Organic Chemistry: Elemental Analysis

Study Notes

Elemental analysis is a crucial step in identifying organic compounds.
Carbon, hydrogen, and oxygen are the primary elements in organic compounds.
Chemical tests are not used to detect carbon, hydrogen, and oxygen.
Nitrogen, halogens (chlorine, bromine, fluorine, iodine), and sulfur are other significant elements.
These elements can be detected using a sodium fusion method.

Organic compounds are nonpolar, making detecting N, S, and X difficult in solution.
Converting these elements into inorganic ions is necessary for testing.
This conversion is done by heating a small amount of the organic compound with an equivalent amount of metallic sodium.
Organic compounds decompose and soluble sodium salts of elements are formed.
The reaction produces sodium thiocyanate (NaSCN) when both sulfur and nitrogen are present in the compounds (in small quantity).
Sodium is highly reactive with water, resulting in an explosion; extreme caution is needed during handling.
Sodium is kept submerged in liquid paraffin to prevent moisture exposure.
Paraffin should be removed before using the sodium.
Touching sodium directly while it is moist can cause burning.
Sodium's shiny surface is oxidized and dulls when exposed to air and moisture.

A small amount of the unknown substance is placed in a clean dry test tube with a small piece of sodium metal.
The test tube is held vertically by a clamp.
Gradually heat the lower part of the test tube until the sodium melts.
Further heating is necessary to prevent vapor loss of the reaction products.
Heat the bottom of the test tube until it turns red (approximately five minutes) after the sodium melts.
Carefully place the hot test tube into a beaker containing 20 mL of distilled water.
If the tube does not break immediately, use a glass rod to break it.
The resulting solution is heated almost to boiling and filtered.
The colorless filtrate is used for confirming the presence of elements.

Add alcohol (either ethanol or methanol) to remove excess sodium by reacting it with the excess sodium metal.
The reaction of sodium with alcohol produces sodium alkoxide.

To 3 mL of the filtrate, add 4 drops of ferrous sulfate (FeSO4) solution.
Adjust the solution's basicity using sodium or potassium hydroxide solution (10%).
Heat the solution to boiling for 30 seconds.
Add dilute sulfuric acid to make the solution acidic.
A Prussian blue precipitate confirms nitrogen's presence.
The reaction involves producing sodium ferrocyanide using sodium cyanide and ferrous sulfate.

Acidify 2 mL of the filtrate using dilute acetic acid.
Add 5 drops of lead acetate solution.
A black precipitate of lead sulfide confirms sulfur.
Acetic acid is preferred over other acids to prevent formation of insoluble white precipitates.

In compounds containing nitrogen and sulfur, acidify 3 mL of the filtrate with dilute nitric acid (added drop-wise until acidic).
Boil for 5 minutes, then add silver nitrate solution dropwise.
White or yellow precipitates indicate the presence of halogens.