3.3.6 Organic Analysis

• Testing for alkenes:

  • Shake with bromine water
  • result is bromine water is decolourised (orange to colourless)
• Testing for Haloalkanes:

  • Add NaOH (aq) and warm, acidify with HNO3 , add AgNO3 (aq)
  • Result: precipitate of AgX (for Cl=white, for Br=cream, for I=yellow)
• Testing for alcohols:

  • Add acidified K2Cr2O7 (potassium dichromate(VI)) and heat
  • Result: colour change from orange to green for 1 0 and 20 alcohols (note: no change for 30 alcohols)
• Testing for aldehydes (2 ways):

  1. Warm with Fehling’s solution, result: brick red ppt forms (from blue solution)
  2. Warm with Tollens’ reagent, result: “silver mirror” (Ag(s) ppt) forms
• How do you test for carboxylic acids?

  • Add Na2CO3 (aq)
  • result: CO2 (g) given off - effervescence
• Mass spectrometry: Used to find the relative molecular masses of organic compounds.
• Mass spectrometry process:

  • compound is dissolved in solution, ionised by a high voltage supply (to mostly 1+ ions), accelerated by a negatively charged plate,
  • becomes a beam of ionised molecules, reach detector and cause a current to flow.
  • Time of flight used to work out m/z value and plot graph.
• How does gas chromatography work and what does it tell you?

  • A stream of gas carries a mixture of vapours through a column packed with solids, different compounds move through at different speeds, so they are separated.
  • The amount of each compound can then be measured
• Why do atoms and chemical bonds absorb infrared radiation?

  
  

  They are constantly vibrating - they can absorb infrared radiation that is the same frequency as their frequency of vibration
• What effect do heavier atoms have on the frequency of vibration? Vibrate slower
• What do the troughs on an infrared spectrum show? The frequencies where radiation has been absorbed - match to table to find out which bonds they represent
• What is the fingerprint region?

  • Area of the infrared spectrum below wavenumber of 1500cm-1
  • Many peaks, caused by complex vibrations of the whole molecule.
  • Unique to every compound, so can be used to identify compounds
• What does GCMS stand for? Gas chromatography linked to mass spectrometry
• What effect does a stronger bond have on the frequency of vibration? Vibrate faster
• How does infrared spectroscopy work?

  • Every bond has a unique vibration frequency in the infrared region of the EM spectrum
  • Bonds absorb radiation that has the same frequency as their frequency of vibration
  • Infrared radiation emerged from a sample is missing the frequencies that have been absorbed → this information can be used to identify the compound’s functional group
• How do you test for aldehydes? What is the result?

  1. Warm with Fehling’s solution, result: brick red ppt forms (from blue solution)
  2. Warm with Tollens’ reagent, result: “silver mirror” (Ag(s) ppt) forms