29.1 - chromatography and functional group analysis

• Chromatography


  Chromatography allows us to separate and identify individual components from a mixture of substances. All forms of chromatography have a stationary phase and a mobile phase.
• Stationary phase in thin layer chromatography


  The phase that does not move.

  - Thin layer of alumina or silica mounted on a glass/metal plate.
• What happens in the stationary phase of thin layer chromatography?


  - The solute molecules adsorb onto the surface.

  - The relative adsorptions of the separated components determines how far they will travel along the stationary phase.

  - The more they interact with the stationary phase, the more they will 'stick' to it.

  - The more adsorbent a substance, the less distance it will travel.
• Mobile phase in thin layer chromatography


  The phase that does move.

  - Flows over the stationary phase.

  - It is a polar or non-polar liquid (solvent) that carries components of the compound being investigated.

  - Polar solvents - water or alcohol.

  - Non-polar solvents - alkanes.
• What is chromatography used for?


  Used in analysis of drugs, plastics, flavourings, air samples, and has applications in forensic science.
• Advantages of thin layer chromatography (TLC)


  Quick and inexpensive.
• Thin layer chromatography (TLC)


  An analystic technique that indicates how many components are in a mixture.
• TLC plate


  A plastic sheet or glass coated with a thin layer of a solid adsorbent substence - usually silica.
• Adsorbent

  The stationary phase.
• How does TLC work?


  The different components in the mixture have different affinities for the absorbent and bind with differing strengths to its surface.
• Adsorption


  The strength that the stationary phase (silica) holds the different substances in the mixture to its surface.
• Steps of carrying out TLC


  1. Draw a baseline in pencil on the TLC plate.

  2. Spot a small amount of the sample on the baseline.

  3. Place the TLC plate in a beaker with some solvent, making sure the baseline is above the solvent. And add a watch glass on top.

  4. As the solvent reaches the top, remove the TLC plate and draw a solvent front line in pencil.

  5. If spots are visible, circle them with a pencil. If they're not visible, place the TLC plate under some UV light and circle any spots you can see.
• What value can a TLC plate be used to calculate for compounds?

  Retention factor (Rf) values.
• How to calculate Rf values


  Distance travelled by component / distance travelled by solvent.
• Why must the baseline be drawn in pencil?

  Any other medium would interact with the sample and solvent.
• What is gas chromatography useful for?


  Separating and identifying volatile organic compounds present in a mixture.
• Stationary phase in gas chromatography


  A high boiling liquid adsorbed onto an inert solid support (capillary column).
• Mobile phase in gas chromatography


  An inert carrier gas, such as helium or neon, moves the sample molecules through the stationary phase.
• How does gas chromatography work?


  - The sample is injected into the gas chromatogram.

  - The mobile carrier gas carries the components through the capillary column.

  - The components slow down as they interact with the liquid stationary phase.

  - The more soluble the component is in the liquid stationary phase, the slower it moves.

  - Components are separated depending on their solubility in the liquid stationary phase.

  - Compounds in the mixture reach the detector at different times.

  - The compound retained in the column for the shortest time is detected first and has the lowest retention time.
• Retention time


  The time taken for each component to travel through the column.
• What two pieces of information can be obtained from a gas chromatogram?


  1. Retention times can be used to identify the components present in the sample by comparing these to retention times for known components.

  
  

  2. Peak integrations (the area under each peak) can be used to determine the concentrations of components in the sample.
• How are concentrations of components found from a gas chromatogram?


  By comparing components peak integration (area) with values obtained from standard solutions of the component.
• Steps to find the concentration of components in a gas chromatogram using standard solutions?


  1. Prepare standard solutions of known concentrations of the compounds being investigated,

  
  

  2. Obtain gas chromatograms for each standard solution.

  
  

  3. Plot a calibration curve of peak area against concentration. This is called external calibration and offers a method for converting peak area into concentration.

  
  

  4. Obtain a gas chromatogram of the compound being investigated under the same conditions.

  
  

  5. Use the calibration curve to measure the concentration of the compound.

  
• Qualitative analysis of organic functional groups


  Reactions can be carried out on a test-tube scale to identify the functional groups in organic compounds.
• Chemical test for alkenes and observations


  Add bromine water drop-wise. Bromine water decolourised from orange to colourless.
• Chemical test for haloalkane and observations

  Add silver nitrate and ethanol and warm to 50°C in a water bath.

  • Chloroalkane = white precipitate (AgCl)
  • Bomoalkane = cream precipitate (AgBr)
  • Iodoalkane = yellow precipitate (AgI)
• Chemical test for carbonyls and observations

  Add 2,4-DNP (I.e. Brady's reagent). Orange precipitate.
• Chemical test for aldehydes and observations


  Add Tollens' reagent and warm. Silver mirror.
• Chemical test for primary and secondary alcohol, and aldehyde and observations

  Add acidified potassium dichromate (VI) and warm in a water bath. Colour change from orange to green.
• Chemical test for carboxylic acid and observations


  Add aqueous sodium carbonate. Effervescence.
• Chemical test for phenols and observations.

  Add aqueous sodium carbonate. Phenols have weak acidity so no reaction occurs.