Organic Analysis

• What is potassium dichromate (VI) ?

  Oxidising agent- oxidises primary alcohols to aldehydes & secondary alcohols to ketones. Tertiary alcohols can't be oxidised. Aldehydes easily oxidised to carboxylic acids.
• How do you test for alcohols?

  Add 10 drops alcohol to 2cm3 of acidified potassium dichromate solution. Warm gently in hot water bath. Primary & secondary alcohols: solution turns from orange to green. (No colour change with tertiary alcohols).
• What are Fehling's & Benedict's solutions?


  Blue solution of complexed copper(II) ions dissolved in sodium hydroxide. Benedict's=dissolved in sodium carbonate.
• How do you test for aldehydes & ketones with F & B solution?

  Add 2cm3 Fehling's or Benedict's solution to tube. Add 5 drops aldehyde or ketone. Warm in hot water bath for few minutes. B or F solution will reduce to brick red Cu2O precipitate when warmed with aldehyde. No reaction with ketone (stays blue).
• How do you test for aldehydes & ketones using Tollens' reagent?

  Put 2cm3 of 0.10 mol dm-3 silver nitrate solution in test tube. Add few drops dilute sodium hydroxide solution-light brown precipitate forms. Add drops of dilute ammonia solution until brown precipitate dissolves completely (this is Tollens' reagent). Silver ions reduced to silver metal when warmed with aldehyde, but not with ketone. Silver coats apparatus to form silver mirror. Warm with water bath as they are flammable.
• Test for carboxylic acids:


  Add 2cm3 solution to be tested. Add 2cm3 sodium carbonate solution (or 1 spatula solid sodium carbonate). Solution fizzes-bubble gas produced through limewater-turns cloudy as CO2 produced. Limewater turns cloudy as calcium carbonate precipitate formed.
• Test for alkenes:

  Add 2cm3 solution to be tested. Add 2cm3 bromine water. Shake test tube. Bromine water goes from orange to colourless. (Tests for presence of unsaturation(double bonds)).
• What is high resolution mass spectrometry?

  Measures atomic & molecular masses extremely accurately (to several decimal places).
• How does infrared spectroscopy work?

  Beam of IR radiation passed through chemical sample. IR radiation absorbed by covalent bonds in the molecules, increasing their vibrational energy. Bonds between different atoms absorb different frequencies of IR radiation & bonds in different places in molecule absorb different frequencies.
• What does an IR spectrometer do?

  Produces graph of transmittance (intensity of radiation emerging) against frequency (wavenumber cm-1) of radiation. Peaks show wavelengths of radiation absorbed.
• What is the fingerprint region?

  Region between 1000cm-1 & 1550cm-1. Unique to particular compounds, so can identify unknown substances by comparing its fingerprint region with that of known substances. Can also assess how pure compound is (impurities produce extra peaks in fingerprint region).