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प्रश्न
Describe the reactions involved in the detection of nitrogen in an organic compound by Lassaigne method.
उत्तर
This method involves the conversion of covalently bonded N, S or halogen present in the organic compounds to corresponding water-soluble ions in the form of sodium salts For this purpose a freshly cut piece of Na of the size of a paper, dried it by pressing between the folds of a filter paper is taken in a fusion tube in an iron tand clamping it just near the upper end and it is gently heated.
When it melts to a shiñing globule, put a pinch of the organic compound on it. Heat the tube with the tip of the flame till all reaction ceases and it becomes red hot. Now plunges it in about 50 mL of distilled water taken in a china dish and break the bottom of the tube by striding against the dish. Boil the contents of the dish for about 10 mts and filter. This filtrate is known as lassaignes extract or sodium fusion extract and it used for qualitative analysis of nitrogen, sulfur and halogens present in organic compounds.
Test for Nitrogen:
If nitrogen is present it gets converted to sodium cyanide which on reaction with freshly prepared ferrous sulphate and ferric ion followed by cone. HCl gives a Prussian blue color or green color or precipitate. It confirms the presence of nitrogen. HCl is added to dissolve the greenish precipitate of ferrous hydroxide produced by the excess of NaOH on FeSO4 which would otherwise mark the Prussian blue precipitate. The following reaction takes part in the formation of Prussian blue.
\[\ce{Na + C + N -> NaCN}\]
from organic compounds
\[\ce{FeSO4 + 2NaOH -> Fe(OH)2 + Na2SO4}\]
(from excess of sodium)
\[\ce{6 NaCN + Fe(OH)2 -> \underset{\text{sod. ferrocycanide}}{Na4 [Fe(CN)6]} + 2NaOH}\]
\[\ce{\underset{\text{ferric ferrocyanide}}{3Na4[Fe(CN)6]} + FeCl3 -> \underset{\text{Prussian blue or green ppt}}{Fe4[Fe(CN)6]3} + 12NaCl}\]
In casr if both N and S arepresent, a blood red color instead of blue or green color will be obtained. This is because of the following reactions.
\[\ce{Na + C + N + S ->[Heat] \underset{\text{(sodium sulphocyanide)}}{NaCNS}}\]
\[\ce{NaCNS + FeCl3 -> \underset{\text{(Blood red colour)}}{\underset{\text{ferric sulphocyanide}}{Fe(CNS)_3}} + 3NaCl}\]
