Method of Preparation of Phenols: The molecular formula of phenol is \({{\rm{C}}_6}{{\rm{H}}_5}{\rm{OH}}\). A phenyl group \(\left( { – {{\rm{C}}_6}{{\rm{H}}_5}} \right)\) is connected to a hydroxy group \(( – {\rm{OH}})\) in this molecule. It is an aromatic organic compound and a flammable white crystalline substance. It’s mildly acidic and should be handled with caution because it can cause chemical burns. the substance should be handled with care and the preparation process should be understood properly before starting laboratory experiments.

Phenols were originally discovered after the distillation of coal tar and after the molecular structure was determined, commercial large scale production of phenols began. It forms the basis for many useful chemicals and materials. It has become a crucial industrial commodity because of the large number of applications. This article covers all the details about phenols, their types, preparation methods and applications. Continue reading to know more.

Methods of Preparation of Phenols

The methods of preparation of phenols are explained below:

From Benzene Sulphonic Acid

At \(573 – 623\;{\rm{K}}\), sodium phenoxide is produced by fusing the sodium salt of benzene sulphonic acid with sodium hydroxide. Sodium phenoxide is converted to phenol when it is treated with dilute acid or carbon dioxide. This is one of the laboratory methods for the preparation of phenol.

From Diazonium Salts

This is one of the most convenient methods for the preparation of phenols. When benzene diazonium chloride solution is warmed with water or dilute acids, phenol is formed with the evolution of nitrogen gas.

From Grignard Reagent

In the presence of dry ether, chlorobenzene or bromobenzene is first transformed into phenyl magnesium halide. The Grignard reagent produces phenol when it reacts with oxygen and is then hydrolysed by a mineral acid.

By Decarboxylation of Salicylic Acid

Decarboxylation of the sodium salt of salicylic acid with soda lime is followed by acidification with dilute hydrochloric acid to produce phenol.

From Dakin Reaction

This reaction occurs when \({\rm{o – }}\) and \({\rm{p – }}\) hydroxy aldehydes or ketones (or \({\rm{o – }}\) and \({\rm{p – }}\) hydroxy amino aldehydes) combine with alkaline hydrogen peroxide, followed by hydrolysis to produce \({\rm{o – }}\) and \({\rm{p – }}\) hydroxyphenol (or \({\rm{o – }}\) and \({\rm{p – }}\) aminophenol)

Industrial Preparation Methods of Preparation of Phenols

The industrial preparation methods of preparation of phenols are explained below:

1. From Coal Tar

Phenol is extracted from the middle oil fraction for commercial purposes \((443 – 503\;{\rm{K}})\). Phenol, cresol, and naphthalene are the main ingredients. When the naphthalene separates off, the fraction is cooled. Sulphuric acid is used to eliminate any leftover contaminants from the remaining oil. After that, dilute caustic soda is used to extract the phenol. Sulphuric acid or carbon dioxide are used to separate the aqueous layer and produce phenol. Fractional distillation is used to separate the oily layer. The phenol is the fraction obtained at \((454 – 455\;{\rm{K}})\).

2. Raschig’s Process

The reaction of benzene, hydrogen chloride and air at \(523\;{\rm{K}}\) in the presence of catalyst cupric chloride and ferric chloride produce chlorobenzene. At \(698\;{\rm{K}}\), it is hydrolysed by superheated steam, yielding phenol and \({\rm{HCl}}\). This is one of the most recent phenol production methods. More benzene can be converted to chlorobenzene using the \({\rm{HCl}}\) produced.

3. Dow Process

The chlorobenzene is hydrolysed in an alkaline environment. Chlorobenzene is heated with a \(10\% \) solution of caustic soda or sodium carbonate at \(62\;{\rm{K}}\) under high pressure to produce large quantities of phenol.

4. From Cumene

Phenol is prepared commercially from cumene which is also known as isopropyl benzene.  Cumene is prepared by Friedel – Craft alkylation of benzene with propane in the presence of phosphoric acid. This on aerial oxidation forms cumene hydroperoxide, which upon subsequent hydrolysis with an aqueous acid gives phenol and propanone

The cumene method, in comparison to most others, uses very mild conditions and low-cost raw ingredients. Both phenol and the acetone by-product must be in demand for the process to be profitable. In \(2010\), global demand for acetone was at \(6.7\) million tonnes, with acetone produced by the cumene process satisfying \(83\%\) of that need.

The mechanisms of cumene hydroperoxide production and degradation deserve further investigation. A radical chain reaction results in the creation of hydroperoxide. A radical initiator removes a hydrogen-radical from the molecule, resulting in the formation of a tertiary radical. The reaction begins with the formation of the tertiary radical.

The radical is then attracted to an oxygen molecule in the next phase. The cumene peroxide radical is formed as a result of this attraction.

Finally, a hydrogen-radical from the second molecule of cumene is abstracted by the cumene peroxide radical, resulting in cumene hydroperoxide and a new tertiary radical.

The cumene hydroperoxide is degraded through a carbocation process. In the first stage, an oxonium ion is formed when a pair of electrons on the oxygen of the hydroperoxide’s “hydroxyl group” are attracted to a proton of the \({{\rm{H}}_3}{{\rm{O}}^ + }\) molecule.

When the positively charged oxygen exits in the form of a water molecule, the oxonium ion stabilises. The loss of a water molecule results in the formation of a new oxonium ion.

The positively charged oxygen is stabilised by a phenoxide ion shift to the oxygen atom (creating a tertiary carbocation). (A phenoxide ion is a phenyl group that has an accessible electron bonding pair to create a new link to the ring.)

An acid-base reaction with a water molecule stabilises the carbocation, resulting in the formation of an oxonium ion.

The oxonium ion is stabilised by the loss of a proton. In an acid-base process, a proton is taken up by the ether oxygen, resulting in a new oxonium ion.

The positively charged oxygen of the ether group attracts the electrons in the oxygen-carbon bond, causing the charge to be delocalised across both atoms. The partial positive charge on the carbon pulls the nonbonding electron pair from the \({\rm{OH}}\) group’s oxygen, allowing the electrons in the original oxygen-carbon bond to be released to the more electronegative oxygen atom.

Finally, the protonated acetone molecule loses a proton, resulting in the production of acetone.

Summary

The molecular formula of phenol is \({{\rm{C}}_6}{{\rm{H}}_5}{\rm{OH}}\). It is an aromatic organic compound. Phenol was originally recovered from coal tar, but it is now produced on a huge scale using petroleum-derived feedstocks. There are various methods for the preparation of phenol. Phenol can be prepared from Benzene Sulphonic Acid, Diazonium Salts, Grignard Reagent, Decarboxylation of Salicylic Acid, Dakin Reaction, Raschig’s Process, Dow Process, Oxidation of Benzene, and cumene.

FAQs on Methods of Preparation of Phenol

Q.1. How can phenol be prepared from benzene sulphonic acid?
Ans: Phenol can be prepared from benzene sulphonic acid. At \({\rm{573 – 623K}}\), sodium phenoxide is produced by fusing the sodium salt of benzene sulphonic acid with sodium hydroxide. Sodium phenoxide is converted to phenol when it is treated with dilute acid or carbon dioxide. This is one of the laboratory methods for the preparation of phenol.

Q.2. Which method is used for the commercial preparation of phenols?
Ans: Phenol is obtained commercially from coal tar. Phenol is extracted from the middle oil fraction \((443 – 503\;{\rm{K}})\) for commercial purposes. Phenol, cresol, and naphthalene are the main ingredients. When the naphthalene separates off, the fraction is cooled. Sulphuric acid is used to eliminate any leftover contaminants from the remaining oil. After that, dilute caustic soda is used to extract the phenol. Sulphuric acid or carbon dioxide are used to separate the aqueous layer and produce phenol. Fractional distillation is used to separate the oily layer. The phenol is the fraction obtained at \({\rm{454 – 455K}}\). Phenol is also prepared commercially from cumene.

Q.3. How can phenol be prepared from benzene?
Ans: Phenol can be prepared from the benzene with the help of Raschig’s Process. In this process, the reaction of benzene, hydrogen chloride, and air at \(523\;{\rm{K}}\) in the presence of catalyst cupric chloride and ferric chloride produce chlorobenzene. At \(698\;{\rm{K}}\), it is hydrolysed by superheated steam, yielding phenol and \({\rm{HCl}}\). This is one of the most recent phenol production methods. More benzene can be converted to chlorobenzene using the \({\rm{HCl}}\) produced.

Q.4. How can phenol be prepared from cumene?
Ans: Phenol is prepared from cumene which is also known as isopropyl benzene.  Cumene is prepared by Friedel – Craft alkylation of benzene with propane in the presence of phosphoric acid. This on aerial oxidation forms cumene hydroperoxide, which upon subsequent hydrolysis with an aqueous acid gives phenol and propanone

The cumene method, in comparison to most others, uses very mild conditions and low-cost raw ingredients. Both phenol and the acetone by-product must be in demand for the process to be profitable. In \(2010\), global demand for acetone was at \(6.7\) million tonnes, with acetone produced by the cumene process satisfying \(83\%\) of that need.

Q.5. What is the Dow process for the preparation of phenol?
Ans: The chlorobenzene is hydrolysed in an alkaline environment. Chlorobenzene is heated with a \(10\%\) solution of caustic soda or sodium carbonate at \(623\;{\rm{K}}\) under high pressure to produce large quantities of phenol.

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