Chapter 12 Organic Chemistry Some Basic Principles and Techniques Solutions
Question - 21 : - Discuss the chemistry ofLassaigne’s test.
Answer - 21 : -
Lassaigne’s test
This test is employed todetect the presence of nitrogen, sulphur, halogens, and phosphorous in anorganic compound. These elements are present in the covalent form in an organiccompound. These are converted into the ionic form by fusing the compound withsodium metal.
The cyanide, sulphide,and halide of sodium formed are extracted from the fused mass by boiling it indistilled water. The extract so obtained is called Lassaigne’s extract. ThisLassaigne’s extract is then tested for the presence of nitrogen, sulphur,halogens, and phosphorous.
(a) Test for nitrogen
Chemistry of the test
In the Lassaigne’s testfor nitrogen in an organic compound, the sodium fusion extract is boiled withiron (II) sulphate and then acidified with sulphuric acid. In the process,sodium cyanide first reacts with iron (II) sulphate and forms sodiumhexacyanoferrate (II). Then, on heating with sulphuric acid, some iron (II)gets oxidised to form iron (III) hexacyanoferrate (II), which is Prussian bluein colour. The chemical equations involved in the reaction can be representedas
(b) Test for sulphur
Chemistry of the test
In the Lassaigne’s testfor sulphur in an organic compound, the sodium fusion extract is acidified withacetic acid and then lead acetate is added to it. The precipitation of leadsulphide, which is black in colour, indicates the presence of sulphur in thecompound.
Chemistry of the test
The sodium fusionextract is treated with sodium nitroprusside. Appearance of violet colour alsoindicates the presence of sulphur in the compound.
If in an organiccompound, both nitrogen and sulphur are present, then instead of NaCN,formation of NaSCN takes place.
Na + C + N + S → NaSCN
This NaSCN (sodiumthiocyanate) gives a blood red colour. Prussian colour is not formed due to theabsence of free cyanide ions.
(c) Test for halogens
Chemistry of the test
In the Lassaigne’s testfor halogens in an organic compound, the sodium fusion extract is acidifiedwith nitric acid and then treated with silver nitrate.
If nitrogen and sulphurboth are present in the organic compound, then the Lassaigne’s extract isboiled to expel nitrogen and sulphur, which would otherwise interfere in thetest for halogens.
Question - 22 : - Differentiate betweenthe principle of estimation of nitrogen in an organic compound by (i) Dumasmethod and (ii) Kjeldahl’s method.
Answer - 22 : -
In Dumas method, a knownquantity of nitrogen containing organic compound is heated strongly with excessof copper oxide in an atmosphere of carbon dioxide to produce free nitrogen inaddition to carbon dioxide and water. The chemical equation involved in theprocess can be represented as
The traces of nitrogenoxides can also be produced in the reaction, which can be reduced to dinitrogenby passing the gaseous mixture over a heated copper gauge. The dinitrogenproduced is collected over an aqueous solution of potassium hydroxide. Thevolume of nitrogen produced is then measured at room temperature andatmospheric pressure.
On the other hand, inKjeldahl’s method, a known quantity of nitrogen containing organic compound isheated with concentrated sulphuric acid. The nitrogen present in the compoundis quantitatively converted into ammonium sulphate. It is then distilled withexcess of sodium hydroxide. The ammonia evolved during this process is passedinto a known volume of H2SO4. The chemical equationsinvolved in the process are
The acid that is leftunused is estimated by volumetric analysis (titrating it against a standardalkali) and the amount of ammonia produced can be determined. Thus, thepercentage of nitrogen in the compound can be estimated. This method cannot beapplied to the compounds, in which nitrogen is present in a ring structure, andalso not applicable to compounds containing nitro and azo groups.
Question - 23 : - Discuss the principle ofestimation of halogens, sulphur and phosphorus present in an organic compound.
Answer - 23 : -
Estimation of halogens
Halogens are estimatedby the Carius method. In this method, a known quantity of organic compound isheated with fuming nitric acid in the presence of silver nitrate, contained ina hard glass tube called the Carius tube, taken in a furnace. Carbon andhydrogen that are present in the compound are oxidized to form CO2 andH2O respectively and the halogen present in the compound isconverted to the form of AgX. This AgX is then filtered, washed, dried, andweighed.
Let the mass of organiccompound be m g.
Mass of AgX formed= m1 g
1 mol of Agx contains 1mol of X.
Therefore,
Mass of halogen in m1 g of AgX 
Estimation of Sulphur
In this method, a knownquantity of organic compound is heated with either fuming nitric acid or sodiumperoxide in a hard glass tube called the Carius tube. Sulphur, present in thecompound, is oxidized to form sulphuric acid. On addition of excess of bariumchloride to it, the precipitation of barium sulphate takes place. This precipitateis then filtered, washed, dried, and weighed.
Let the mass of organiccompound be m g.
Mass of BaSO4 formed= m1 g
1 mol of BaSO4 =233 g BaSO4 = 32 g of Sulphur
Therefore, m1 gof BaSO4 contains 
Estimation of phosphorus
In this method, a knownquantity of organic compound is heated with fuming nitric acid. Phosphorus,present in the compound, is oxidized to form phosphoric acid. By adding ammoniaand ammonium molybdate to the solution, phosphorus can be precipitated asammonium phosphomolybdate.
Phosphorus can also beestimated by precipitating it as MgNH4PO4 by addingmagnesia mixture, which on ignition yields Mg2P2O7.
Let the mass of organiccompound be m g.
Mass of ammoniumphosphomolybdate formed = m1 g
Molar mass of ammoniumphosphomolybdate = 1877 g
If P is estimated as Mg2P2O7,
Question - 24 : - Explain the principle ofpaper chromatography.
Answer - 24 : -
In paper chromatography,chromatography paper is used. This paper contains water trapped in it, whichacts as the stationary phase. On the base of this chromatography paper, thesolution of the mixture is spotted. The paper strip is then suspended in asuitable solvent, which acts as the mobile phase. This solvent rises up thechromatography paper by capillary action and in the procedure, it flows overthe spot. The components are selectively retained on the paper (according totheir differing partition in these two phases). The spots of differentcomponents travel with the mobile phase to different heights. The paper soobtained (shown in the given figure) is known as a chromatogram.
Question - 25 : - Why is nitric acid addedto sodium extract before adding silver nitrate for testing halogens?
Answer - 25 : -
While testing theLassaigne’s extract for the presence of halogens, it is first boiled withdilute nitric acid. This is done to decompose NaCN to HCN and Na2Sto H2S and to expel these gases. That is, if any nitrogen andsulphur are present in the form of NaCN and Na2S, then they areremoved. The chemical equations involved in the reaction are represented as
Question - 26 : - Explain the reason forthe fusion of an organic compound with metallic sodium for testing nitrogen,sulphur and halogens.
Answer - 26 : -
Nitrogen, sulphur, andhalogens are covalently bonded in organic compounds. For their detection, theyhave to be first converted to ionic form. This is done by fusing the organiccompound with sodium metal. This is called “Lassaigne’s test”. The chemicalequations involved in the test are
Carbon, nitrogen,sulphur, and halogen come from organic compounds.
Question - 27 : - Name a suitabletechnique of separation of the components from a mixture of calcium sulphateand camphor.
Answer - 27 : -
The process ofsublimation is used to separate a mixture of camphor and calcium sulphate. Inthis process, the sublimable compound changes from solid to vapour statewithout passing through the liquid state. Camphor is a sublimable compound andcalcium sulphate is a non-sublimable solid. Hence, on heating, camphor willsublime while calcium sulphate will be left behind.
Question - 28 : - Explain, why an organicliquid vaporises at a temperature below its boiling point in its steamdistillation?
Answer - 28 : -
In steam distillation,the organic liquid starts to boil when the sum of vapour pressure due to theorganic liquid (p1) and the vapour pressure due to water (p2)becomes equal to atmospheric pressure (p), that is, p = p1 + p2
Since p1 < p2,organic liquid will vapourise at a lower temperature than its boiling point.
Question - 29 : - Will CCl4 givewhite precipitate of AgCl on heating it with silver nitrate? Give reason foryour answer.
Answer - 29 : -
CCl4 willnot give the white precipitate of AgCl on heating it with silver nitrate. Thisis because the chlorine atoms are covalently bonded to carbon in CCl4.To obtain the precipitate, it should be present in ionic form and for this, itis necessary to prepare the Lassaigne’s extract of CCl4.
Question - 30 : - Why is a solution ofpotassium hydroxide used to absorb carbon dioxide evolved during the estimationof carbon present in an organic compound?
Answer - 30 : -
Carbon dioxide is acidicin nature and potassium hydroxide is a strong base. Hence, carbon dioxidereacts with potassium hydroxide to form potassium carbonate and water as
Thus, the mass of theU-tube containing KOH increases. This increase in the mass of U-tube gives themass of CO2 produced. From its mass, the percentage of carbonin the organic compound can be estimated.