6.3.1 - qualitative analysis

• To test for alkenes you shake with bromine water. Positive result is decolourised from orange to colourless
• To test for haloalkanes you add warm NaOH and acidify with HNO3 and AgNO3.

  Positive result is a precipitate of AgX either white (AgCl), cream (AgBr) or yellow (AgI)
• To test for alcohols add acidified potassium dichromate (VI) and heat

  positive result colour changes from orange to green for primary and secondary alcohols. there is no change for tertiary Alcohols.
• To test for aldehydes you can either

  add fehlings solution and warm, a positive result forms a brick red precipitate

  add tollens reagent and warm, a positive result forms a silver mirror.
• To test for carboxylic acids you add aqueous sodium carbonate and the positive result gives off carbon dioxide effervescence
• To test for phenols (by weak acidity) you add NaOH which gives a neutralisation reaction but when reacted with a carbonate there is no reaction.
• the principles of chromatography are separation techniques with the principle that a mixture is separated if it is dissolved in a solvent and this mobile phase is passed over a solid (stationary phase)
• The mobile phase carries the soluble components of the mixture
• more soluble components or ones that have more affinity to the solvent, will move faster.
• The stationary phase holds back components of the mixture that are attracted to it.
• A sample moves slower when there is more affinity to the stationary phase. Often attracted By hydrogen bonding.
• Substances are separated by chromatography because the affinity for the mobile phase and the stationary phase are different for each component. So they move at different rates and are separated over time.
• Different substances have different rf values as different substances have different polarities so have different retention times or rf values and are attracted more or less stronger to the stationary phase.
• TLC is thin layer chromatography
• The stationary phase in TLC is a plastic, glass, metal sheet coated in silica or alumina
• Advantages of TLC over paper chromatography are:

  • Runs faster
  • smaller amounts of a mixture can be separated
  • TLC plates are more robust
• To observe colourless spots in TLC you can either shine UV light on them.

  Or spray with a developing agent (eg, ninhydrin) which turns amino acid spots from colourless to purple
• Rf value = distance moved by solute / distance moved by solvent
• rf value stands for retention factor
• retention factor is a measure of the rate of movement through chromatography; rate of movement of the solvent and that component
• To confirm the identity of a substance from its rf compare it to data book values for samples run in the same solvent.
• The stationary phase in GLC is powder coated in oil. It is packed into a long thin capillary tube ( 100m long and 0.5mm diameter). It is coiled and placed in an oven so the temperature can be varied.
• The mobile phase in GLC is a carrier or inert gas eg, N2 or He
• In GLC you measure retention time. Since different components of the mixture take different amounts of time to move through.
• Advantages of GLC

  very sensitive

  GC can detect minute traces of substances
• GLC is used to test athletes blood and urine for drugs
• GC or GCMS can be used to identify substances by matching the gas chromatograph to that of a known substance. Substances identity can then be confirmed by mass spec, nmr or IR
• GCMS works as gas chromatography is run with retention time recorded. Then the mixture is run through a mass spectrometer and fragmentation pattern and molecular ion peak confirms identity.