21 Chemistry -- Fundamental Principles of Organic Chemistry

What is the composition of Lassaigne's extract if the organic compound contain Nitrogen? How presence of Nitrogen detected?

1 Answers

If the organic compound contains nitrogen as foreign element the sodium extract must have the following composition:

Na + C + N-------------------------> NaCN
(sodium cyanide)

Detection of Nitrogen by Lassaigne's extract:
After the preparation of the lassaigne's solution is made alkaline by adding few drops of dilute of sodium hydroxide solution if the lassaigne's solution is not alkaline. A small amount of freshly prepared ferrous sulphate solution is added which yields green precipitate. It is boiled and then cooled. Now concentrated hydrochloric acid is added to dissolve the precipitate followed by the addition of few drops of ferric chloride solution. The appearance of purssain blue colour confirms the presence of nitrogen. The chemical reactions involved in the process are:

Na + C + N-----------------------------------> NaCN
NaOH + FeSO₄ ----------------------------> Fe(OH)₂ +Na₂SO₄
(green ppt.)
NaCN + Fe(OH)₂--------------------------> Fe(CN)₂ + NaOH
Fe(CN)₂ + 4NaCN-------------------------> Na₄[Fe(CN)₆] + NaCl
(Sodium Ferro cyanide)
Na₄[Fe(CN)₆] + FeCl₃--------------------> Fe₄[Fe(CN)₆]₃ + NaCl
(Purssain blue)
Fe(OH)₂ + Conc. HCl----------------------> FeCl₂ + H₂O

What is the IUPAC name of this compound.?

Why is it necessary to prepare sodium extract for the detection of foreign elements in organic compound?

Organic compounds are covalent compounds and they do not prefer to react with inorganic or ionic reagents. Hence, they should be converted into ionic compound. Also the Organic compound performs molecular reaction which can consume huge time. since sodium is highly reactive electropositive metal, it easily reacts with elements of organic compounds to form the ionic compounds.

Na + C + N ------------------> NaCN
2Na + S ----------------------> Na₂S
...

Why is lassaigne's extract alkaline in nature?

If sodium metal is not completely fused it reacts with water to form sodium hydroxide. Therefore the sodium extract is usually alkaline in nature. On the other hand, it becomes alkaline due to hydrolysis of NaCN, Na₂S etc. except of sodium.

Why does one need to use freshly prepared Ferrous Sulphate solution in detection of Nitrogen by Lassaigne's test?

We need to use freshly prepared Ferrous Sulphate solution in detection of Nitrogen by Lassaigne's because when FeSO4 is kept for a long time it gets oxidizedto ferric sulphate by atmospheric oxygen.

(+) 2-butanol has specific rotation of + 13.9° when measured in pure form. A sample of 2-butanol was found to have an optical rotation of –3°. What is the stereomeric composition of this mixture ?

Let x is the % of (+) 2-butanol.

13.9 x – 13.9 (100 – x) = – 300.

x = 39.2, % of d form = 39.2, % of l form = 60.8.

How many isomers are there corresponding to the formula C4H10O ?

There are 7—1-butanol, 2-butanol, 2-methyl-1-propanol, 2-methyl-2-propanol, diethyl ether, methyl propylether, and methylisopropyl ether.