ORGANIC cHEMISTRY - III

Experiment 6

Aim of the Experiment

To determine the functional groups present in the given unknown organic compound

Theory

PTHALEIN TEST

Phenol on heating with phthalic anhydride in the presence of concentrated sulfuric acid forms a colourless condensation compound called phenolphthalein. On further reaction with dilute sodium hydroxide solution gives a pink colour fluorescent compound called fluorescein. Characteristic colours are produced by different phenolic compounds which can be viewed under white background.

The colours produced by different phenolic compounds in phthalein dye test is listed below.

Phenol------Reddish pink

o-cresol------Red

m-cresol------blue or violet blue

1-naphthol------green

2-naphthol------faint green

Resorcinol------yellow-green fluorescence

Hydroquinone------deep purple

IODOFORM TEST

When Iodine and sodium hydroxide are added to a compound that contains either a methyl ketone or a secondary alcohol with a methyl group in the alpha position, a pale yellow precipitate of iodoform or triiodomethane is formed. It can be used to identify aldehydes or ketones. If an aldehyde gives a positive iodoform test, then it must be acetaldehyde since it is the only aldehyde with a CH3C=O group.

PHENYL HYDRAZINE TEST

Aldehydes and ketones have cabonyl group and on condensation with phenyl hydrazine gives crystalline phenyl hydrazones

2,4 dinitro phenyl phenyl hydrazine is a better reagent and gives crystalline derivative even with alipahtic aldehydes and ketones

TOLLEN’S TEST

Tollens Reagent Preparation

Since Tollens Reagent has a relatively short shelf life, the reagent is not commercially sold. Therefore, this reagent is often prepared directly in the laboratory. One such method for the preparation of Tollens Reagent is described below:

Step 1: A few drops of dilute NaOH are introduced to an aqueous solution of silver nitrate. The aqueous solution of silver nitrate contain silver aquo complexes wherein water acts as a ligand. The hydroxide ions now convert these silver aquo complexes into silver oxides. This silver oxide (given by Ag₂O) precipitates as a brown solid from this solution. The reaction can be written as follows.

2AgNO₃ + 2NaOH → Ag₂O (brown ppt) + 2NaNO₃ + H₂O

Step 2: The brown precipitate of silver oxide generated in step 1 is now dissolved with aqueous ammonia. The solution which formed from this addition of aqueous ammonia contains the [Ag(NH₃)₂]⁺ complex. This complex is the primary component of Tollens Reagent. The reaction can be written as:

Ag₂O (brown ppt) + 4NH₃ + 2NaNO₃ + H₂O → 2[Ag(NH₃)₂]NO₃ + 2NaOH

When an aldehyde is introduced to the Tollens reagent, two things occur:

The aldehyde is oxidized by the Tollens reagent and forms a carboxylic acid

The silver ions present in the Tollens reagent are reduced into metallic silver. Generally, the Tollens Test is carried out in clean test tubes made of glass. This is because the reduction of the silver ions into metallic silver form a silver mirror on the test tube

Tollens test is commonly referred to as the Silver Mirror test due to the formation of this layer of metallic silver on the test tube.

MULLIKEN AND BARKER TEST

Compounds with nitro group are generally yellow coloured and on reduction with zinc dust and ammonium chloride solution give hydroxylamine derivative which gives test with Tollen's reagent

Materials required

sodium hydroxide solution , silver nitrate solution , ammonium hydroxide solution , pthalic anhydride , concentrated sulphuric acid , iodine solution , zinc dust, ammonium chloride solution etc...

Procedure

Procedure to perform Tollen's test

Take 2-3 ml of the the unknown organic compound in the test tube.

Add tollens reagent to the organic compound and heat it slightly

The presence of silver mirror indicates the presence of aldehyde

Procedure to perform Pthalein test

In a clean and dry test tube take your unknown organic compound

Take equal amounts of phthalic anhydride solution and moisten it with few drops of concentrated sulphuric acid solution

Now heat it gently , cool it and pour few drops of the mixture in excess of sodium hydroxide solution

Characteristic colours produced indicates the presence of phenolic compound

Procedure to perform iodoform test

Take your unknown organic compound in the clean and dry test tube

Add equal amounts of sodium hydroxide solution

Now start adding iodine solution dropwise

Keep on adding iodine solution till the colour of the iodine solution persists

Heat the solution for 10-15 seconds and don't heat above 60 degree Celsius

Now cool the solution

Appearance of yellow crystals indicates presence of methyl ketones .

Procedure to perform phenyl hydrazine test

Take 5 ml of distilled water and add equal amounts of glacial acetic acid

Add 0.5 ml of phenyl hydrazine and shake

Now add your unknown organic compound and shake well of 1-2 minutes

Warm for few minutes in water bath if essential

Flocculent precipitates of phenyl hydrazone indicates presence of aldehyde or ketone

Procedure to perform Mulliken Baker's test

Dissolve the unknown organic compound in 4-5ml of 1:1 alcohol solution

Now add zinc dust and ammonium chloride solution

Now heat to boiling for about 5 minutes and cool and filter

Add little of this filtrate to Tollen's reagent

Black or grey precipitates indicates presence of nitro group

Result

Yellow crystals in iodoform test indicates presence of methyl ketones or acetaldehyde

Silver mirror formation in tollen's test indicates presence of aldehydes

Black or grey precipitates in Mulliken Baker test indicates presence of nitro group

Floculent precipitates in phenyl hydrazine test indicates presence of aldehyde and ketone

Questions

1.What is the principle of iodoform test ?

2.What is the principle of Tollen's test ?

3.What is the principle of Mulliken Baker's test ?