What is Lassaigne test for organic compounds?

Lassaigne's test:

• Lassaigne's test is the most common test for detecting halogens, sulphur, and nitrogen is this one. In this experiment, sodium and the organic component are combined to produce the corresponding, readily recognisable sodium salts. As a result, sodium cyanide, sodium sulphide, and sodium halide are produced from nitrogen, sulphur, and halogens, respectively.

         Na + C + N →  NaCN

         2Na + S   →  Na₂S

         Na + X    →   NaX (X = Cl, Br, I)

         Na + C + N + S   →  NaSCN

Procedure:

• Take a tiny amount of dry, clean sodium (about 50–75 mg) and place it in a dry, clean ignition tube. Now heat tube slowly. Take it out of the heat, then add the compound (20-25 mg or 1-2 drops). (To limit its loss by evaporation, a paste made of the liquid chemical and sodium carbonate may be employed, or solid sodium carbonate may be added after the liquid compound.) Heat it up until it turns red hot by first heating it on a low flame and then increasing the heat. Once the compound has fully reacted, let it in the red-hot state for 1-2 minutes before dropping it into 10 ml of distilled water that has been maintained in a porcelain bowl. Do a second fusion in a different ignition tube, then submerge it in the identical porcelain basin. Boil the porcelain basin's contents for two to three minutes before filtering. Now filtrate the solution and use filtrate for testing the additional elements.(sometimes referred to as lassaigne's extract).

Precautions

1. Use forceps to handle the sodium rather than your fingers.
2. Compounds including nitroalkanes, azides, diazonium salts, chloroform, and carbon tetrachloride may cause a minor explosion. It is advisable to wear safety glasses to prevent mishaps. 
3. Unreacted sodium should be decomposed carefully by reacting with a small amount of ethyl alcohol.

Test for Nitrogen

(A) Reaction with Ferrous sulphate:

• Add solid ferrous sulphate (1-2 ml) to the lassaigne's or sodium extract (50-75 mg). If sulphur is present, a black precipitate of ferrous sulphide is produced instead of the expected dark greenish grey precipitate of ferrous hydroxide. Gently boil the liquid for one minute, then add some diluted sulfuric acid to make it acidic (3-4 ml). The presence of nitrogen in the compound is indicated by a blue precipitate of Fe4[Fe(CN)6]3 (Prussian blue) or by a blue colour.

         6NaCN+ FeSO4 →  Na[Fe(CN)6]   + Na2SO4

         

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

• In situ oxidation of alkaline ferrous salt solution in the presence of atmospheric oxygen yields Fe2(SO4)3.

(B) Benzidine-Copper Sulphate Test:

Acidify the sodium extract (1-2 ml) with acetic acid (3-4 drops). Add freshly prepared benzidine solution (1% in 50% acetic acid, 2-3 drops) to the above solution and shake the mixture. Treat with copper sulphate solution (1%, 1-2 drops). Appearance of a blue colour or a precipitate indicates the presence of nitrogen. If iodine is also present along with nitrogen, a greenish precipitate is obtained in this test.

(C) Reaction with PNB and ODNB :

By treating the Lassaigne's extract with p-nitrobenzaldehyde (PNB) and o-dinitrobenzene (ODNB), nitrogen can be identified. The appearance of a purple colour signifies a positive test. Purple colour is obtained due to the formation of o-nitrophenyl hydroxylamine from ODNB and its conversion to the o-quinonoid form (A) in alkaline medium.

Procedure: Take 1 ml. of the alkaline lassignes extract (If extract is not alkaline add few drops of solution of NaOH) and add p-nitrobenzaldehyde solution 4-5 drops and o-nitrobenzene solution 4-5 drops. Formation of purple colour indicates the nitrogen is present.

Note: Use 0.1 M solution of PNB and ODNB in methyl cellosol (2-methoxyethanol).

Test for Sulphur

(A) Lead Acetate Test: Take sodium extract (~ 2 ml.) and acidify it with acetic acid (2 ml). After that add a few drops (4-5 drops) of lead acetate solution in it. The presence of sulphur is indicated by the appearance of a black precipitate (PbS).

         

                                Na₂S + (CH₃COO)₂Pb →   PbS + 2CH₃COONa

(B) Sodium Nitroprusside Test: To the sodium extract (1-2 ml.) add a 1-2 drops freshly prepared aqueous solution of Na₂[Fe(CN)₅NO] (sodium nitroprusside). A deep reddish violet colouration of Sodium sulphonitroprusside (  Na₄[Fe(CN)₅NOS]  ) indicates the presence of sulphur.

                              Na₂S+ Na₂[Fe(CN)₅NO] → Na₄[Fe(CN)₅NOS]

Test for Nitrogen and Sulphur Present Together:

(A) Test With FeCl3: