Qualitative Analysis of Organic Compounds

Detection of Elements in Organic Compounds

An organic compound always contains carbon and hydrogen. Beyond these two, it may also contain oxygen, nitrogen, sulphur, halogens (fluorine, chlorine, bromine, iodine), and phosphorus. The challenge in detecting these elements is that organic compounds are covalent and do not readily release their constituent atoms as ions in solution. A direct chemical test on the compound itself will not work — you must first break the molecule down into simpler, ionic forms.

The standard method for doing this is sodium fusion (also called Lassaigne's test). A small piece of sodium metal is heated with the organic compound in a fusion tube until the tube is red hot. The tube is then plunged into distilled water. The violent reaction converts the elements present into water-soluble sodium salts. The resulting solution, called the sodium fusion extract (or Lassaigne's extract), is then used for individual tests.