Detection of Extra elements in the organic compound

 

Dr. pooja bhagat

ASSOCIATE PROFESSOR

Department Of Chemistry

Acharya narendra dev college

university of Delhi

 

 

In an organic compound, elements carbon, hydrogen and oxygen are assumed to be present. Elements other than these elements i.e. Nitrogen, sulphur and halogens (chlorine, bromine and iodine) may also be present in an organic compound. As you have known earlier that chemical analysis is of two types: Qualitative analysis and Quantitative analysis. The detection of extra elements in a given organic compound is a type of qualitative analysis since here we are dealing with the composition of the compound. This experiment is to be done very carefully as further analysis of the organic compound is based on the extra element present in it.

 

Lassaigne’s Test

 

These extra elements are usually detected by Lassaigne’s test. In this test, the organic compound is fused with metallic sodium to convert these elements into water soluble sodium salts.And then this extract is used to perform the tests.  

 

I Preparation of lassaigne’s extract

 

Cut a small piece of sodium metal*a with metallic spatula and dry it by pressing between the folds of filter paper. Put this piece of sodium metal into an ignition tube. The ignition tube is heated slowly till the sodium metal turns into shining globule. Remove it from flame and add the compound (a pinch of solid compound or 2-3 drops of liquid compound*b). First heat it gently and then strongly until it becomes red hot. Plunge it into 10 ml of distilled water contained in a china dish. And cover the china dish immediately with wire gauze to stop the flames. The same procedure is repeated by 3 more ignition tubes . The ignition tubes if not broken completely, are crushed with the help of a glass rod. Boil the contents of the china dish for about 5 minutes and filter. The filtrate is called lassaigne’s extract or sodium extract. The filtrate should be colourless. Filtrate will be coloured when fusion is incomplete. So, if it is coloured, whole procedure should be repeated again.

 

*a While using the sodium metal, following precautions should be taken into account:

  1. Donot touch the sodium metal with your fingers, always use forceps to handle it.
  2. Never throw the sodium metal into sink.
  3. Unreacted  sodium metal should be carefully decomposed by adding small amount of ethyl alcohol into it.

 

*b When the liquid compound gets evaporated very rapidly before its fusion with sodium metal, a paste of the compound with sodium carbonate may be used or after the addition of  liquid compound in the ignition tube, solid sodium carbonate may be added.

 

The reactions involved in the preparation of Lassiagne’s extract are:

 

Nitrogen, Na + C + N      NaCN

                                      (sodium cyanide)

So, nitrogen will be present in the form of sodium cyanide in the Lassiagne’s extract.

  

Sulphur, 2Na + S        Na2S

                                 (sodium sulphide)

Thus, sulphur will be present  in the form of sodium sulphide in the Lassiagne’s extract.

 

 

Nitrogen and sulphur both, Na + C + N + S   NaSCN

                                                                        (sodium thiocyanate)

And so, when nitrogen and sulphur are present together in the organic compound, they will be present  in the form of sodium thiocyanate in the Lassiagne’s extract.

 

 

Halogens, Na + X      NaX   (X=Cl, Br and I)

                                   (sodium halide)

So, halogens will be present  in the form of sodium halide (if X=Cl, then sodium chloride, if X= Br, then sodium bromide and if X= I, then sodium iodide) in the Lassiagne’s extract.

 

I.1 Test for Nitrogen

 

Take 1-2 ml of the Lassaigne’s (sodium) extract in the test tube add freshly prepared ferrous sulphate solution into it. A dirty green precipitate of ferrous hydroxide is obtained. If no precipitate is formed, add a few drops of dilute sodium hydroxide solution to get the precipitate. Now heat this mixture gently with shaking for 1 minute and add dilute sulphuric acid into it. A Prussian blue colour confirms the presence of Nitrogen in the compound.

 

Reactions of Nitrogen test:

 

Ferrous sulphate reacts with the NaOH present in the extract to give a dirty green colour precipitate.

 

FeSO4   + 2NaOH      Fe(OH)2  +  Na2SO4

                                (ferrous hydroxide)

                                  (dirty green ppt.)

 

NaOH is formed in the Lassiagne’s extract by the reaction between unreacted sodium metal and water. Thus, if no green precipitate is obtained that means NaOH is not present in the extract so few drops of it is added in the test tube to get the precipitate.

 

Ferrous ions react with sodium cyanide (ionic form of nitrogen in the extract) to give sodium ferricyanide.

 

Fe(OH)2  +  6NaCN         Na4Fe(CN)6    +   2NaOH

                                   (sodium ferrocyanide)

 

On boiling the alkaline solution of ferrous salt, some ferric ions are inevitably produced by the aerial oxidation. And ferrous and ferric hydroxides are dissolved on the addition of dilute sulphuric acid.  

 

Sodium ferrocyanide reacts with ferric salt to give ferriferrocyanide complex which is Prussian blue colour complex.

 

3Na4Fe(CN)6  +  2Fe2(SO4)3  Fe4[Fe(CN)6]3  + 6Na2SO4

                                              (Ferri-ferrocyanide)

                                                  (Prussian blue)

This Prussian blue colour indicates the presence of nitrogen in the given compound.

 

I.2  Test for Sulphur

 

Sodium Nitroprusside Test

 

Take 1-2 ml of  the sodium extract in the test tube, add 1ml of freshly prepared sodium nitroprusside solution into it. A violet colour indicates the presence of sulphur in the compound.

Rection of Sulphur test:

 

Sodium sulphide (ionic form of sulphur in the extract) reacts with sodium nitroprusside to give sodium sulphonitroprusside which is violet in colour.

 

Na2S   +   Na2 [Fe(CN)5NO]        Na4[Fe(CN)5NOS]

             (sodium nitroprusside)    (sodium sulphonitroprusside)

                                                               (violet colour)

The violet colour indicates the presence of sulphur in the compound.

 

I.3 Test for Nitrogen and Sulphur present together

 

Take 1-2 ml of sodium extract in the test tube and acidify it with dilute hydrochloric acid. Add 2-3 drops of ferric chloride solution. A blood red colour indicates the presence of Nitrogen and Sulphur together.

 

Reaction of Nitrogen and Sulphur test together:

 

Sodium thiocyanate (ionic form of nitrogen and sulphur together in the extract) reacts with ferric chloride to give ferric thiocyanate, a blood red colour complex.

 

3NaSCN   +   FeCl3         Fe(SCN)3     +    3NaCl

                                     (ferric thiocyanate)

                                      (blood red colour)

So,the blood red colour indicates the presence of nitrogen and sulphur together.

 

During the preparation of Lassaigne’s extract, if the organic compound containing both nitrogen and sulphur is fused with an excess of sodium metal, instead of sodium thiocyanate, sodium cyanide and sodium sulphide are formed.

 

NaSCN  +  2Na    NaCN  +  Na2S

 

In such cases, the tests for NaCN and Na2S can be performed. Thus in such cases individual test for nitrogen and sulphur will be positive.

 

I.4 Test for Halogens

 

Halogens are detected by two tests:

(A) Silver nitrate test                      (B) Layer test

 

(A) Silver Nitrate Test

 

(a) When nitrogen and / or sulphur are absent

 

Take 1-2 ml of sodium extract in test tube and acidify it with dilute nitric acid. Add silver nitrate solution (0.5ml). A white precipitate soluble in Ammonium hydroxide indicates the presence of chlorine. A pale yellow precipitate soluble in excess of Ammonium hydroxide solution indicates the presence of bromine. And a yellow precipitate insoluble in Ammonium hydroxide indicates the presence of iodine.

 

Reaction of Halogens:

 

Sodium halide (ionic form of halogen in extract) reacts with silver nitrate to give the precipitate of silver halide.

 

NaX   +  AgNO3    AgX↓  +  NaNO3

                                  (ppt)

 

If X=Cl, then AgCl (silver chloride) will be formed which is white colour precipitate soluble in Ammonium hydroxide.

 

If X=Br, then AgBr (silver bromide) will be formed which is pale yellow colour precipitate soluble in excess of Ammonium hydroxide.

 

If X=I, then AgI (silver iodide) will be formed which is yellow colour precipitate insoluble in Ammonium hydroxide.

 

(b) When nitrogen and / or sulphur are present

 

Take 1 ml of sodium extract in test tube and acidify it with conc. nitric acid (2-3ml). Boil the mixture to reduce the original volume to half to get rid of HCN and / or H2S. And now perform the silver nitrate test.

 

When the silver nitrate test is positive (precipitate is obtained) then the layer test is to be performed.

 

The positive silver nitrate test only indicates the presence of halogens. And which of the halogen is present is confirmed by the Layer test.

 

(B)Layer test:

 

Take 1-2 ml of sodium extract in a test tube and add 1 ml of carbon tetrachloride or chloroform. Now, add 1 ml of chlorine water and shake the test tube well. If the organic layer turns orange in colour, it indicates the presence of bromine.While if the organic layer turns violet in colour, it indicates the presence of iodine. If no colour change in the organic layer, it indicates the presence of chlorine in it.

 

Reaction of Layer test:

 

Chlorine water is used as an oxidizing agent and Br-(ionic form of bromine in extract) gets converted into Br2 .

 

2NaBr  + Cl2    2NaCl   +  Br2

 

And Br2 gives orange colour to organic layer.

Chlorine water oxidises I- (ionic form of iodine in extract) into I2 .

 

2NaI  + Cl2    2NaCl   +  I2

 

And I2 gives voilet colour to organic layer.

 

SUMMARY

 

We have learnt about the detection of following extra elements in the given organic compound:

 

         nitrogen,

         sulphur,

         chlorine,

         bromine and

         Iodine.

 

 

How To record

 

AIM: To detect the presence of extra element(s) in the given organic compound.

 

APPARATUS REQUIRED: China dish, funnel, test tubes, ignition tubes, capillary tubes, glass rod, wire gauze, pair of tongs, test tube holder, dropper, spatula

 

CHEMICALS REQUIRED: Organic compound whose extra element is to be determined, sodium metal, ferrous sulphate, sodium nitroprusside, ferric chloride solution, silver nitrate, aq. NaOH, dil. Sulphuric acid, dil. Nitric acid, dil. Hydrochloric acid, conc. nitric acid

 

PROCEDURE:

Cut a small piece of sodium metal with metallic spatula and dry it by pressing between the folds of filter paper. Put this piece of sodium metal into an ignition tube. The ignition tube is heated slowly till the sodium metal turns into shining globule. Remove it from flame and add the compound (a pinch of solid compound or 2-3 drops of liquid compound). First heat it gently and then strongly until it becomes red hot. Plunge it into 10 ml of distilled water contained in a china dish. And cover the china dish immediately with wire gauze to stop the flames. The same procedure is repeated by 3 more ignition tubes . The ignition tubes if not broken completely, are crushed with the help of a glass rod. Boil the contents of the china dish for about 5 minutes and filter. The filtrate is called lassaigne’s extract or sodium extract. The filtrate should be colourless. Filtrate will be coloured when fusion is incomplete. So, if it is coloured, whole procedure should be repeated again.

 

This filterate is used to perform the test for extra elements.

 

OBSERVATION TABLE:

S.No.

Experiment

Observation

Inference

1

Test for Nitrogen

To the Lassaigne’s or sodium extract (1-2 ml), add sodium ferrous sulphate (50-75mg).

 [If no precipitate is formed, add a few drops of dilute sodium hydroxide solution to get the precipitate.]

Now heat this mixture gently with shaking for 1 minute and add dilute sulphuric acid into it.

 

 

dirty green precipitate is obtained.

 

 

 

 

 Prussian blue colour

 

__

 

 

 

 

 

 

Nitrogen is present.

2

Test for Sulphur

 

Sodium Nitroprusside Test

 

To the sodium extract (1-2 ml) add 1ml of freshly prepared sodium nitroprusside solution.

 

 

 

 

Voilet colour

 

 

 

 

Sulphur is present

3

Test for Nitrogen and Sulphur present together

 

Take 1-2 ml of sodium extract and acidify it with dilute hydrochloric acid. Add 2-3 drops of ferric chloride solution.

 

 

 

blood red colour

 

 

 

Nitrogen and Sulphur are present together.

4

 

(A)

 

(a)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(b)

Test for Halogens

 

Silver Nitrate Test

 

When nitrogen and / or sulphur are absent

 

Acidify 1-2ml of sodium extract  with dilute nitric acid . Add silver nitrate solution (0.5ml).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

When nitrogen and / or

sulphur are present

 

Acidify 1 ml of sodium extract with dilute nitric acid (2-3ml). Boil the mixture to reduce the original volume and now perform the silver nitrate test.

 

 

 

 

 

 

 

 White precipitate soluble in Ammonium hydroxide solution

 

 Pale yellow precipitate soluble in excess of Ammonium hydroxide solution

 

 Yellow precipitate insoluble in Ammonium hydroxide

 

 

 

 

 

 

 

 

Chlorine is present

 

 

 

 

Bromine is present

 

 

 

 

 

Iodine is present

 

RESULT: The given organic compound no. --- contains --------- as an extra element.

 

PRECAUTIONS:

 

1)      Donot touch the sodium metal with your fingers, always use forceps to handle it.

2)      Never throw the sodium metal into sink.

3)      Unreacted  sodium metal should be carefully decomposed by adding small amount of ethyl alcohol into it.

4)      Always use freshly prepared ferrous sulphate  and sodium nitroprusside solution.

5)      Fuse 4 ignition tubes so that the extract should be concentrated.

6)      Remove sodium cyanide and sodium sulphide before perfoming the test for halogens.

7)      In layer test, shake the test tube by putting thumb on the mouth of the test tube.