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Phenols are a class of organic compounds that has a hydroxyl group (-OH) directly attached to aromatic hydrocarbon rings. These are widely used in the medicinal field and food industry. Hence, they are widely synthesized at the industrial level. Phenols are produced by plants and microorganisms as well. There can be an aromatic hydrocarbon ring with one or more phenolic groups attached to it. It is important to identify phenol in an organic aromatic compound, in order to know its various characteristics which help in the formation of drugs and disinfectants. They are acidic and this property of phenols helps in tests for phenols. There are various methods of tests for phenolic groups in an aromatic compound.
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Keywords: Phenols, hydrocarbon rings, atoms, phthalic anhydride, litmus test, ferric chloride, libermann’s test, Organic aromatic compound, hydrocarbon
Litmus Test
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The litmus test can be performed in the following way:
Aim
Detection of a phenol functional group in a given sample.
Theory
The litmus paper changes colour if the solution is acidic or basic. An acidic solution turns blue litmus paper into red and a basic solution turns red litmus paper into blue. Since phenols are acidic, they show the litmus test.
Materials Required
Test tube, blue litmus paper, dropper, etc.
Procedure
Taking the given solution in a test tube and placing 1-2 drops of it on the blue litmus paper.
Observation
Colour of the litmus paper changes from blue to red.
Result
The above solution may contain phenol.
Precautions
The solution is poisonous and corrosive so must be handled carefully.
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Ferric Chloride Test
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Ferric Chloride Test can be used to test for phenolic group in the following way:
Aim
Detection of a phenol functional group in a given sample.
Theory
The test is built on the fact that phenols give a coloured complex with neutral ferric chloride solution.
Material Required
Test tube, ferric chloride solution, dropper etc.
Procedure
- In order to prepare neutral ferric chloride solution, we add diluted solution of NaOH to ferric chloride solution slowly until a permanent brown precipitate is produced.
- After that we filter it and remove the precipitate. This clear filtrate is used for the test of phenols.
- Then we take 3 ml of the given solution in a test tube and add freshly prepared neutral ferric chloride solution in it dropwise.
- Now, we note the colour change.
Observation
- In case the colour of the solution becomes blue, green, violet, or red, it would indicate the presence of a phenol group.
- While ortho, meta or para – cresol, resorcinol gives violet or blue colour X, β - naphthol gives green colour and α - naphthol gives pink colour.
- Formic acid and acetic acid give a deep red colour.
Result
It can be concluded that the given solution contains phenol.
Precautions
We should use freshly prepared, dilute, and neutral solution of ferric chloride.
Phthalein Dye Test
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Phenolic group can be also be tested through the phthalein dye test.
Aim
Detection of a phenol functional group in a given sample.
Theory
The solution gets condensed on heating with phthalic anhydride in the presence of conc. Pink-coloured compounds are formed by Phenolphthalein on reaction with a limited amount of sodium hydroxide while more than sodium hydroxide it gives a colourless compound.
Material Required
Given organic compound, test tube, phthalic anhydride, conc. sulfuric acid, sodium hydroxide, distilled water, burner etc.
Procedure
- Add 100 mg of the given organic compound to a dry test tube.
- Then add 100 mg of phthalic anhydride to the test tube.
- Now, add 2 to 3 drops of concentrated sulfuric acid to the same test tube where phthalic anhydride and sample have been added.
- For 2 minutes, heat the test tube in an oil bath.
- Then, cool the mixture.
- Then pour the mixture in the test tube in the beaker that contains 15ml dilute sodium hydroxide.
Observation
In case 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. But here we 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.
Result
The given organic compound contains phenol group.
Libermann’s Test
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Libermann’s test can be performed in the following way:
Aim
Detection of phenol functional groups in a given sample.
Theory
When the solution is dissolved in concentrated Sulfuric acid and sodium nitrite are added to it. After that in this mixture, sodium nitrite reacts with concentrated sulfuric acid and gives nitrous acid and sodium sulphate. Then, nitrous acid reacts with phenol and gives p- nitrophenol. And this, p- nitrophenol over phenol and on dilution, gives indophenol complex which is red in colour.
Observation
On dilution of the solution with water, the solution turns red and by adding NaOH to it, the solution turns blue (deep blue) in colour.
Result
A deep blue colour confirms the presence of phenol in the given sample.
2NaNO3 + H2SO4 → 2HNO2 + Na2SO4
Precautions
It must be handled carefully as they are strong acids that contain an electron-withdrawing group and get dissolved in sodium hydrogen carbonate solution as well.
Bromine Water Test
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Bromine water test helps to identify the phenolic group in a sample.
Aim
Detection of phenol functional groups in a given sample.
Theory
The test is based on the electrophilic substitution reaction of phenol with bromine. When the solution reacts with bromine water gives a poly-bromo derivative. Bromine atoms replace all hydrogen atoms present at ortho and para position in phenol concerning the -OH group. So, by reaction of phenol with bromine white precipitate of tribromo phenol is formed.
Material Required
Bromine water, glacial acetic acid, given organic sample, test tubes, distilled water, etc.
Procedure
- Dissolve the solution in glacial acetic acid in a clean and dry test tube.
- Then add bromine water to the test tube slowly.
- Now observe the colour change.
Observation
The colour changes from orange-red to colourless and a white precipitate is formed.
Result
A white precipitate confirms the presence of phenol in the given sample.
Precautions
We must handle Bromine water carefully as it may cause eyes and skin irritation.
Things to Remember
- Detection of phenol functional groups can be done by the following tests:
- Litmus test
- Ferric chloride test
- Phthalein dye test
- Libermann’s test
- Bromine water test
- When we prepare a neutral ferric chloride solution, we add a diluted solution of sodium hydroxide to the ferric chloride solution slowly until a permanent brown precipitate is produced.
- There can be an aromatic hydrocarbon ring with one or more phenolic groups attached to it.
- An acidic solution turns blue litmus paper into red and a basic solution turns red litmus paper into blue. Since phenols are acidic, they show the litmus test.
- Pink-coloured compounds are formed by Phenolphthalein in reaction with a limited amount of sodium hydroxide.
- Phenols are widely used in the medicinal field and food industry.
Also Read:
Sample Questions
Ques. What are phenols? (2 marks)
Ans. Phenols are a class of small molecules with at least one phenol unit in their structure. They are separated into subgroups based on their chemical structures. Like phenolic acids, flavonoids, tannins, coumarins, lignans, quinones, stilbenes, and curcuminoids.
Ques. What are polyphenols? (2 marks)
Ans. Polyphenols are secondary metabolites with antibacterial, antimutagenic, anticancer, antitumor, and anti-inflammatory properties as a result of their bioactivity. These can also act as ion or hydrogen donors, scavenge radicals, chelate metals, quench oxygen atoms, and scavenge radicals.
Ques. What is the nutritional aspect of phenols? (3 marks)
Ans. Phenolic compounds are secondary metabolites in coffee beans, cereals, fruits, olives, vegetables, and tea leaves. Components like 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. On the other hand Epigallocatechin, catechin, rutin, ferulic acid, and cinnamic acid, were able to resist pH-induced destruction
Ques. State the effect of HPP on Protein - Phenolic compound interactions. (3 marks)
Ans. They are healthy components found primarily in fruits and vegetables that are known for their antioxidant properties. Effect of high-pressure treatment on apple juice was investigated there was discovered a possible connection between phenolic component oxidation and pectin methyl esterase (PME) activity. Pectin methyl esterase activity was shown to be reduced following pressurization at pressures below 300 MPa but increased at 400 MPa.
Ques. How do you confirm the presence of phenol in a solution by Libermann’s test? (3 marks)
Ans. The given solution is dissolved in concentrated Sulfuric acid and sodium nitrite are added to it. After that in this mixture, sodium nitrite reacts with concentrated sulfuric acid and gives nitrous acid and sodium sulphate. On dilution of the solution with water, the solution turns red and by adding NaOH to it, the solution turns blue (deep blue) in colour. A deep blue colour confirms the presence of phenol in the given sample.
Ques. How do you confirm the presence of phenol in a solution using bromine water test? (3 marks)
Ans. Bromine atoms replace all hydrogen atoms present at ortho and para position in phenol concerning -OH group. So, by reaction of phenol with bromine white precipitate of tribromo phenol is formed. We dissolve the solution in glacial acetic acid in a clean and dry test tube. Then we add bromine water to the test tube slowly.
The colour changes from orange-red to colourless and a white precipitate is formed.
A white precipitate confirms the presence of phenol in the given sample.
Ques. How do you confirm the presence of phenol in a solution using the Phthalein dye test? (5 marks)
Ans. When phenol gets condensed on heating with phthalic anhydride in the presence of concentrated pink-coloured compounds are formed by phenolphthalein on reaction with a limited amount of sodium hydroxide while more than sodium hydroxide it gives a colourless compound. When 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. But here we 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.
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