organic analysis

• Potassium dichromate(VI)


  Oxidising agent
• Oxidation of alcohols by potassium dichromate(VI)

  1. Oxidises primary alcohols to aldehydes

  2. Oxidises secondary alcohols to ketones

  3. Tertiary alcohols cannot be oxidised
• Oxidation of alcohols by potassium dichromate(VI)

  Potassium dichromate(VI) is reduced, causing a colour change from orange to green
• Test for primary, secondary and tertiary alcohols

  1. Add 10 drops of alcohol to 2cm3 of acidified potassium dichromate solution

  2. Warm the mixture gently in a water bath

  3. If primary/secondary alcohol: orange ➜ green

  4. If tertiary alcohol: no colour change - remains orange
• Distinguishing primary and secondary alcohols

  1. Oxidise the alcohol under reflux

  2. If it tests positive for being a carboxylic acid, then it's a primary alcohol

  3. If it tests positively for being an aldehyde, then it's a primary alcohol

  4. If it tests positive for being a ketone, then it's a secondary alcohol
• Aldehydes and ketones


  Flammable, so use a water bath instead of a Bunsen burner
• Fehling's solution test for aldehydes and ketones

  1. Add 2cm3 of Fehling's solution to a test tube

  2. Add 5 drops of unknown substance

  3. Place in hot water bath for 5 minutes

  4. If aldehyde: clear blue ➜ brick red

  5. If ketone: no colour change - remains clear blue
• Tollens' reagent (silver mirror) test for aldehydes and ketones


  1. Put 2cm3 of 0.10 mol dm-3 silver nitrate solution in a test tube

  2. Add dilute sodium hydroxide, then dilute ammonia to form Tollens' reagent

  3. Put in hot water bath, then add unknown substance

  4. If aldehyde: colourless ➜ silver mirror

  5. If ketone: no colour change - remains colourless
• Test for carboxylic acids

  1. Add 2cm3 of unknown substance to a test tube

  2. Add solid sodium carbonate (or 2cm3 of sodium carbonate solution)

  3. If it fizzes and limewater goes from colourless ➜ cloudy, it's a carboxylic acid

  4. If it doesn't fizz and limewater remains colourless, it's not a carboxylic acid
• This test will give a positive result with any acid, so can only be used to distinguish organic compounds if you already know one is a carboxylic acid
• Test for alkenes


  1. Add 2cm3 of unknown solution to a test tube

  2. Add 2cm3 of bromine water

  3. Shake the test tube

  4. If alkene: orange ➜ colourless

  5. If not alkene: no colour change - remains orange
• Mass spectrometry


  Produces peaks in the spectrum due to the original molecule and fragments of the molecule
• Molecular ion peak


  The last major peak at the highest m/z value, giving the relative molecular mass (Mr) of the compound
• High resolution mass spectrometry

  Can measure relative atomic and molecular masses to up to 4 decimal places, useful for identifying compounds with the same Mr when rounded to the nearest whole number
• Infrared spectrometry


  Bonds in a molecule absorb IR radiation at specific frequencies depending on the bond's vibrational frequency
• Infrared spectrum


  Each trough (peak) represents the energy absorbed by a particular bond, causing it to vibrate
• Infrared spectrum


  • Can be split into two regions: below 1500 cm-1 (fingerprint region) and above 1500 cm-1 (functional group region)
• Fingerprint region (below 1500 cm-1)


  Contains many signals, making functional group identification difficult, but is unique for each molecule and can be used for identification
• Functional group region (above 1500 cm-1)


  Contains characteristic signals for functional groups, such as C-H, O-H, and C=O
• Greenhouse gases


  Absorb infrared radiation emitted by the Earth, contributing to global warming