A class of organic compounds that 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 the 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 help in the formation of drugs, disinfectants etc. Phenols are acidic 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 the presence of phenol functional groups in an aromatic compound.
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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 colour if the solution is acidic or basic. Acidic solutions turn blue litmus paper into red and basic solutions turn red litmus paper into blue. Phenols are acidic, so they show the litmus test.
Here you need to keep in mind that carboxylic acids also give this test. So, to distinguish between the 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, the 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- The colour of litmus paper changes from blue to red.
Result- The given solution may contain phenol.
Precautions- Phenol is poisonous and corrosive 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 coloured complex with neutral ferric chloride solution. Phenol group reacts with the ferric ion of ferric chloride and forms a coloured complex such as \[C_{6}H_{5}OH\] (simplest phenol) reacts with ferric chloride and forms a violet coloured complex \[[Fe(C_{6}H_{5}O)_{6}]^{3-}\]. The reaction is given below –
\[ 6C_{6}H_{5}OH + FeCl_{3} \rightarrow [Fe(C_{6}H_{5}O)^{3-} + 3HCl + 3H^{+} \]
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 colour change.
Observation- If the colour 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 colour. β - naphthol gives green colour and α - naphthol gives pink color. Formic acid and acetic acid give a deep red colour.
Reaction- Reaction involved in the experiment can be written as follows –
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 forms phenolphthalein. Phenolphthalein gives pink-coloured compounds on reaction with a limited amount of sodium hydroxide while more than sodium hydroxide it gives a colourless compound.
Materials Required- test tube, given organic compound, phthalic anhydride, conc. Sulfuric acid, sodium hydroxide, distilled water, burner, oil bath, dropper et
Procedure- Take a clean dry test tube. Add 100 mg of the given organic compound to this test tube.
Now add 100 mg of phthalic anhydride to the same test tube.
Add 2 – 3 drops of concentrated sulfuric acid in the same test tube in which sample and phthalic anhydride have 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 phenyl 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 the disappearance of colour and we get a colourless compound.
Color Produced by Various Phenol Compounds in Phthalein Dye Test is Given Below –
Reactions Involved- Reactions involved in the experiment can be written as follows –
Result- Given organic compound contains phenol group.
Precautions
Phenol is poisonous and corrosive so must be handled carefully.
Conc. Sulfuric acid should be handled very carefully.
Some phenols are strong acids that 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 groups in a given sample.
Theory- When phenol is dissolved in conc. Sulfuric acid and sodium nitrite are added to 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- nitrophenol. This, p- nitrophenol over phenol and on dilution, gives indophenol complex which is red in color. When the indophenol complex reacts with strong alkali (such as NaOH) it forms a blue-colored indophenol anion. Reactions involved can be written as follows –
\[2NaNO_{3} \, \, \, \, + \, \, \, \, H_{2}SO_{4} \, \, \, \, \rightarrow \, \, \, \, 2HNO_{2} + \, \, \, \, Na_{2}SO_{4}\]
Sodium nitrite Sulfuric acid Nitrous acid Sodium sulfate
Materials Required- test tube, test tube holder, given organic compound, conc. Sulfuric acid, sodium hydroxide, sodium nitrite, distilled water, burner, etc.
Procedure-
Take a clean dry test tube.
Place a small amount of sodium nitrite crystals in it.
Now add 100 mg of the given phenolic compound to it and heat the test tube for about 30-35 seconds gently.
Allow the test tube to cool.
Now, add 1 ml of the conc. Sulfuric acid and shake the test tube gently and carefully to mix the contents of it.
Observe the colour change and dilute the solution with water. (If a phenol group is present in the given compound then on dilution, the red-colored compound is formed.)
Now, add sodium hydroxide in the red solution and observe the change in color.
Observation- When we dilute the solution with water, the solution turns red and by adding NaOH to it, the solution turns blue (deep blue) in colour.
Result- Deep blue colour confirms the presence of phenol in the given sample.
Precautions-
Concentrated sulfuric acid should be handled carefully.
Phenols should be handled carefully.
Color change should be noticed vigilantly.
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 groups 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 concerning -OH group are replaced by bromine atoms. Thus, by reaction of phenol with bromine white precipitate of tribromo phenol is formed. The reaction is given below –
Materials Required- Bromine water, glacial acetic acid, given organic sample, test tubes, distilled water, etc.
Procedure
Dissolve the given sample in a small quantity of glacial acetic acid in a clean and dry test tube.
Now add bromine water to the test tube slowly.
Observe the colour change.
Observation- Color changes from orange-red to colourless and a white precipitate is formed.
Result- White precipitate confirms the presence of phenol in the given sample.
Precautions- Bromine water should be handled carefully as it may cause eyes and skin irritation.
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 the 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.
FAQs on Test for Phenolic Group
1. What are phenolic compounds?
Phenolic compounds are a class of small molecules with at least one phenol unit in their structure. Phenolic chemicals are separated into subgroups based on their chemical structures, such as phenolic acids, flavonoids, tannins, coumarins, lignans, quinones, stilbenes, and curcuminoids.
In the plant kingdom, on the other hand, phenolic chemicals are generally found in either a soluble or a bound state. Plants' internal endoplasmic reticulum synthesizes and stores the majority of soluble phenolic chemicals in vacuoles. Visit our website to learn more.
2. What are the other reactions involving phenols?
Esterification occurs with phenols. Phenol esters are active esters because they are susceptible to hydrolysis. Phenols are oxidation-reactive compounds. Cleavage of 1,2-dihydroxybenzene to the monomethyl ester of 2,4 hexadecenoic acid with oxygen and copper chloride in pyridine is an example of oxidative cleavage. The Teuber reaction and ozone are examples of oxidative de-aromatization to quinones. In the reaction shown below, singlet oxygen created from oxone/sodium carbonate combines with 3,4,5-trimethylphenol to form a para-peroxyquinole in an acetonitrile/water mixture. With sodium thiosulfate, the hydroperoxide is converted to quinoa.
In the Elbs persulfate oxidation, phenols are converted to hydroquinones. In the Bucherer carbazole synthesis, naphthols and hydrazines react with sodium bisulfite.
3. What are polyphenols?
Phenolic chemicals can be found in a wide variety of plants. They are secondary metabolites with antibacterial, antimutagenic, anticancer, antitumor, and anti-inflammatory properties as a result of their bioactivity. They can also act as ion or hydrogen donors, scavenge radicals, chelate metals, quench oxygen atoms, and scavenge radicals.
Some phenolic chemicals have strong antioxidant action when used as pure compounds in foods, whereas others rely on synergism to provide protection. This is both a disadvantage and an opportunity for the sector.
4. What is the effect of HPP on Protein - Phenolic compound interactions?
Phenolic chemicals are healthy components found primarily in fruits and vegetables that are known for their antioxidant properties. The effect of high-pressure treatment (200–400 MPa, 20°C, for 0–10 min) on apple juice was investigated by Baron, Dénes, and Durier (2006), who discovered a possible connection between phenolic component oxidation and pectin methylesterase (PME) activity. PME activity was shown to be reduced following pressurization at pressures below 300 MPa but increased at 400 MPa.
The greater phenolic component content (such as hydroxycinnamic acid, procyanidins, and catechins) after juice pressurization at 200–300 MPa for 10 minutes inhibited PME, lowering its activity in this pressure range compared to the control.
5. Describe the nutritional aspect of phenols.
Secondary metabolites in coffee beans, cereals, fruits, olives, vegetables, and tea leaves are known as phenolic compounds. Caffeic, chlorogenic, and gallic acid were not stable at high pH in a study on the stability of structurally distinct phenolic compounds in buffers in the pH range 3–11. Epigallocatechin, catechin, rutin, ferulic acid, and cinnamic acid, on the other hand, were able to resist pH-induced destruction.
The findings show that if a phenolic molecule is found to be unstable during food processing, it may not be useful as an antioxidant, anticarcinogen, or antibiotic in foods exposed to heat or high pH. The Vedantu app and website provide free study materials.
