Nitrogen, sulphur and halogens in an organic compound are detected by Lassaigne's test through Lassaigne's extract. Organic compounds contain these elements mostly linked through covalent bonds. In order to make them free, we convert them into ionic form by treating the compound with sodium. The ionic compound formed are water-soluble and releases the element in ionic form, which is easy to detect.
`text(Preparation of Lassaigne's Extract)` : A small piece of dry sodium metal is heated gentaly in a fusion tube till it melts to a shining globule. Then, a small amount of organic substance is added and the tube is heated strongly till it becomes red hot. The red hot tube is plunged into distilled water contained in a china dish. The fusion tube should break into pieces on adding to water. If it is not broken, it is broken using a glass rod. Then the solution is boiled for some time. It is then cooled and filtered. The filtrate is known as sodium extract or Lassaigne's extract.
Sodium reacts with elements of the organic compound to give following reactions :
`C + N + Na -> NaCN`
`S + 2Na -> Na_2S`
`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 + S + C+ N -> NaSCN`
All the sodium salts being soluble in water, can be easily detected.
`text(Detection of nitrogen)` : If the organic compound contains nitrogen, the sodium fusion extract will contain sodium cyanide(`NaCN`). A small portion of the extract is boiled with ferrous sulphate solution and acidified with dilute sulphuric acid. A blue precipitate of Prussian blue(ferric ferrocyanide) confirms the presence of nitrogen.
`2NaCN +FeSO_4 -> Fe(CN)_2 +B=Na_2SO_4`
`Fe(CN)_2 + undersettext[(excess)][4NaCN] -> Na_4[Fe(CN)_6]`
`FeSO_4 underset(H_2SO_4) oversettext(oxidised by) -> Fe_2(SO_4)_3`
`3Na_4[Fe(CN)_6] + 2Fe_2(SO_4)_3 -> undersettext(Prussian blue)(Fe_4[Fe(CN)_6]_3) + 6Na_2SO_4`
If the organic compound contains sulphur along with nitrogen and the sodium, present is in insufficient amount then sodium thiocynate is formed which gives blood red colouration by reaction with ferric ions.
`3NaSCN + Fe^(3+) -> undersettext(Blood red colour)([Fe(SCN)_3]) + 3Na^(+)`
However, the absence of blood red colouration does not necessarily mean that sulphur is absent in the organic compound because `Na`, `C`, `N`, `S` combines with `Na` to give `NaCN` and `Na_2S`
`NaSCN + HCl -> HSCN + NaCl`
`4HSCN + Co(NO_3)_2 -> undersettext(Blue colour)(H_2[Co(SCN)_4]) + 2HNO_3`
`text(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.
`text(Sodium Nitroprusside Test)` : Add freshly prepared sodium nitroprusside solution to one portion. A deep violet colour confirms the presence of sulphur.
`Na_2S + Na_2[Fe(CN)_5NO] -> undersettext(violet colour)(Na_4[Fe(CN)_5NOS])`
`text(Lead Acetate Test)` : Acidify the second portion with acetic acid and then add lead acetate solution. A black precipitate of lead sulphide confirms the presence of sulphur.
`Na_2S +undersettext(lead acetate)((CH_3COO)_2Pb) -> underset(black ppt)(PbS downarrow) + 2CH_3COONa`
`text(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 + AgNO_3 -> AgX downarrow + NaNO_3`
(a) White precipitate, soluble in aqueous ammonia indicates chlorine.
(b) Light yellow precipitate, sparingly soluble in aqueous ammonia indicates bromine.
(c) Pale yellow precipitate, insoluble in aqueous ammonia indicates iodine.
`text(Detection of Phosphorus)` : The organic compound containing phosphorus is fused with sodium peroxide and extracted with water. The aqueous solution is boiled with concentrated nitric acid and ammonium molybdate is added. A canary yellow precipitate indicates the presence of phosphorus.
Nitrogen, sulphur and halogens in an organic compound are detected by Lassaigne's test through Lassaigne's extract. Organic compounds contain these elements mostly linked through covalent bonds. In order to make them free, we convert them into ionic form by treating the compound with sodium. The ionic compound formed are water-soluble and releases the element in ionic form, which is easy to detect.
`text(Preparation of Lassaigne's Extract)` : A small piece of dry sodium metal is heated gentaly in a fusion tube till it melts to a shining globule. Then, a small amount of organic substance is added and the tube is heated strongly till it becomes red hot. The red hot tube is plunged into distilled water contained in a china dish. The fusion tube should break into pieces on adding to water. If it is not broken, it is broken using a glass rod. Then the solution is boiled for some time. It is then cooled and filtered. The filtrate is known as sodium extract or Lassaigne's extract.
Sodium reacts with elements of the organic compound to give following reactions :
`C + N + Na -> NaCN`
`S + 2Na -> Na_2S`
`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 + S + C+ N -> NaSCN`
All the sodium salts being soluble in water, can be easily detected.
`text(Detection of nitrogen)` : If the organic compound contains nitrogen, the sodium fusion extract will contain sodium cyanide(`NaCN`). A small portion of the extract is boiled with ferrous sulphate solution and acidified with dilute sulphuric acid. A blue precipitate of Prussian blue(ferric ferrocyanide) confirms the presence of nitrogen.
`2NaCN +FeSO_4 -> Fe(CN)_2 +B=Na_2SO_4`
`Fe(CN)_2 + undersettext[(excess)][4NaCN] -> Na_4[Fe(CN)_6]`
`FeSO_4 underset(H_2SO_4) oversettext(oxidised by) -> Fe_2(SO_4)_3`
`3Na_4[Fe(CN)_6] + 2Fe_2(SO_4)_3 -> undersettext(Prussian blue)(Fe_4[Fe(CN)_6]_3) + 6Na_2SO_4`
If the organic compound contains sulphur along with nitrogen and the sodium, present is in insufficient amount then sodium thiocynate is formed which gives blood red colouration by reaction with ferric ions.
`3NaSCN + Fe^(3+) -> undersettext(Blood red colour)([Fe(SCN)_3]) + 3Na^(+)`
However, the absence of blood red colouration does not necessarily mean that sulphur is absent in the organic compound because `Na`, `C`, `N`, `S` combines with `Na` to give `NaCN` and `Na_2S`
`NaSCN + HCl -> HSCN + NaCl`
`4HSCN + Co(NO_3)_2 -> undersettext(Blue colour)(H_2[Co(SCN)_4]) + 2HNO_3`
`text(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.
`text(Sodium Nitroprusside Test)` : Add freshly prepared sodium nitroprusside solution to one portion. A deep violet colour confirms the presence of sulphur.
`Na_2S + Na_2[Fe(CN)_5NO] -> undersettext(violet colour)(Na_4[Fe(CN)_5NOS])`
`text(Lead Acetate Test)` : Acidify the second portion with acetic acid and then add lead acetate solution. A black precipitate of lead sulphide confirms the presence of sulphur.
`Na_2S +undersettext(lead acetate)((CH_3COO)_2Pb) -> underset(black ppt)(PbS downarrow) + 2CH_3COONa`
`text(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 + AgNO_3 -> AgX downarrow + NaNO_3`
(a) White precipitate, soluble in aqueous ammonia indicates chlorine.
(b) Light yellow precipitate, sparingly soluble in aqueous ammonia indicates bromine.
(c) Pale yellow precipitate, insoluble in aqueous ammonia indicates iodine.
`text(Detection of Phosphorus)` : The organic compound containing phosphorus is fused with sodium peroxide and extracted with water. The aqueous solution is boiled with concentrated nitric acid and ammonium molybdate is added. A canary yellow precipitate indicates the presence of phosphorus.