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Detection of elements

Lassaigne’s test: Nitrogen, sulphur, and halogens in an organic compound are detected by Lassaigne’s test through Lassaigne’s extract. This extract is prepared using the following method:
Sodium reacts with elements of the organic compound to give the following reaction:
C + N + Na NaCN; S + 2Na Na2S
X + Na NaX (where X = Cl, Br, or I)
When nitrogen and sulphur both are present in the organic compound, then sodium thiocynate is formed.
Na + C + N + S NaSCN
All the sodium salts being soluble in water can be easily detected.
  1. Detection of nitrogen
    2NaCN + FeSO4Fe(CN)2 + Na2SO4
    Description: 51636.png
    Description: 51643.png
    Description: 51651.png
  2. Detection of sulphur: If the organic compound contains sulphur, the sodium fusion extract will contain sodium sulphide. It is divided into two portions, and following tests are performed:
    • Sodium nitroprusside test
      Description: 51658.png
    • Lead acetate test
      Description: 51667.png
  3. Detection of halogens: If the organic compound contains halogen, the sodium fusion extract will contain sodium halide. The sodium fusion extract is boiled with dilute nitric acid to decompose sodium cyanide or sodium sulphide (if present), otherwise a white precipitate of silver cyanide or silver sulphide will be formed even in the absence of halogen. The solution is then cooled and silver nitrate solution is added. The characteristic precipitate confirms the presence of a halide.
    NaX + AgNO3AgX + NaNO3
    • White precipitate soluble in aqueous ammonia indicates chlorine.
    • Light yellow precipitate sparingly soluble in aqueous ammonia indicates bromine.
    • Pale yellow precipitate insoluble in aqueous ammonia indicates iodine.

Detection of functional groups

  1. Tests for carboxylic acid group
    • Sodium bicarbonate test: Add a small quantity of the organic compound to sodium bicarbonate solution taken in a test tube. Compound dissolves with brisk effervescences.
  2. Tests for aldehyde group
    • Schiff’s test
      Description: 53436.png
    • Fehling’s test
      Description: 51678.png
    • Tollen’s test
      Description: 51685.png
      Description: 51754.png
  3. Tests for ketone group: Ketones, unlike aldehydes, do not restore the pink color of Schiff’s reagent nor do they reduce Fehling’s solution or ammoniacal silver nitrate solution. Ketones yellow or red crystalline precipitate with 2,4-dinitrophenylhydrazine and also with sodium bisulphite reagent.
    Description: 53447.png
    Ketones add on sodium hydrogen sulphite to form crystalline bisulphite compounds.
    Description: 53457.png
  4. Tests for phenol group
    • Neutral FeCl3 test: When phenols are treated with neutral ferric chloride solution, they form colored complexes. The color of the complex may be violet, red, blue, or green. This is a characteristic reaction of compounds having enolic group (=C–OH). All stable enols respond to this test.
    • Bromine water test
      Description: 53474.png
    • Ceric ammonium nitrate test
      Description: 51706.png
    • Liebermann’s test: Take a small amount of the compound and fuse with a few crystals of NaNO2 in a test tube. Cool the test tube and add some concentrated H2SO4. A deep green color is obtained and when poured into large excess of water, green color changes to red. When a little NaOH solution is added to the aqueous solution, the solution becomes deep blue colored.
  5. Tests for alcohol group: Alcohols may be considered as neutral compounds. They are soluble in water or dioxane.
    • Sodium test
      Description: 51714.png
    • Ceric ammonium nitrate test: This test is useful only when the compound contains less than 10 carbon atoms per molecule.
      Description: 51722.png
  6. Tests for primary amines (NH2)
    • Carbylamines test
      Description: 51730.png
  7. Tests for secondary amines (NH)
    • Liebermann’s nitroso test: Dissolve some organic compound in concentrated HCl, and then add a small amount of water. Cool the solution in ice-cold water bath and then add cold dilute NaNO2 solution. A yellow oily emulsion is produced. Take this emulsion in a test tube and add phenol and concentrated H2SO4 to it. Green color appears. Addition of water changes green color to red, which changes to deep blue on adding NaOH solution.
  8. Tests for nitro group
    • Reduction test
      Zn + 2HCl ZnCl2 + 2[H]
      C6H5NO2 + 6[H] C6H5NH2 + 2H2O
      NaNO2 + HCl HNO2 + NaCl
      C6H5NH2 + HNO2 + HCl C6H5N2+Cl + 2H2O
      Description: 53485.png
    • Mulliken’s test
      Description: 51737.png
      Description: 51746.png

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