Organic analysis

• Mass Spectrometry


  Technique used to determine the molecular formula of a compound from the accurate mass of the molecular ion
• Molecular ion


  • The peak with the highest mass/charge ratio, due to the original molecule that hasn't fragmented
  • As the charge of the ion is +1, the mass/charge ratio is equal to Mr
• Molecular ion formation


  Molecule M → [M]+. + e–
• Molecular ion


  The molecule with one electron knocked off (It is both an ion and a free radical)
• High resolution mass spectroscopy


  • Can measure the mass to 5 decimal places, helping to differentiate between compounds with similar Mr
• Accurate masses of atoms: H = 1.0078, C = 12.0000, O = 15.9949, N = 14.0031
• Determining molecular formula from accurate mass


  1. Calculate Mr to 4 decimal places

  2. Compare to expected molecular formulas
• If a compound contains a chlorine or bromine atom, two molecular ion peaks (M and M+2) will occur due to isotopes
• The ratio of heights of M:M+2 peaks for a compound with one chlorine or bromine atom will be 3:1 or 1:1 respectively
• If a compound contains two chlorine or bromine atoms, a M+2 and M+4 peak will occur
• The ratio of heights of M:M+2:M+4 peaks for a compound with two chlorine or bromine atoms will be 9:6:1 or 1:2:1 respectively
• Infrared spectroscopy


  • Certain groups in a molecule absorb IR radiation at characteristic frequencies
  • Complicated spectra can provide information about the types of bonds present
• Fingerprinting region (below 1500 cm-1)


  Complicated spectra that are unique for every compound, can be used to identify the compound by comparison to a database
• Functional group identification region (above 1500 cm-1)


  Use an IR absorption table to deduce presence or absence of particular bonds or functional groups
• Functional group identification


  • C=O 1680-1750 cm-1
  • O-H (acid) 2500-3000 cm-1
• Absorption of IR radiation by bonds is the same as causes the greenhouse effect
• Functional group identification tests

  • Alkene + bromine water
  • Aldehyde + Fehling's solution
  • Aldehyde + Tollens' reagent
  • Carboxylic acid + sodium carbonate
  • 1o/2o alcohol, aldehyde + sodium dichromate/sulfuric acid
  • Chloroalkane + warm silver nitrate
• Tollens' reagent


  Aqueous ammonia and silver nitrate, oxidises aldehydes to carboxylic acids, forming a silver mirror
• Fehling's solution


  Contains blue Cu2+ ions, oxidises aldehydes to carboxylic acids, forming a red Cu2O precipitate
• Mechanism of greenhouse effect

  1. UV radiation heats Earth's surface

  2. Earth radiates IR radiation

  3. CO2 bonds absorb IR, transferring energy to atmosphere by collisions, warming it