What is a phenol?
An organic compound is aromatic in nature with the structural formula C6H5OH. It is a white crystalline rock that is volatile in nature. The molecule has a phenyl group (−C6H5) attached to a hydroxyl group (−OH). It is slightly acidic and needs careful handling due to its tendency for causing chemical burns.
Image: Structure of Phenol
Phenol was first mined from coal tar, but today is manufactured on a large scale (around 7 billion kg/year) from petroleum. It is an important industrial product as a pioneer to various materials and useful compounds. It is principally used to manufacture plastics and related materials. Phenol and its chemical products are very important for the production of Bakelite, polycarbonates, detergents, nylon, epoxies, herbicides such as phenoxy herbicides, and many pharmaceutical drugs.
Properties
Some properties of phenol are listed below.
Reactions of phenol
Phenol is extremely reactive to electrophilic aromatic substitution as the oxygen atom's pi electrons give electron density into the ring. By this overall approach, several groups can be attached to the ring, through halogenation, sulfonation, acylation, and other methods.
However, phenol's ring is so powerfully activated—second only to aniline-that chlorination or bromination of phenol will lead to replacement on all carbon atoms para and ortho to the hydroxyl group, not only on one carbon. It reacts with dilute nitric acid at room temperature to produce a mixture of 2-nitrophenol and 4-nitrophenol while with concentrated nitric acid, many nitro groups get replaced on the ring to produce 2, 4, 6-trinitrophenol which is also known as picric acid.
The aqueous mixture of phenol is weakly acidic and changes blue litmus somewhat to red. It is easily neutralized by sodium hydroxide giving sodium phenate but is weaker than carbonic acid, it cannot be neutralized by sodium bicarbonate or sodium carbonate to release carbon dioxide.
C6H5OH + NaOH → C6H5ONa + H2O
When a mixture of phenol and benzoyl chloride is shaken in the occurrence of dilute sodium hydroxide solution, phenyl benzoate is produced. This is a case of the Schotten-Baumann reaction:
C6H5OH + C6H5COCl → C6H5OCOC6H5 + HCl
Phenol is transformed to benzene when it is distilled with zinc dust, or when phenol vapour is passed over grains of zinc at 400°C:
C6H5OH + Zn → C6H6 + ZnO
In the presence of boron trifluoride (BF3) phenol is reacted with diazomethane and as a result, anisole is obtained as the main product and nitrogen gas as a byproduct.
C6H5OH + CH2N2 → C6H5OCH3 + N2
When phenol reacts with iron(III) chloride solution, a powerful violet-purple solution is produced.
Preparation of Phenols
Preparation of phenols from diazonium salts, benzene sulphonic acid, haloarenes, cumene. They are also known as carbolic acids. They are weak acids and mostly form phenoxide ions by dropping one positive hydrogen ion (H+) from the hydroxyl group. Phenol was mainly manufactured from coal tar. Nowadays, with developments in technologies, some new methods have come up for the making of phenols in laboratories. In laboratories, phenol is mainly created from benzene derivatives. Some of the approaches of preparation of phenols are explained below:
Preparation of Phenols From Haloarenes:
Chlorobenzene is an example of haloarenes which is made by mono replacement of the benzene ring. When chlorobenzene is reacted with sodium hydroxide at 623K and 320 atm sodium phenoxide is formed. Finally, sodium phenoxide on acidification makes phenols.
Image: An example of preparation of Phenols from haloarenes
Preparation of Phenols from Benzene Sulphonic Acid:
Benzenesulphonic acid can be acquired from benzene by reacting it with oleum. Benzenesulphonic acid, hence formed, is fused with molten sodium hydroxide at a very high temperature which leads to the development of sodium phenoxide. Lastly, sodium phenoxide on acidification gives phenols.
Image: Reaction of preparation of Phenols from Benzene Sulphonic Acid
Preparation of Phenols From Diazonium Salts:
When an aromatic primary amine is fused with nitrous in the presence of HCl(NaNO2 + HCl) acid at 273 – 278 K, diazonium salts are gained. These diazonium salts are extremely reactive in nature. Upon warming with water, these diazonium salts, to end hydrolyze to phenols. Phenols can also be acquired from diazonium salts by treating it with dilute acids.
Image: Reaction of preparation of Phenols from Diazonium Salts
Preparation of Phenols From Cumene:
Cumene is an organic compound acquired by Friedel-Crafts alkylation of benzene with propylene. On oxidation of cumene (isopropylbenzene) in the presence of air, cumene hydroperoxide is found. Upon further action of cumene hydroperoxide with dilute acid, phenols are produced. Acetone is also made as one of the by-products of this reaction in large quantities. Therefore, phenols prepared by these techniques need purifications.
Image: Reaction of Preparation of Phenols from cumene
Synthesis of Phenols by the pyrolysis of the sodium salt of benzene sulphonic acid
You can produce phenols in large amounts by the pyrolysis of the sodium salt of benzene sulphonic acid, by a process known as the Dow process, and by the air oxidation of cumene. Each of these methods is described below. You can also make small amounts of phenol by the peroxide oxidation of phenylboronic acid and the hydrolysis of diazonium salts.
In this method, benzene sulfonic acid is reacted with aqueous sodium hydroxide. The resulting salt is mixed with solid sodium hydroxide and reacted at a high temperature. The product of this reaction is sodium phenoxide, which is acidified with aqueous acid to make phenol.
Image: Reaction of synthesis of phenols by the pyrolysis of the Na salt of benzene sulphonic acid
Dow Process
In this process, chlorobenzene is reacted with dilute sodium hydroxide at a temperature of about 300°C and 3000 psi pressure. The following figure exemplifies the Dow process.
image: Reaction of preparation of phenol by Dow process
Air Oxidation of Cumene
The oxidation of cumene in the presence of air (isopropylbenzene) will lead to the making of both acetone and phenol, as shown in the following figure.
Image: The overall reaction of air oxidation of cumene
The mechanisms for the development and degradation of cumene hydroperoxide need closer looks, which are delivered following the figure.
Cumene Hydroperoxide Formation- The development of hydroperoxide continues by a free radical chain reaction. A radical initiator extracts a hydrogen‐free radical from the molecule, making a tertiary free radical. The formation of the tertiary free radical is the first step in the reaction.
Image: The formation of the tertiary free radical
Further, the free radical is attracted to an oxygen molecule. This attraction yields the hydroperoxide free radical.
Image: The reaction of formation of hydroperoxide free radical
Lastly, the hydroperoxide free radical extracts a hydrogen-free radical from a molecule of cumene to produce cumene hydroperoxide and a new tertiary free radical.
Image: Reaction of formation of cumene hydroperoxide and a tertiary free radical
Cumene Hydroperoxide Degradation- The degradation of the cumene hydroperoxide continues through a carbocation mechanism. In the 1st step, a pair of electrons on the oxygen of the hydroperoxide's “hydroxyl group” is attracted to a proton of the H3O + molecule, making an oxonium ion.
Image: Reaction of formation of an oxonium ion
Next, the oxonium ion develops stability when the positively charged oxygen leaves in a water molecule. This loss of a water molecule yields a new oxonium ion.
Image: Reaction of formation of new oxonium ion
A phenide ion move to the oxygen atom (which makes a tertiary carbocation) stabilizes the positively charged oxygen. (A phenide ion is a phenyl group with an electron bonding pair accessible to produce a new bond to the ring.)
Image: The reaction of formation of a tertiary carbocation from phenyl ion
The carbocation is stabilized via an acid‐base reaction with a water molecule, leading to the development of an oxonium ion.
Image: The reaction of formation of oxonium ion
The stability of the oxonium ion is by the loss of a proton.
Image: The loss of an electron from oxonium ion
Afterwards, a proton is selected by the ether oxygen in an acid‐base reaction, producing a new oxonium ion.
Image: Reaction to form new oxonium ion
The positively charged ether oxygen attracts the electrons in the oxygen‐carbon bond toward itself, thus delocalizing the charge over both of the atoms. The fractional positive charge on the carbon attracts the nonbonding electron pair from the oxygen of the OH group, letting the electrons in the original oxygen‐carbon bond be released back to the extra electronegative oxygen atom.
Image: The reaction of formation of phenol
Finally, a proton is lost from the protonated acetone molecule, leading to the development of acetone.
Image: The reaction of formation of acetone
Application of Phenol
Some of the applications of phenol are listed below.
The main uses of phenol, consuming two-thirds of its making, include its transformation to precursors for plastics.
Condensation with acetone develops bisphenol-A, a key precursor to epoxide resins and polycarbonates.
Condensation of alkylphenols, phenol, or diphenols with formaldehyde will give phenolic resins, a well-known example of which is Bakelite.
Partial hydrogenation of phenol will give cyclohexanone, a precursor to nylon. Nonionic detergents are formed by alkylation of phenol to give the alkylphenols, e.g., nonylphenol, which is then exposed to ethoxylation.
Medical
Phenol is also a useful precursor to a huge collection of drugs, most notably aspirin but also several herbicides and pharmaceutical drugs.
Phenol is an element in the liquid/liquid phenol-chloroform abstraction technique used in molecular biology for the procurement of nucleic acids from tissues or cell culture samples. Depending on the pH of the solution either DNA or RNA can be mined
Niche Use
Phenol is so low-priced that it attracts many small-scale uses. It is a part of industrial paint strippers used in the aviation industry for the removal of polyurethane, epoxy and other chemically resistant coatings.
Phenol byproducts have been used in the making of cosmetics including hair colourings, sunscreens, skin lightening preparations, as well as in skin toners or exfoliators. Still, due to safety reasons, phenol is banned from use in cosmetic merchandise in the European Union and Canada.
Hence, phenol is an essential chemical which is used for different purposes. In this article, properties along with the important methods of preparation of phenol with reactions and its applications are provided which is helpful for students to understand the important chemicals of organic chemistry.
