Question

What are the tests for phenols and the chemical reactions related to it?
• $\bold{Do\;not\;spam\;or\;copy.}$
• $\bold{Need\;quality\;answer.}$​

Answer 1

Phenols are compounds with the -OH group directly linked to aromatic nucleus. Pure phenols are colorless solids or liquids, but turn reddish on atmospheric oxidation. They show strong IR absorption band in the range $\sf{3580-3650\:\: cm^{-1}}$ and the C-O stretching bands are observed in $\sf{1200\:\: cm^{-1}}$ region. Phenols are mildly acidic which on reaction with alkali metal hydroxides, form phenoxides. These phenoxides are decomposed by acids, to give back free phenols. Acid used here can be mineral acid, carboxylic acids and also carbonic acid. So, phenols are less acidic than these acids. $\textsf{(Refer to Attachment - 1) }$ Phenols are soluble in organic solvents but insoluble in water, but their salts are soluble in water but insoluble in organic solvents.

The acidic character of phenols, greater acidity of carboxylic acids compared to them and solubility of phenoxides in water are all of analytical importance and these ideas are used in detection of phenolic -OH group in compounds and also for its separation and purification.

$\underline{\underline{\textbf{\textsf{The tests for phenols are as follows,}}}}$

• Reaction with Ferric chloride : Phenols give characteristic colors like green, blue, violet or red on reaction with FeCl₃.This is due to the formation of complexes. Catechol gives green color and resorcinol gives violet color with FeCl₃.This coloration is a diagnostic test for phenols. The exact structure of the complex forming is not certain.
• Lieberman Nitro reaction : Phenol on reaction with sodium nitrite and concentrated H₂SO₄ forms a deep blue or green color which on careful dilution with water turns red. Treating this with NaOH, brings back the green or blue color. The series of reactions occurring is as shown in the reaction.
• Phthalein reaction  : Phenols when heated with phthalic anhydride in the presence of anhydrous zinc chloride, phthalein with characteristic color depending on the $\sf{P^H}$ of the medium are formed. This is another diagnostic test.  Similarly, resorcinol and phthalic anhydride gives fluorescein, which dissolves in dilute alkali to give green fluorescence. At the same time, catechol condenses with phthalic anhydride in presence of acid to give alizarin, which is used as a dye.

Answer 2

Phenols are compounds with the -OH group directly linked to aromatic nucleus. Pure phenols are colorless solids or liquids, but turn reddish on atmospheric oxidation. They show strong IR absorption band in the range  and the C-O stretching bands are observed in  region. Phenols are mildly acidic which on reaction with alkali metal hydroxides, form phenoxides. These phenoxides are decomposed by acids, to give back free phenols. Acid used here can be mineral acid, carboxylic acids and also carbonic acid. So, phenols are less acidic than these acids.  Phenols are soluble in organic solvents but insoluble in water, but their salts are soluble in water but insoluble in organic solvents.

The acidic character of phenols, greater acidity of carboxylic acids compared to them and solubility of phenoxides in water are all of analytical importance and these ideas are used in detection of phenolic -OH group in compounds and also for its separation and purification.