organic analysis

• High resolution mass spectrometry


  Can be 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.
• When organic molecules are passed through a mass spectrometer

  It detects both the whole molecule and fragments of the molecule
• 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 d.p. This can help differentiate between compounds that appear to have similar Mr (to the nearest whole number)
• Accurate masses of atoms: H = 1.0078, C = 12.0000, O = 15.9949, N = 14.0031
• Example 1: Calculating Mr to 4 d.p. to determine molecular formula

  1. CH3CH2OH = 46.0417

  2. H2NCH2NH2 = 46.0530
• Carbon has a value of 12.0000 as it is by definition the standard reference
• Mass spectrum for butane

  • M+2 peak
• M+2 peak


  Occurs if a compound contains a chlorine or bromine atom, due to the two naturally occurring isotopes
• Ratios of M:M+2 peaks

  • CH3Cl = 3:1
  • CH3Br = 1:1
  • C2H4Cl2 = 9:6:1
  • C2H4Br2 = 1:2:1
  • C2H3Cl3 = 27:27:9:1
• Infrared spectroscopy


  Certain groups in a molecule absorb infra-red radiation at characteristic frequencies

  
• Below 1500 cm-1


  Complicated spectra that provide information about the types of bonds present in a molecule
• Above 1500 cm-1

  Use an IR absorption table to deduce presence or absence of particular bonds or functional groups

  
• Functional groups identified by IR

  • C=O 1680 – 1750 cm-1
  • O-H (acid) 2500- 3000 cm-1
• 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 green house effect.

  
• Functional group identification by test-tube reactions

  • Alkene + Bromine water
  • Aldehyde + Fehling's solution
  • Aldehyde + Tollens' reagent
  • Carboxylic acid + Sodium carbonate
  • 1o 2o alcohol and aldehyde + Sodium dichromate and sulfuric acid
  • Chloroalkane + Warm with silver nitrate
• Tollens' Reagent


  Formed by mixing aqueous ammonia and silver nitrate, the active substance is the complex ion of [Ag(NH3)2]+
• Tollens' Reagent reaction


  Aldehydes are oxidised into a carboxylic acid, silver(I) ions are reduced to silver atoms
• Fehling's Solution


  Contains blue Cu2+ ions
• Fehling's Solution reaction

  Aldehydes are oxidised into a carboxylic acid, copper(II) ions are reduced to copper(I) oxide
• UV wavelength radiation passes through the atmosphere to the Earth's surface

  Heats up Earth's surface
• The Earth radiates out infrared long wavelength radiation

  The C=O Bonds in CO2 absorb infrared radiation so the IR radiation does not escape from the atmosphere

  
• The energy absorbed by CO2 is transferred to other molecules in the atmosphere by collisions


  The atmosphere is warmed