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Organic Chemistry: Some Basic Principles and Techniques — Chemistry STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceChemistryOrganic Chemistry: Some Basic Principles and Techniques5 Marks
Question
Discuss the chemistry of Lassaigne's test.
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Answer

The elements nitrogen, sulphur and halogens are tested in an organic compound by Lassaigne’s test. The organic compound (N, S or halogens) is fused with sodium metal as to convert these elements into ionisable inorganic substances, i.e., nitrogen into sodium cyanide, sulphur into sodium sulphide and halogens into sodium halides.
$2\text{Na}+\text{S}\xrightarrow{\Delta}\text{Na}_2\text{S}$
$\text{Na}+\text{C}+\text{N}\xrightarrow{\Delta}\text{NaCN}$
$\text{Na}+\text{X}\xrightarrow{\Delta}\text{NaX}(\text{x}=\text{Cl},\text{Br},\text{I})$
Once the ions are formed, the inorganic tests can be applied to them and the compound can be analysed.
  1. Test for Nitrogen: The sodium fusion extract is boiled with iron(II) sulphate and then acidified with acid. The formation of Prussian blue colour confirms the presence of nitrogen.
$6CN^- + Fe^{2+} \rightarrow [Fe(CN)6]^{4-}$
$3[Fe(CN)_6]^{4-} + 4Fe^{3+} \rightarrow Fe_4[Fe(CN)_6]_3.xH_2O$
  1. Test for Sulphur: The sodium fusion extract is acidified with acetic acid and lead acetate is added to it. Ablack precipitate of lead sulphide indicates the presence of sulphur.
$\text{S}^{2+}+\text{Pb}^{2+}\rightarrow\text{PbS}\downarrow \\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Black}$
  1. Test for Halogens: The sodium fusion extract is acidified with nitric acid and then treated with silver nitrate. A white precipitate, soluble in ammonium hydroxide shows the presence of chlorine, a yellowish precipitate, sparingly soluble in ammonium hydroxide shows the. presence of bromine and a yellow precipitate, insoluble in ammonium hydroxide shows the presence of iodine.
$X^- + Ag^+ \rightarrow AgX$
X represents a halogen -Cl, Br or I.
  1. Test for Phosphorus: The compound is heated with an oxidising agent (sodium peroxide). The phosphorus present in the compound is oxidised to phosphate. The solution is boiled with nitric add and then treated with ammonium molybdate. A yellow colouration or precipitate indicates the presence of phosphorus.
$Na_3PO_4 + 3HNO_3 \rightarrow H_3PO_4 + 3NaNO_3​​​​​​​$

$\text{H}_3\text{PO}_4 \ \ \ \ + \ \ \ \ 12(\text{NH}_4)_2\text{MoO}_4 \ \ \ \ + \ \ \ \ 21\text{HNO}_3 \ \ \ \ \rightarrow \ \ \ \ (\text{NH}_4)_3\text{PO}_4.12\text{MoO}_3 \ \ \ \ + \ \ \ \ 21\text{NH}_4\text{NO}_3 \ \ \ \ + \ \ \ \ 12\text{H}_2\text{O} \\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Ammonium molybdate} \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Ammonium phosphomoybdate} \\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{(yellow ppt.)}$

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