BP208P – Organic Chemistry Lab Manual

Carbon, hydrogen, oxygen, nitrogen, sulfur, and halogen elements are the most frequent elements found in organic compounds; less common elements include phosphorus, arsenic, antimony, mercury, and other metals that can be found as organic acid salts. There is no direct means of detecting oxygen. Detecting components such as nitrogen (N), sulphur (S), halogens (Cl, Br, I), and phosphorus (P) in organic compounds is an important aspect of organic qualitative analysis. The Lassaigne’s test, often known as the sodium fusion test, is a conventional method for this purpose. Organic compounds commonly contain covalently bonded components such as nitrogen, sulphur, halogens, and phosphorus. To detect these elements, the molecule is fused with metallic sodium, which changes the covalently bonded elements to their respective ionic forms (cyanide, sulphide, halide, and phosphate). These ionic species are soluble in water and may be discovered by certain chemical methods.

To identify nitrogen, sulfur, and halogen in organic compounds, they must first be converted into ionisable inorganic chemicals that may be tested using ionic inorganic analytical methods. This may be achieved in numerous ways, but the best approach is to fuse the organic chemical with metallic sodium (Lassaigne’s test). If the ingredients listed above are present, sodium cyanide, sodium sulphide, and sodium halides are generated, which are easily identifiable. It is essential to employ an excess of sodium; otherwise, if sulphur and nitrogen are present, sodium thiocyanate, NaCNS, may be formed; this may cause a red coloring with iron(III) ions but will not react to tests for cyanide or sulphide ions. With an excess of sodium, the thiocyanate will be degraded as follows:

NaCNS + 2Na → NaCN + Na₂S

The Prussian Blue test can identify cyanide ions, and hence nitrogen, in the material. The filtered alkaline solution formed by the action of water on sodium fusion is treated with iron(II) sulphate, yielding sodium hexacyanoferrate(II). When the alkaline iron(II) salt solution is boiled, some iron(III) ions are inevitably produced by the action of air; when dilute sulphuric acid is added, dissolving the iron(II) and (III) hydroxides, the hexacyanoferrate(II) reacts with the iron(III) salt, producing iron(III) hexacyanoferrate(II) and giving the solution a Prussian blue colour.

FeSO₄ + 6NaCN → Na₄[Fe(CN)₆] + Na₂SO₄

3Na₄[Fe(CN)₆] + 2Fe₂(SO₄)₃ → Fe₄[Fe(CN)₆]₃ + 6Na₂SO₄

Hydrochloric acid should not be used to acidify an alkaline solution because the yellow color from the iron(III) chloride generated causes the Prussian blue to appear greenish. For the same reason, iron(III) chloride should not be added, as is commonly recommended: air oxidation of the hot alkaline solution produces an adequate concentration of iron(III) ions. When the fusion solution is treated with an alkaline p-nitrobenzaldehyde-O-dinitrobenzene reagent, a rich purple color forms, which is an alternate and more sensitive test for cyanide ions.

Sulphur, as a sulphide ion, can be identified by precipitation as black lead sulphide with lead acetate solution and acetic acid, or by the purple color generated by the addition of disodium pentacyanonitrosoferrate (III).

Na₂S + Pb(CH₃COO)₂ → PbS↓ + 2CH₃COONa

The addition of weak nitric acid and a silver nitrate solution detects halogens as typical silver halides. Cyanide and sulphide ions both interfere with the halide test by generating precipitates of silver cyanide and silver sulphide. If nitrogen or sulphur are discovered, the interfering ions must be eliminated by boiling the acidified fusion solution before adding the silver nitrate solution to detect the halogen.

NaX + AgNO₃ → AgX↓ + NaNO₃