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Discuss the chemistry of Lassalgne's test |
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Answer» Solution :Nitrogen (N), sulphur (S), Halogens (Cl, Br, I) and phosphorus (P) present in an organic compound are detected by "Lassaigne.s test... (a) The elements (N, X, S) present in the compound are converted from covalent form into the ionic from by fusing the compound with sodium metal. Following reactions take place (i) `Na + C + underset("Nitrogen")(N) overset(Delta)rarr underset("Sodium cyanide")(NaCN)` (ii) `2Na + underset("Sulphur")(S) overset(Delta)rarr underset("Sodium sulphide")(Na_(2)S)` (iii) `Na + underset("Halogen")(X ) overset(Delta)rarr underset("Sodium halide")(NaX) (X= Cl, Br or I)` Cyanide, sulphide and halide of sodium so formed on sodium fusion are extracted from the fused mass by boiling it with distilled water. This extract is known as sodium fusion extract. (B) Test for Nitrogen: Procedure: The sodium fusion extract is boiled with iron (II) sulphate and then acidified with concentrated SULPHURIC acid. The formation of Prussian blue colour confirms the presence of nitrogen. Sodium cyanide first reacts with iron (II) sulphate and forms sodium hexacyanido- ferrate (II).On heating with concentrated sulphuric acid some iron (II) ions are oxidised to iron (III) ions which react with sodium hexacyanidoferrate (II) to produce iron (III) hexacyanidoferrate (II) (ferriferrocyanide) which is Prussian blue in colour. `underset("Cyanide")(6CN_((aq))^(-)) + underset("Ferrous ion")(Fe_((aq))^(2+)) rarr underset("Hexacyanidoferrate-(II) iron")([Fe(CN)_(6)]_((aq))^(+))` `3[Fe(CN)_(6)]_((aq))^(-) + underset("Iron (III)")(4Fe^(3+)) overset(xH_(2)O)rarr underset(underset("Ferriferrocyanide")("Prussian blue"))(Fe_(4)[Fe(CN)_(6)]_(3). H_(2)O)` (c ) Test for sulphur: FOllowing two test are occurs for DETECTION of sulphur. (i) The sodium fusion extract is acidified with acetic acid and lead acetate is added to it. A black precipitate of lead sulphide indicates the presence of sulphur. `underset("Sulphide ion")(S_((aq))^(2-)) + underset("Lead")(Pb_((aq))^(2+)) rarr underset("Black precipitate (lead sulphide)")(PbS_((s)))` (ii) On treating sodium fusion extract with sodium nitroprusside, appearance of a violet colour further indicates the presence of sulphur. `underset("Sulphide ion")(S_((aq))^(2-)) underset("Nitroprusside ion")([Fe(CN)_(5)NO]_((aq))^(2-))rarr underset("Violet solution")([Fe(CN)_(5).NOS]_((aq))^(4-))` In case, nitrogen and sulphur both are present in an organic compound: In sodium thiocyanate (NaSCN) is formed `Na + C + N + S rarr NaSCN` Which is heated with iron (II) sulphate and BLOOD red colour complex but not prussian blue. Hear, the reaction occurs between iron (III) ion and thiocyanate ion `(SCN^(-))`. `underset("Ferric ion")(Fe_((aq))^(3+)) + underset("Thiocyanate ion")(SCN_((aq))^(-)) rarr underset("Ferric thiocyanate ion (Blood red)")([Fe(SCN)]^(2+))` Note: If sodium fusion is carried out with excess of sodium, the thiocyanate decomposes to yield cyanide and sulphide. Those ions give their usual tests. `NaSCN + 2Na rarr NaCN + Na_(2)S` (d) Test for halogens: The fusion of organic with sodium metal and form sodium halide (NaX). This solution is basic. This sodium fusion extract is acidified with NITRIC acid and them treated with silver nitrate, the percipitate of AgXis formed. `Ag^(+) + X^(-) rarr AgX darr` (i) If a white precipitate is formed and soluble in ammonium hydroxide shows the presence of chlorine. (ii) If a yellowish precipitate is form, sparingly soluble in ammonium hydroxide shows the presence of bromine and (iii) If a yellow precipitate, in soluble in ammonium hydroxide shows the presence of iodine. Note: If nitrogen or sulphur is also present in the compound, the sodium fusion extract is first boiled with concentrated nitric acid to decompose cyanide or sulphide of sodium formed during Lassaigne.s test. These ions would otherwise interfere with silver nitrate test for halogens. (e ) Test for Phosphorus: The compound is heated with an oxidising agent sodium peroxide `(Na_(2)O_(2))`. The phosphorus present in the compound, is oxidised to phosphate. The solution is boiled with nitric acid and then treated with ammonium molybdate. A yellow colouration or precipitate indicates the presence of phosphorus. `2P + 3Na_(2)O_(2) O_(2) overset(Delta)rarr 2Na_(3)PO_(4)` `Na_(3)PO_(4) + 3HNO_(3) overset(Delta)rarr H_(3)PO_(4) + 3NaNO_(3)` `H_(3) PO_(4) _ underset("Ammonium molybdate")(12(NH_(4))MoO_(4)) + 21HNO_(3)rarr underset("Ammonium phosphomolybdate (Yellow ppts)")((NH_(4))_(3)PO_(4). 12MoO_(3)) + 21NH_(4)NO_(3) + 12H_(2)O` |
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