Preparation of Phenols

Phenols are a class of organic compounds characterized by the attachment of the hydroxyl group (-OH) to the aromatic benzene ring. In other words, they provide the backbone and core of organic chemistry and numerous industries. These compounds are way more than just a simple class of chemicals; they play a very vital role in various industries, from pharmaceuticals and plastics to agriculture. Phenols are in antiseptics and disinfectants every day; they are present in dyes. The ability to turn them into almost everything, from medical treatments to industrial materials, is among those very special chemical properties that make them special. Several different methods have been worked out for phenol preparation; all of them have their advantages and applications. Controlled paths of phenol synthesis for research and small-scale production remain as laboratory techniques of hydroxylation of aromatic compounds. In sharp contrast, on the other hand, are industrial techniques; for instance, operation at the Cumene process scale speaks of size and efficiency if products are to be commercialized. This paper shall review these various methods of preparation in some detail, first of all looking at the laboratory-scale techniques for the synthesis of phenol from aromatic compounds, then moving on to the industrial production of phenol from cumene.

PREPARATION OF PHENOL—HYDROXYLATION OF AROMATIC COMPOUNDS (I)

The preparation of phenol in most cases involves the hydroxylation of an aromatic compound, usually benzene. Essentially, what this implies is that a hydroxyl (-OH) group should be introduced into the ring structure of the compound; however, this only apparently materializes under a number of chemical reactions. Laboratory conditions usually employ the Friedel-Crafts technique of hydroxylation. The reaction describes the oxidation of the benzene molecule to an oxidizing agent like nitric acid, HNO₃, in the presence of a catalyst like sulfuric acid, H₂SO₄. This converts the benzene into phenol by the insertion of a hydroxyl group in it.

The other is the hydrolysis of aryl halides, where haloaromatic compounds are treated with a strong base like sodium hydroxide to yield phenol by replacing the halogen with a hydroxyl group. This process is known as the Reimer-Tiemann reaction, and this reaction comes quite in handy to synthesize phenolic compounds from aryl halides.

Phenol is manufactured from the hydrocarbon, cumene. Cumene (isopropylbenzene) is oxidized in the presence of air to cumene hydroperoxide. It is converted to phenol and acetone by treating it with dilute acid. Acetone, a by-product of this reaction, is also obtained in large quantities by this method. The reaction occurs as follows:

Preparation of Phenol II: Industrial Synthesis from Cumene

The main industrial method of production of phenol is scaled for large production and is called the Cumene process. There are two major steps: alkylation of benzene and subsequent oxidation, followed by cumene cleavage.

Step 1: Alkylation of Benzene: In the presence of an acid catalyst, usually aluminum chloride, AlCl₃, benzene reacts with propene. The product is cumene, isopropylbenzene. Although this is rather a key stage in phenol production, it is only a pretreatment stage.

Step 2: Oxidation and Cleavage: First, cumene gets oxidized to cumene hydroperoxide; after that, it will further cleave in the presence of an acid catalyst. Then, the final products for this cleavage will include phenol and acetone. This is widely applied since this process is so effective in producing large quantities of phenol.

From Haloarenes
Chlorobenzene is fused with NaOH at 623K and 320 atmospheric pressure. Phenol is obtained by acidification of sodium phenoxide so produced. The reaction occurs as follows.

From Benzene sulphonic acid
Benzene is sulfonated with oleum and benzene sulphonic acid so formed is converted to sodium phenoxide on heating with molten sodium hydroxide. Acidification of the sodium salt gives phenol. The reaction occurs as follows:

From Diazonium salts
A diazonium salt is formed by treating an aromatic primary amine with nitrous acid (NaNO₂ + HCl) at 273-278 K. Diazonium salts are hydrolysed to phenols by warming with water or by treating with dilute acids. The reaction occurs as follows.

Relevance and Applications

Uses of phenols differ from one industry to another due to the range of characteristics they exhibit. They are employed in pharmaceutical industries for the manufacture of antiseptics and disinfectants and also as precursor products in the synthesis of medicines. For example, phenol is used as a precursor in the synthesis of aspirin and acetaminophen. The chemical industry uses phenols in the manufacture of plastics, resins, and dyes. They are also used in making explosives as well as in polymer additives so that they may act as stabilizers.

One of the key reasons why phenol preparation is important academically is for the understanding of principles behind the reactions and mechanisms of Organic Chemistry, particularly in respect to aromatic compounds. Familiarity with the above methods is of basic importance for any student or investigator dealing with phenolic compounds and their derivatives.

Some methods of phenol preparation by the Cumene process represent well the strength of organic chemistry in efficiently making compounds of value. In doing so, such methods show not only chemical transformations but also the extent to which production has to be scaled up to meet industrial demand.

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Some Solved Examples

Example 1
Question:

P&Q respectively are

1)

2)

3) (correct)

4)

Solution:

Therefore, option (3) is correct.

Example 2
Question:

The main products formed during a reaction of 1-methoxy naphthalene with hydroiodic acid are:

1)

and CH₃OH

2)

and CH₃I

3) (correct)

and CH₃I

4)

and CH₃OH

Solution:

The reaction will be -

Therefore, the Correct option is (3)

Example 3
Question:

Toluene is catalytic oxidizing by air in the presence of cupric salt and gave A. What is A?

1)Phenol

2)Cumene

3)sodium phenoxide

4)None

Solution:

As we have learned,

Therefore, option (1) is correct.

Summary

Methods of phenol preparation vary from laboratory direct to industrial techniques. Laboratory methods, such as Friedel-Crafts hydroxylation and the hydrolysis of aryl halides, provide routes that offer a controlled way of producing phenols for research work and small applications. The cumene process illustrates this aspect of the practical application of organic chemistry in the effective manufacturing of phenol at an industrial scale. Hence, their preparation methods are of significance not only for academics but also for industrial applications. It substantiates the role of phenol in various industries and its role in discovering the processes and products of chemicals, along with their perfection.