Question

A violet color compound is formed in detection of $S$. Which of the following compound is formed in this process.
A. $N{a_4}[Fe{(CN)_5}NOS]$
B. $N{a_3}[Fe{(CN)_5}NOS]$
C. $N{a_2}[Fe{(CN)_5}NOS]$
D. $Na[Fe{(CN)_5}NOS]$

Answer 1

The elements like carbon, hydrogen, and oxygen are assumed to be present in any organic compound. Nitrogen, sulphur and the halogens are the elements which may be present in that. So for these extra elements, we do some tests to confirm the presence of them.

Complete answer:
Detection of extra elements is for the detection of presence of elements like nitrogen, sulphur, and halogens in the given organic compound. Since organic compounds generally are bonded by covalent bonds. So for their detection we need those elements in ionic form. Therefore first, we fuse the organic compound with sodium metal to give the corresponding easily identifiable sodium salts which are ionic.
Thus nitrogen gives sodium cyanide, sulphur gives sodium sulphide on reacting with sodium metal. This solution is called as Lassaigne’s extract
The reactions are as follows:
$Na + C + N \to NaCN \\\ 2Na + S \to Na2S \\\ Na + X \to NaX(X = Cl,Br,I) \\\ Na + C + N + S \to NaCNS$
Now the elements came in the solution in the form of sodium salts, we’ll do some specific test for the confirmation of those elements. For each salt, we have different types of tests.
In the case of sulphur, we have two tests. First we have a sodium nitroprusside test. We need to add freshly prepared aqueous sodium nitroprusside to the extract. Appearance of deep violet coloration indicates the presence of sulphur. The reaction is a s follows,
$\rm{N{a_2}S + N{a_2}[Fe{(CN)_5}NO] \;\;\;\longrightarrow\;\;\;\;\; N{a_4}[Fe{(CN)_5}NOS]}$
$\;\;\;\;\;\;\;\;\;\;\;\;\;\rm{Sodium \,nitroprusside} \:\;\;\;\;\;\;\;\;\rm{sodium\, sulpho \,nitroprusside}$
$\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\rm{Reddish \,violet \,coloration}$

**Hence the correct option is A.

Note:**
The second test for sulphur detection in lead acetate test in which the Lassaigne’s extract reacts with lead acetate. The appearance of black precipitate indicates the presence of sulphur in the organic compound. The black precipitate is of lead sulphate.