Question 12.21 Discuss the chemistry of Lassaigne’s test.
Class XI Organic Chemistry : Some Basic Principles and Techniques Page 363
Class XI Organic Chemistry : Some Basic Principles and Techniques Page 363
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step 1 :- The elements present in the compound are converted from covalent to ionic form,
Na + C + N ------> NaCN
2Na + S --------> Na2S
Na + X ---------> NaX
these ionic compounds are extracted by boiling them with distilled water. this extract is called sodium fusion extract.
now the sodium fusion extract is treated with different reagents to test the different elements ,
step 2: Test for nitrogen
the sodium fusion extract is boiled with FeSO4 and then concentrated H2SO4 is added.
Fe²+ + 6CN– -------> [Fe(CN)6]⁴-
3[Fe(CN)6]e- + 3Fe³+ -----> Fe4[Fe(CN)6]3.xH2O { Prussian blue confirm the presence of N}
step 3 : Test for sulphur
the sodium fusion extract is added to CH3COOH and (CH3COO)2Pb.
Na2S + (CH3COO)2Pb ------>PbS (black ppt)
S²- + [Fe(CN)6NO]²- --------> [Fe(CN)5NOS]⁴-{violet (confirm the presence of S )}
step 4 :- Test for Both N and S
Na + C + N + S --------> NaSCN
Fe³+ + SCN– --------> [Fe(SCN)]²+ {blood red (confirm the presence of both N and S )}
step 5 :- Test for halogens
the sodium fusion extract is acidified with HNO3 followed by the addition of AgNO3.
Ag+ + Cl- ------> AgCl↓ ( white ppt)
Ag+ + Br- ------> AgBr ↓ ( yellow ppt )
Ag+ + I- --------> AgI↓ ( yellow ppt ) insoluble in NH4OH
step 6 :- Test for phosphorus
the compound is oxidised to phosphate . then it is boiled with HNO3 followed by the addition of (NH4)2MoO4 .
Na3PO4 + 3HNO3 -----> H3PO4 + 3NaNO3
H3PO4 + 12(NH4)2MoO4 + 21HNO3 ----> (NH4)2PO4.12MoO3{(yellow ppt) confirm the presence of P } + 21NH4NO3 + 12H2O
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