chemistry

Detection of Carbon, Hydrogen , Nitrogen ,Oxygen, Sulphur ,Halogens in Organic compounds

Detection of Carbon, Hydrogen , Nitrogen ,Oxygen, Sulphur ,Halogens in Organic compounds

Detection of Elements in Organic compounds:

100% Organic compounds have carbon. Beside carbon they also have hydrogen, oxygen, nitrogen, Sulphur and halogens.

For detections of these elements chemical test are performed as follows:

I)   Detection of Carbon:- 

                Take a little amount of organic compound and comparatively larger amount of dry CuO in a test tube.

Heat the test tube strongly. The gas produced as a result of heating is passed through the lime water taken in a test tube. If lime water turns milky then carbon is present in the compound.  

C          +        2CuO            →          CO2         +        2Cu

       Ca (OH)2 (aq)    +   CO2    →    CaCO3     +  H2O

     Aqueous calcium hydroxide (lime water)           Calcium carbonate (white ppt)

On heating strongly the compound burns and carbon changes to carbon dioxide which on passing through lime water reacts and produce white precipitate of calcium carbonate.  

II) Detection of Hydrogen:- 

Take a little amount of organic compound and comparatively larger amount of dry CuO in a test tube.

Heat the test tube strongly. The gas and vapours produced as a result of heating are passed over anhydrous copper sulphate. If water drops appeared on cooler parts of the test tube or if anhydrous copper sulphate turned blue then hydrogen is present in the compound.

       CuO           +      2H                  →            H2O     +        Cu 

          Cupric oxide               Organic compound                       

         CuSO4     +         5H2O         →       CuSO4.5H2 O

       Anhydrous copper sulphate                                 hydrated copper sulphate (Blue vitriol)

(white)                                                                                (blue)

On heating strongly the compound burns and hydrogen of the compound changes to water which on passing over anhydrous copper sulphate is absorbed and converts anhydrous copper sulphate to hydrated copper sulphate which has blue colour.

III)  Detection of Nitrogen, Sulphur and Halogens:-

          To detect nitrogen, sulphur and halogens in the given organic compound first Lassaigne’s solution is prepared.

Preparation of Lassaigne’s solution:

It is based upon the fact that when organic compound containing nitrogen, sulphur and halogen is strongly heated with metallic sodium, its nitrogen converts to ionic NaCN (sodium cyanide), sulphur to ionic NaS (sodium sulphide) and halogen to ionic NaX (sodium halide) if they are present.

A small piece of sodium metal is taken in a fusion tube and is melted. A pinch of organic compound is dropped on molten sodium in the fusion tube and the tube is heated to red hotness. The red hot tube is broken in distilled water contained in a china dish. The mixture is boiled and then filtered. The filtrate is called Lassaigne’s solution.

Carbon and nitrogen from organic compound react with sodium to form sodium cyanide.

                     Na           +         C             +          N           →            NaCN

                                                                                                   Sodium cyanide (ionic)

Sulphur from organic compound reacts with sodium to form sodium sulphide.

                                Na           +              S                  →          NaS

                                                                                                  Sodium sulphide (ionic)

Halogen from organic compound reacts with sodium to form sodium halide.

                Na           +              X          →                             NaX                     Where X = (Cl, Br, I)

                                                                                    Sodium halide (ionic)

(A)  Detection of Nitrogen:

A few mls of Lassaigne’s solution are taken in a test tube. Then a few drops of NaOH solution and few mls of freshly prepared FeSO4 are added to Lassaigne’s solution. The solution is boiled and cooled. Now a few drops of FeCl3 solution and little HCl is added. Formation of Prussian blue or green colouration or precipitate confirms presence of Nitrogen.

                                Na  +  C  +  N                  →                   NaCN

                                FeSO4  +  2NaOH            →                Na2SO4  +  Fe(OH)2

                                Fe(OH)2  + 6NaCN           →                  Na4[Fe(CN)6]  +  2NaOH

                                                                                              Sodium ferrocyanide

                                                                                         Sodium hexa cyano ferrate (II)

                                3Na4[Fe(CN)6] +4FeCl3          →            Fe4[Fe(CN)6]3  + 12NaCl

                                                                                                Ferri ferrocyanide

                                                                                           (Prussian blue or green)

(B) Detection of sulphur:

A few mls of Lassaigne’s solution are taken in a test tube and acetic acid is added to it. Then lead acetate is added to the solution. Appearance of black precipitate of lead sulphide indicates presence of sulphur in the compound.

                2Na        +              S                 →                          Na2S

                Na2S + (CH3COO)2Pb                →                       PbS         +              2CH3COONa

                Sodium sulphide   Lead acetate                       Lead sulphide       sodium acetate

                                                                                                (Black ppt)

(C)   Detection of Halogens:

  A few mls of Lassaigne’s solution are taken in a test tube and 1-2 drops of conc HNO3 are added to it. The solution is boiled and cooled. Now silver nitrate is added to the solution.

  • Appearance of white precipitate of silver chloride soluble in NH4OH solution indicates presence of chlorine in the compound.

                                Na  +  Cl       →        NaCl

                                NaCl  +  AgNO3        →      AgCl  +  NaNO3

                                                                        Silver chloride

                                                                      (White precipitate)

  • Appearance of pale yellow precipitate of silver bromide partially soluble in NH4OH solution indicates presence of bromine in the compound.

                                Na  +  Br               →                    NaBr

                                NaBr  +  AgNO3         →                    AgBr  +  NaNO3

                                                                                        Silver bromide

                                                                               (Pale yellow precipitate)

  • Appearance of yellow precipitate of silver iodide insoluble in NH4OH solution indicates presence of iodide in the compound.

                                Na  +  I                     →                         NaI

                                NaI  +  AgNO3       →                          AgI  +  NaNO3

                                                                                             Silver iodide

                                                                                             (Yellow precipitate)

IV)   Detection of Oxygen:-

     Method-I:

    The compound is heated in a dry test tube in atmosphere of nitrogen. Formation of water droplets on the cooler part of the test tube indicates presence of oxygen in the compound.

   Method-II:

All other elements are detected and estimated. If sum of percentages of elements is not equal to 100 then remaining amount is of oxygen.

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