to calculate ratios