Phenol is an antiseptic and disinfectant. It is active against a wide range of micro-organisms including some fungi and viruses but is only slowly effective against spores. Phenol has been used to disinfect skin and to relieve itching. Pure phenol is used in certain medical procedures and as an ingredient in numerous treatments and laboratory applications. There are various ways of preparing Phenol. In this article, we will discuss the various phenolic preparations and their uses in our daily lives.

What is Phenol?

Phenol is an aromatic organic compound with the chemical formula \({{\rm{C}}_6}{{\rm{H}}_5}{\rm{OH}}\) in which the \( – {\rm{OH}}\) group or the hydroxyl group is attached to an aromatic benzene ring. Phenol is a mildly acidic, toxic, white crystalline solid obtained as a byproduct in coal tar distillation.

It is highly hygroscopic and volatile. It has a sickly sweet smell and a sharp burning taste. On a large scale, Phenol is obtained from petroleum-derived feedstocks. It is the precursor to the production of many essential commodities such as plastics and antiseptics.

Preparation of Phenol from Cumene

Cumene, isopropyl benzene, is an organic compound obtained by Friedel-Crafts alkylation of benzene with propylene.

Air oxidation of Cumene leads to the production of Cumene hydroperoxide. When an alkaline solution of Cumene in sodium carbonate is oxidised, passing air in the presence of cobalt naphthalate as a catalyst, cumene hydroperoxide is formed. On treating cumene hydroperoxide with dilute acid, Phenol along with acetone is obtained. As acetone is a byproduct of this process, phenols obtained by this method need purification.

Phenol Preparation Mechanism

The formation of Phenol from Cumene takes place in two stages-

1. Formation of Cumene Hydroperoxide – Chain Initiator Reaction

Step 1: The formation of the cumene hydroperoxide proceeds by a radical chain reaction. A radical initiator abstracts a hydrogen‐radical from the cumene molecule, creating a tertiary radical. It is the initial step of the reaction.

Step 2: Chain Propagation

The tertiary radical is attracted to an oxygen molecule to produce a hydroperoxide radical.

In the final step, the hydroperoxide radical abstracts a hydrogen-radical from a second molecule of Cumene to form cumene hydroperoxide and a new tertiary radical.

2. Degradation of Cumene Hydroperoxide

The degradation of the cumene hydroperoxide proceeds via a carbocation mechanism.

Step 1: In the first step, a pair of electrons on the oxygen of the hydroperoxide’s “hydroxyl group” is attracted to a proton of the \({{\rm{H}}_3}{{\rm{O}}^ + }\) molecule, forming an oxonium ion.

Step 2: The oxonium ion formed in step 1 is stabilised by the loss of a water molecule, producing a new oxonium ion.

Step 3: A tertiary carbocation is formed when a phenide ion shifts to the positively charged oxygen atom. This shift stabilises the positively charged oxygen atom. (A phenide ion is a phenyl group with an electron bonding pair available to form a new bond to the ring.)

Step 4: The carbocation formed in step 3 is stabilised by an acid‐base reaction with a water molecule, leading to the formation of an oxonium ion.

Step 5: Loss of proton takes place, which stabilises the oxonium ion.

Step 6: A proton is picked up by the ether oxygen in an acid‐base reaction, yielding a new oxonium ion.

Step 7: The positively charged ether oxygen pulls the electrons in the oxygen‐carbon bond toward itself by delocalised the charge over both atoms.

The partial positive charge on the carbon attracts the nonbonding electron pair from the oxygen of the \({\rm{OH}}\) group.

It allows the electrons in the original oxygen‐carbon bond to be released back to the more electronegative oxygen atom—the electron shift results in the formation of Phenol.

Step 8: A proton is lost from the protonated acetone molecule, leading to the formation of acetone.

Preparation of Phenol from Diazonium Salt

The diazonium ion is present in the benzene diazonium chloride solution. The diazonium ion reacts with the water in the solution to form Phenol. It is obtained either in solution or as a black oily liquid (depending on how much phenol is formed), evolving Nitrogen gas. The diazonium ion formed immediately reacts with the water in the solution to give Phenol.

Preparation of Phenol from Benzene Sulphonic Acid

Benzene sulphonic acid can be obtained from benzene by reacting it with Oleum. The acid is initially treated with an aqueous solution of \({\rm{NaOH}}\) to form sodium benzene sulphonate. It is then mixed with solid \({\rm{NaOH}}\) and fused at \(573\;{\rm{K}}\) to form a sodium phenoxide ion. The resulting product, sodium phenoxide ion, is then acidified to yield Phenol.

Preparation of Phenol from Grignard Reagent(RMgX)

Haloarenes, when treated with magnesium in dry ether, forms phenyl magnesium bromide. The product reacts with oxygen in the air to form phenyl oxymagnesium bromide. Phenyl oxymagnesium bromide on warming with water yields phenol.

Preparation of Phenol from Benzene (Raschig Process)

The Raschig–Hooker process is a two-step chemical process for the production of Phenol.

Step 1: Production of Chlorobenzene from benzene.

In the first step, benzene is heated with \({\rm{HCl}}\) in the air at \({400^ \circ }{\rm{C}}\). Copper or iron chloride is used as a catalyst. Chlorobenzene is produced with the release of a water molecule.

Step 2: Formation of Phenol from Chlorobenzene.

In the second step, the resulting chlorobenzene is introduced to steam at \({450^ \circ }{\rm{C}}\) over a silicon catalyst that hydrolyses the chlorobenzene, giving Phenol and hydrogen chloride. Due to the two-step nature, the Raschig–Hooker process is used to produce either chlorobenzene or phenol.

Preparation of Phenol from Aniline

Diazonium salts are obtained on treating primary aromatic amines with nitrous \(\left( {{\rm{NaN}}{{\rm{O}}_2} + {\rm{HCl}}} \right)\) acid at \(273 – 278\;{\rm{K}}\).

On warming with water, these diazonium salts finally hydrolyse to phenols. Phenols can also be obtained from diazonium salts by treating them with dilute acids.

Phenol Preparation by Decarboxylation of Sodium Salt of Salicylic Acid

Carboxylic acids, when heated with soda lime, undergoes decarboxylation. Salicylic acid is a carboxylic acid that reacting with soda lime, undergoes decarboxylation to yield Phenol. The mixture of \({\rm{CaO}}\) and \({\rm{NaOH}}\) is known as soda lime. The byproducts obtained after decarboxylation of salicylic acid is Phenol, sodium carbonate and water.

Preparation of Phenol from Haloarenes

Chlorobenzene is an example of a haloarene formed by the monosubstitution of chlorine to the benzene ring. Chlorobenzene fused with sodium hydroxide at \(623\;{\rm{K}}\) and \(300{\rm{ atm }}\) produces sodium phenoxide ion. The sodium phenoxide ion on acidification gives Phenol.

Preparation of Phenol by Dow’s Process

Dow’s process is a chemical process to manufacture Phenol. It is carried out by reacting chlorobenzene with molten sodium hydroxide at extreme conditions of temperature. Chlorobenzene fuses with sodium hydroxide at \({350^ \circ }{\rm{C}}\), producing sodium phenoxide ion. Sodium phenoxide ion upon acidification gives Phenol. The reaction occurs through an elimination addition reaction via benzyne intermediate.

Preparation of Phenol from Coal Tar

Phenol is commercially prepared from the middle oil fraction \({\rm{ (443 – 503K) }}\) of coal tar distillate. The middle fraction of coal tar distillate mainly contains Phenol, naphthalene and bases like pyridine. As naphthalene has a low melting point, it is removed by chilling the fraction. The remaining oil is now treated with acids such as \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\) to remove basic impurities like pyridine. Phenol is then extracted with dilute caustic soda.

Preparation of Phenol from Benzene

Benzene upon hydroxylation with hydrogen peroxide\(\left( {{{\rm{H}}_2}{{\rm{O}}_2}} \right)\) in the presence of \({\rm{HS}}{{\rm{O}}_3}\;{\rm{F}}\) gives Phenol.

Benzene on oxidation in the presence of \({{\rm{V}}_2}{{\rm{O}}_5}\) catalyst at \({315^ \circ }{\rm{C}}\) yields Phenol.

Preparation of Phenolphthalein Indicator

Phenolphthalein is often used as an indicator in acid–base titrations. For this application, it turns colourless in acidic solutions and pink in basic solutions. It is a weak acid, which can lose \({{\rm{H}}^ + }\) ions in the solution. The synthesis of phenolphthalein is carried out by electrophilic aromatic substitution of phthalic anhydride. Two equivalents of Phenol are treated with phthalic anhydride in the presence of concentrated methanesulfonic acid at \({90^ \circ }{\rm{C}}\) to yield the product phenolphthalein.

FAQs on Preparation of Phenol

Q.1. Why does Phenol turn pink after long-standing?
Ans: Phenol turns pink in colour on exposure to air due to its slow oxidation to quinone. Quinone combines with the Phenol to give an additional product known as phenoquinone, which is pink in colour.

Q.2. What is the industrial preparation of Phenol?
Ans: The Hock process (cumene-phenol process, cumene process) is an industrial process for developing Phenol. It is obtained by the air oxidation of Cumene. Acetone is a major byproduct obtained in this method.

Q.3. How will you prepare phenol from benzene?
Ans: Phenol is prepared from benzene through the Raschig–Hooker process. Benzene is heated with \({\rm{HCl}}\) in the air at \({400^ \circ }{\rm{C}}\) in the presence of Copper or iron chloride. Chlorobenzene so produced is introduced to steam at \({450^ \circ }{\rm{C}}\) over a silicon catalyst that hydrolyses the chlorobenzene, giving Phenol and hydrogen chloride. Due to the two-step nature, the Raschig–Hooker process is used to produce either chlorobenzene or phenol.

Q.4. How will you prepare phenol from Grignard reagent?
Ans: Haloarenes, when treated with magnesium in dry ether, forms phenyl magnesium bromide. The product reacts with oxygen in the air to form phenyl oxymagnesium bromide. Phenyl oxymagnesium bromide on warming with water yields phenol.

Q.5. How do you separate Phenol and carboxylic acid?
Ans: Phenol and carboxylic acid can be separated by sodium bicarbonate. Carboxylic acid will give a brisk effervescence of carbon dioxide on treatment with sodium bicarbonate. In contrast, Phenol does not show any such change.

Q.6. How is Phenol obtained from the cumene process?
Ans: Air oxidation of Cumene leads to the production of Cumene hydroperoxide. When an alkaline solution of Cumene in sodium carbonate is oxidised, passing air in the presence of cobalt naphthalate as a catalyst, cumene hydroperoxide is formed. On treating cumene hydroperoxide with dilute acid, Phenol along with acetone is obtained.

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