Answer to Question #129146 in General Chemistry for 250300183747

Question

QUALITATIVE ANALYSIS

1)-Test(Lassaigne’s extract 

(Sodium fusion 

extract) + Ferrous 

sulphate (FeSO4) )

-Observation (Blue precipitate was observed)

-inference(what is the inference?)

2)-Test(Potassium 

permanganate 

[KMnO4] + A)

-observation (Colourless solution was

observed)

-inference (what is the inference?)

3)Test(Organic compound + 

Na metal)

-observation (Lassaigne’s extract)

-inference(what is the inference?)

4)Test(Butane + KMnO4)

-observation (what is the observation?)

-inference (what is the inference?)

Answer 1

(1) (what is the inference?)

Lassaigne’s Test for detection of Nitrogen in Organic compound involves

first the formation of NaCN (IONIC) in the Sodium fusion extract, then its treatment to fresh FeSO4+H2SO4.

Initial product is Na4[Fe(CN)6] (Iron in II oxidation state), which after adding H2SO4 forms Ferri ferrocyanide (Prussian Blue Colour).{ Fe4[Fe(CN)6]3}

So, we see the initial product formation is detrimental for the coloured compound but it needs Fe2+.

Fe2+ has great tendency to form Fe3+ (as Fe3+ has d5 config while Fe2+ has d6, former being half filled and hence more stable). [Facts are agreeable to Reduction Potential Values too.]

So, in fresh sol of FeSO4 isnt used, sol will have predominant Fe3+ which wont give the test satisfactorily. Thus is the need to use Fresh Sol of FeSO4.

(2)  (what is the inference?)

The KMnO4 Test

Three drops of the compound to be tested are dissolved in 2 ml of water or aqueous ethanol. 2% KMnO4 solution (a purple solution) is added dropwise and the solution is shaken. Indications of a positive test: The purple color of the KMnO4 solution disappears and a precipitate of MnO2 is formed.

Explanation :

But colourless solution formed indicate there is no presence of KMnO4.

(3) (what is the inference?)

In the Lassaignes test for sulphur in an organic compound, the sodium fusion extract is acidified with acetic acid and then lead acetate is added to it. The precipitation of lead

sulphide, which is black in colour, indicates the presence of sulphur in the compound. 

Lassaignes extract + Lead acetate + Acetic acid → Black precipitate

S^2- + Pb²⁺ → PbS (Black)

The sodium fusion extract is treated with sodium nitroprusside. Appearance of violet colour also indicates the presence of sulphur in the compound.

S^2- + [Fe(CN)5NO]^2- + 4Fe³⁺ → [Fe(CN)5NOS]^4-

If in an organic compound, both nitrogen and sulphur are present, then instead of NaCN, formation of sodium thiocyanate (NaSCN) takes place.

Na + C + N + S → NaSCN

This NaSCN gives a blood red colour. Prussian colour is not formed due to the absence of free cyanide ions.

Fe³⁺  + SCN^- → [Fe(SCN)]^2- (Red)

(4) (what is the inference?)

In Lassaignes test for halogens in an organic compound, the sodium fusion extract is acidified with nitric acid and then treated with silver nitrate.

X^- + Ag⁺ → AgX  (X = Cl. Br, I)

If nitrogen and sulphur both are present in the organic compound, then the Lassaignes extract is boiled to expel nitrogen and sulphur, which would otherwise interfere in the test for halogens.