Phenols contain a phenyl(–C₆H₅), bonded to a hydroxyl(–OH) group. It is a white crystalline solid at room temperature. It is flammable and has a strong odor. It is produced on a large scale (about 7 billion kg/year) as a precursor to many materials and useful compounds. It is only mildly acidic but requires careful handling due to its propensity to cause burns. Phenol was first extracted from coal tar, and its major uses involve its conversion to plastics or related materials. Phenols are key for building polycarbonates, epoxies, bakelite, nylon, detergents and a large collection of drugs, herbicides and pharmaceuticals.

Natural occurrence:
Phenols are found in the natural world, especially in the plant kingdom. In some cases of natural phenols, they are present in vegetative foliage to discourage herbivory, such as in the case of western poison oak.

Acidic nature of phenol:
Phenols turn blue litmus red and react with metals liberating hydrogen. However, they do not react with carbonates or bi-carbonates. Phenols behave as acids because of the presence of polar O–H group in them. A hydrogen ion can break away from the –OH group and transfer to a base. They ionize in aqueous solutions to give H⁺ ions.
Eg: In water:

Phenol is a very weak acid and the position of equilibrium lies well to the left. Phenols as well as phenoxide ion both are resonance stabilized.

Electron structure of hydroxyl group in phenols:
There is an interaction between the de-localized electrons in the benzene ring and one of the lone pairs on the oxygen atom. This has an important effect on both the properties of the ring and of the –OH group. One of the lone pairs on the oxygen overlaps with the de-localized ring electron system.
The donation of the oxygen's lone pair into the ring system increases the electron density around the ring. That makes the ring much more reactive than it is in benzene itself. It also helps to make the –OH group's hydrogen a lot more acidic than it is in alcohols.

Substituent effects on acidity:
Substituents located at ortho or para to the –OH group, can dramatically influence the acidity of the phenol due to resonance and/or inductive effects. Electron withdrawing substituents enhance the acidity.
Eg: –OH, –Cl, –Br, –NO₂.
These substituents make phenol more acidic by stabilizing the phenoxide ion through delocalization of −ve charge and through inductive effect or mesomeric effect.

Electron donating substituents decrease the acidity.
Eg: –H, –CH₃, –OCH₃, –NH₂.
These substituents make phenol less acidic by destabilizing the phenoxide ion by resonance effect or inductive effect. Positive inductive effect of methyl group destabilize the phenoxide ion.

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