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

Detection of Elements in Organic compounds:

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

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

I)   Detection of Carbon:- 

                Take a little amount of organic compound and a 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 a white precipitate of calcium carbonate.  

II) Detection of Hydrogen:- 

Take a little amount of organic compound and a 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                                

 CuSO₄   +   5H₂O   →    CuSO₄.5H₂ O

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 a 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 an 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.

Material Required for Lassaigne’s Solution preparation:

Following is the list of materials required for the preparation of lassaigne’s solution.

1. Sodium metal
2. Distilled water
3. Organic compound
4. Test tube
5. Fusion tube
6. China dish
7. Funnel
8. Wire gauze
9. Tripod stand’
10. Filter paper
11. Glass rod
12. Bunsen burner

Procedure for the Preparation of Lassaigne’s Solution:

Step 1:

Take a small piece of dry sodium metal in a fusion tube.

Step 2:

Heat the fusion tube gently on the Bunsen burner to melt sodium metal.

Step 3:

Add a small amount of organic compound to a fusion tube.

Step 4:

Heat it first gently & then strongly the tube becomes red hot. 

Step 5:

Break this fusion tube into a china dish containing 20cm³ of distilled water.

Step 6:

Mix ,boil & filter the solution .

Step 7:

This filtrate is called Lassaigne’s solution or sodium extract

Then divide the filtrate (Lassaigne’s solution ) into three parts to test the presence of N, S & halogens.

It is based upon the fact that when an 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.

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

   Na       +     C       +     N     →   NaCN

                                     Sodium cyanide (ionic)

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

     Na           +         S             →        NaS

                                           Sodium sulphide (ionic)

Halogen from organic compounds 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 a 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 a little HCl is added. The formation of Prussian blue or green colouration or precipitate confirms the 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. The appearance of a black precipitate of lead sulphide indicates the presence of sulphur in the compound.

       2Na      +    S         →           Na₂S

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

                                       (Black ppt)

(C)   Detection of Halogens:

  A few mls of Lassaigne’s solution are taken in a test tube and 1-2 drops of concentrated 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 an atmosphere of nitrogen. The formation of water droplets on the cooler part of the test tube indicates the presence of oxygen in the compound.

   Method-II:

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

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