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  4. Correct pair of compounds which gives blue colouration/precipitate and white precipitate, respectively, when their Lassaigne’s test is separately done is

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Q. Correct pair of compounds which gives blue colouration/precipitate and white precipitate, respectively, when their Lassaigne’s test is separately done is

WBJEEWBJEE 2013Organic Chemistry – Some Basic Principles and Techniques

Solution:

image $+na \xrightarrow{\Delta}$ sodium extract $(NaCN)$
$\xrightarrow[{FeCl_{3}}]{FeSO_{4}} \underset{\text{Prussion blue}}{Fe _{4}\left[ Fe ( CN )_{6}\right]_{3}}$
image $+Na \xrightarrow{\Delta}$ sodium extract $(NaCl) \xrightarrow{AgNo_{3}} $
$\underset{\text{White Precipetate }}{AgCl \downarrow+ NaNO _{3}}$
Hydrozine, $NH _{2} \cdot NH _{2}$ does not respond Lassaigne's test because it does not contain any carbon and hence, $NaCN$ is not formed. Compound, $NH_{2} - \underset{\overset{||}{S}}{C} -NH_{2}$
N and S, hence, it will give red colour in Lassaigne test.