While studying organic chemistry, you have come across the term ‘phenol’ many times. Haven’t you? So, what are phenols? In this chapter, we will look at the preparation of phenols and how we can get them on a large scale requirement. We will study the various methods and reactions that we must carry out in order to obtain phenols. However, before we proceed, let us take a quick look at what phenols are. Can you tell us what phenols are?
What is a Phenol?
Phenols are the organic compounds that have a benzene ring bonded to a hydroxyl group. We also know them by the name of carbolic acids. They are weak acids and generally form phenoxide ions by losing one positive hydrogen ion (H+) from hydroxyl group.
In earlier days, people were able to synthesise phenol from coal tar. It was a very complex and lengthy process. It had a lot of risks associated with it as well. Nowadays, with advancements in technologies, however, certain new methods have come up for the preparation of phenols in laboratories.
In laboratories, chemists primarily synthesise and derive phenol from benzene derivatives. In this chapter, we will look at some of the ways in which we can produce phenols commercially in laboratories.
Browse more Topics under Alcohols Phenols And Ethers
- Chemical Reactions of Alcohols, Phenols & Ethers
- Chemical Reactions of Ethers
- Introduction and Classification of Alcohols, Phenols & Ethers
- Physical Properties of Alcohols, Phenols and Ethers
- Preparation of Alcohols
- Physical Properties of Ethers
- Preparation of Ethers
- Some Commercially Important Alcohols
1) Preparation of Phenols from Haloarenes
Chlorobenzene is an example of haloarenes. We can obtain chlorobenzene by the monosubstitution of a benzene ring. When chlorobenzene fuses with sodium hydroxide at 623K and 320 atm, we obtain sodium phenoxide. Finally, sodium phenoxide on acidification gives phenols.
2) Preparation of Phenols from Benzene Sulphonic Acid
We can obtain Benzenesulphonic from benzene by reacting it with oleum. Benzenesulphonic acid, thus formed, is treated with molten sodium hydroxide at high temperature. This process leads to the formation of sodium phenoxide. Finally, sodium phenoxide on acidification gives phenols.
3) Preparation of Phenols from Diazonium Salts
When we treat an aromatic primary amine with nitrous (NaNO2 + HCl) acid at 273 – 278 K, we can easily obtain diazonium salts. These diazonium salts are highly reactive in nature. Upon warming with water, these diazonium salts finally hydrolyse to phenols. We can also obtain phenols from diazonium salts by treating it with dilute acids.
4) Preparation of Phenols from Cumene
Cumene is an organic compound that we can obtain by the Friedel-Crafts alkylation of benzene with propylene. Upon oxidation of cumene (isopropylbenzene) in presence of air, we obtain cumene hydroperoxide.
Upon further treatment of cumene hydroperoxide with dilute acid, we get the phenols. We also produce acetone as one of the by-products of this reaction in large quantities. Hence, phenols prepared by these methods need purifications.
Solved Example for You
Q: Write down the physical properties of phenols.
Ans: The physical properties of phenol are:
- Phenols are usually colourless, toxic, corrosive, needle-shaped solids.
- They can soon liquify due to their highly hygroscopic nature.
- Phenol is less soluble in water but readily soluble in organic solvents.
- Simplest phenols, because of hydrogen bonding have quite high boiling points.
- o-nitrophenol is volatile and also less soluble in water because of intramolecular hydrogen bonding