Test for Phenolic Group

A class of organic compounds which has a hydroxyl group (-OH) directly attached to aromatic hydrocarbon rings are called phenols or phenolics. Phenols are widely used in the medicinal field and food industry. This is the reason they are widely synthesized at industrial level. Apart from this phenol are produced by plants and microorganisms as well. An aromatic hydrocarbon ring may have one or more phenolic groups attached to it. Identification of phenol in an organic aromatic compound is important, to know its various characteristics which helps in formation of drugs, disinfectants etc. Phenols are acidic in nature and this property of phenols helps in tests for phenols. Tests for phenolic groups in an aromatic compound can be done by various methods. In this article we will discuss five chemical tests to detect presence of phenol functional groups in an aromatic compound. 


Test for Phenol 

Detection of phenol functional group can be done by following tests –

  • Litmus test 

  • Ferric chloride test 

  • Phthalein dye test 

  • Libermann’s test 

  • Bromine water test 

Any of the above tests can be used to detect the phenol group in a compound. Let us discuss all these tests in detail one by one –


Litmus Test 

Aim – To detect the presence of a phenol functional group in a given sample.

 

Theory – Litmus paper changes color if the solution is acidic or basic in nature. Acidic solutions turn blue litmus paper into red and basic solutions turn red litmus paper into blue. Phenols are acidic in nature, so they show the litmus test. 

Here you need to keep in mind that carboxylic acids also give this test. So, to distinguish between two, you need to compare carboxylic acid and phenol. Phenol is less acidic in comparison to carboxylic acid that’s why when carboxylic acid reacts with sodium carbonate, it gives effervescence while phenol does not. Thus, litmus test is not a confirmation test to detect the presence of phenol in a solution of organic compounds. 


Materials Required – blue Litmus paper, dropper, test tube etc. 


Procedure – Take the given solution in a test tube and place 1-2 drops of it on the blue litmus paper. 


Observation – Color of litmus paper changes from blue to red. 


Result – The given solution may contain phenol. 


Precautions – Phenol is poisonous and corrosive in nature so must be handled carefully. 


Ferric Chloride Test 

Aim – To detect the presence of a phenol functional group in a given sample. 

Theory – This test is based on the fact that the phenols give a colored complex with neutral ferric chloride solution. Phenol group reacts with ferric ion of ferric chloride and forms a colored complex such as C6H5OH (simplest phenol) reacts with ferric chloride and forms a violet colored complex [Fe(C6H5O)6]3-. Reaction is given below –

6C6H5OH          +                  FeCl3          🡪           [Fe(C6H5O)6]3- + 3HCl + 3H+

Phenol                         Ferric chloride                  Violet complex 


Materials Required – test tube, ferric chloride solution, dropper etc. 


Procedure – Preparation of Ferric chloride solution – To prepare neutral ferric chloride solution, add diluted solution of NaOH to ferric chloride solution slowly until a permanent brown precipitate is produced. Now filter it and remove the precipitate. The clear filtrate thus obtained is used for the test of phenols. It should be prepared freshly for the test. 

Take 3 ml of the given solution in a test tube and add freshly prepared neutral ferric chloride solution in it dropwise. Note the color change. 


Observation – If the color of the solution becomes blue, green, violet, or red, this indicates the presence of a phenol group. Ortho, meta or para – cresol, resorcinol gives violet or blue color. β - naphthol gives green color and α - naphthol gives pink color. Formic acid and acetic acid give deep red color. 


Reaction – Reaction involved in the experiment can written as follows –

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Result – The given solution contains phenol. 


Precautions – 1. Use freshly prepared, dilute, and neutral solution of ferric chloride. 


Phthalein Dye Test 

Most of the phenols give this test accurately and show the result precisely.


Aim – To detect the presence of a phenol functional group in a given sample. 


Theory – Phenol gets condensed on heating with phthalic anhydride in the presence of conc. Sulfuric acid and forms phenolphthalein. Phenolphthalein gives pink colored compounds on reaction with a limited amount of sodium hydroxide while in excess of sodium hydroxide it gives a colorless compound. 


Materials Required – test tube, given organic compound, phthalic anhydride, conc. Sulfuric acid, sodium hydroxide, distilled water, burner, oil bath, dropper etc. 


Procedure –

  • Take a clean dry test tube. Add 100 mg of the given organic compound in this test tube.  

  • Now add 100 mg of phthalic anhydride in the same test tube.

  • Add 2 – 3 drops of concentrated sulfuric acid in the same test tube in which sample and phthalic anhydride has been taken.

  •  Heat the test tube in an oil bath for about 2 minutes.

  • Cool the mixture of the test tube.

  • Pour the mixture of the test tube in the beaker carefully which contains 15 ml of dilute sodium hydroxide solution. 

  • Observe the colour change. 


Observation – If the colour of the reaction mixture changes to pink, red, green or blue then it indicates the presence of phenol group in the given organic compound. However here you need to note that if sodium hydroxide solution is added in large amounts then it may cause disappearance of color and we get a colorless compound. 

Color Produced by Various Phenol Compounds in Phthalein Dye Test are Given Below –

Phenolic Compound 

Color 

o - Cresol

Red 

m – Cresol 

Bluish – purple 

p – Cresol 

No color 

Phenol 

Reddish pink 

1 – naphthol 

Green 

2 – naphthol 

Light green 

Resorcinol 

Yellow – green fluorescence 


Reactions Involved – Reactions involved in the experiment can written as follows –

(image will be uploaded soon)


Result – Given organic compound contains phenol group. 


Precautions – 1. Phenol is poisonous and corrosive in nature so must be handled carefully. 

2. Conc. Sulfuric acid should be handled very carefully. 

3. Some phenols are strong acids which contain an electron withdrawing group and get dissolved in sodium hydrogen carbonate solution as well.


Libermann’s Test 

This test is given by those phenols which have a free para position.  


Aim – To detect the presence of phenol functional group in a given sample.


Theory – When phenol is dissolved in conc. Sulfuric acid and sodium nitrite is added in it. Then in this mixture, sodium nitrite reacts with concentrated sulfuric acid and gives nitrous acid and sodium sulfate. Now, nitrous acid reacts with phenol and gives p- nitrosophenol. This, p- nitrophenol in excess of phenol and on dilution, gives indophenol complex which is red in color. When indophenol complex reacts with strong alkali (such as NaOH) forms a blue colored indophenol anion. Reactions involved can be written as follows –

2NaNO3                 +                 H2SO4               🡪                    2HNO2            +           Na2SO4

Sodium nitrite                       Sulfuric acid                           Nitrous acid            Sodium sulfate 

(image will be uploaded soon)


Materials Required – test tube, test tube holder, given organic compound, conc. Sulfuric acid, sodium hydroxide, sodium nitrite, distilled water, burner etc. 


Procedure – 1. Take a clean dry test tube.

2. Place a small amount of sodium nitrite crystals in it.

3. Now add 100 mg of given phenolic compound in it and heat the test tube for about 30-35 seconds gently. 

4. Allow the test tube to cool. 

5. Now, add 1 ml of the conc. Sulfuric acid and shake the test tube gently and carefully to mix the contents of it. 

6. Observe the change in color and dilute the solution with water. (If a phenol group is present in the given compound then on dilution, red colored compound is formed.)

7. Now, add sodium hydroxide in the red solution and observe the change in color. 


Observation – When we dilute the solution with water, solution turns red in color and on adding NaOH in it, solution turns blue (deep blue) in color.

 

Result – Deep blue color confirms the presence of phenol in the given sample. 


Precautions – 1. Concentrated sulfuric acid should be handled carefully. 

2. Phenols should be handled carefully. 

3. Color change should be noticed vigilantly. 

4. During heating, the test tube must be held by using a test tube holder. 


Bromine Water Test 

Aim – To detect the presence of phenol functional group in a given sample.


Theory – This test is based on the electrophilic substitution reaction of phenol with bromine. When phenol reacts with bromine water gives a poly-bromo derivative. All hydrogen atoms present at ortho and para position in phenol with respect to -OH group are replaced by bromine atoms. Thus, by reaction of phenol with bromine white precipitate of tribromo phenol is formed. Reaction is given below –

(image will be uploaded soon)


Materials Required – Bromine water, glacial acetic acid, given organic sample, test tubes, distilled water etc.

 

Procedure – 1. Dissolve the given sample in a small quantity of glacial acetic acid in a clean and dry test tube. 

2. Now add bromine water in the test tube slowly. 

3. Observe the color change. 


Observation – Color changes from orange – red to colorless and a white precipitate is formed. 


Result – White precipitate confirms the presence of phenol in the given sample. 


Precautions – 1. Bromine water should be handled carefully as it may cause eyes and skin irritation. 

2. Phenolic samples should be handled carefully. 


This ends our coverage on the Test for Phenolic Group. We hope you enjoyed learning and were able to grasp the concepts. We hope after reading this article you will be able to perform the experiments related to the topic easily. If you are looking for solutions to NCERT Textbook problems based on this topic, then log on to Vedantu website or download Vedantu Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.