organic analysis

Cards (23)

  • when organic molecules are passed through a mass spectrometer it detects both the whole molecule and fragments of the molecule
  • on mass spectrometer, peak with highest mass/charge ratio will be normally due to original molecule that has not fragmented (molecular ion)
    as charge of the ion is +1 the mass/charge ratio is equal to Mr
  • molecular ion is the molecule with one electron knocked off
    it is both an ion and free radical
  • molecular ion formed:
    M --> [M]+. e-
  • high resolution mass spectrometry can be used to determine molecular formula of a compound from accurate mass of molecular ion
    can measure mass to 5d.p helping us differentiate
  • if a compound contains a chlorine or bromine atom then two molecular ion peaks will occur
    a M and M+2 peak will occur due to two naturally occurring isotopes of chlorine or bromine
  • chlorine exists as Cl35 (75%) and Cl37 (25%)
    bromine exists as Br79 (50%) and Br81 (50%)
  • certain groups in a molecule absorb infra-red radiation at characteristic frequencies
  • complicated spectra can be obtained then provide information about the types of bonds present in a molecule
  • below 1500cm-1 is fingerprinting
    part of spectrum is unique for every compound so can be used as a fingerprint
    computer compares IR spectra of fingerprint region against database of known pure compounds to identify compound
  • above 1500cm-1 is functional group identification
    infrared spectroscopy is complicated and contains many signals so picking out functional group signals is difficult
  • rogue absorptions can also occur in IR spectroscopy and are indications of impurities
  • The absorption of infra-red radiation by bonds in this type of spectroscopy is the same absorption that bonds in CO2 , methane and water vapour in the atmosphere do - that causes the greenhouse effect
  • mechanisms of green house effect
    UV wavelength radiation passes through Earth's surface to heat it up, Earth radiates out infrared long wavelength radiation
    C=O bonds in CO2 absorb IR radiation so it does not escape from atmosphere, this energy is transferred to other molecules in the atmosphere by collisions so the atmosphere is warmed
  • test functional group alkene with bromine water
    orange colour decolourises
  • test functional group carboxylic acid with sodium carbonate
    effervescence of CO2 formed
  • test functional group primary and secondary alcohol and aldehyde with reagent sodium dichromate and sulfuric acid
    observe orange to green colour change
  • test functional group chloroalkane by warming with silver nitrate acidified with nitric acid
    observe slow formation of white precipitate AgCl
  • test functional group aldehyde
    tollens reagent: silver mirror forms
    fehling's solution: blue solution to red precipitate
    condition is heating gently
  • tollens reagent formed by mixing aqueous ammonia and silver nitrate
    the active substance is the complex ion [Ag(NH3)2]+
  • aldehydes only are oxidised by tollens reagent into a carboxylic acid, the silver (I) ions are reduced to silver atoms
  • aldehydes only are oxidised by Fehling's solution into a carboxylic acid. The copper (II) ions are reduced to copper (I) oxide
    Blue Cu2+ ions in solution change to red precipitate of Cu2O
  • ketones do not react with Fehling's solution or Tollen's reagent