Question

A compound which does not give a positive test in Lassaigne’s test for nitrogen is :
A.Urea
B.Hydrazine
C.Azo benzene
D.Phenyl hydrazine

Answer 1

To detect elements such as nitrogen , sulphur , halogens and phosphorus present in organic compound Lassaigne’s test . For this test we have to prepare sodium extract or Lassaigne’s extract. If an organic compound contains both nitrogen and sulphur then sodium thiocyanate is formed. If this sodium cyanide is not formed then this test is not positive.
$Na + C + N\xrightarrow{{Heating}}NaCN$

Complete step by step answer:
A.Urea :- Urea have formula $\mathop {CH}\nolimits_4 \mathop N\nolimits_2 \mathop O\nolimits_{}$. It contains both carbon and nitrogen so when it reacts with sodium it forms sodium cyanide $\left( {NaCN} \right).$ So it will give positive lassaigne’s test. Hence this option is not correct. .

B.Hydrazine :- It has formula $\mathop {NH}\nolimits_2 .\mathop {NH}\nolimits_2$ . It does not contain carbon in it. So it does not form sodium cyanide . Hence it will not give positive lassaigne’s test. So this option is correct.

C.Azo benzene :- It has formula $\mathop C\nolimits_{12} \mathop H\nolimits_{10} \mathop N\nolimits_2$. It contains both carbon and nitrogen so it reacts with sodium. It leads to formation of sodium cyanide. It does not give positive lassaigne’s test. Hence this option is not correct.

D.Phenyl hydrazine :- it has formula $\mathop C\nolimits_6 \mathop H\nolimits_8 \mathop N\nolimits_2$.It contains both carbon and nitrogen so when reacts with sodium. It leads to formation of sodium cyanide. It does not give positive lassaigne’s test. Hence this option is not correct.

So our required answer is option B .

Note:
For the preparation of sodium extract we take a Small piece of a dry sodium metal in a fusion tube 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 and boiled for some time . It is then cooled and filtered . This filtrate liquid is known as sodium extract.