91Ó°ÊÓ

Aryl alcohols are called phenols. They have a hydroxyl group attached to the benzene ring.

The acidity of phenols is greater than alcohols due to resonance in the conjugate base formed after the removal of a proton.

Hydroxyl groups are ortho-para directing groups. Therefore, they make these positions electron-rich and favorable for electrophilic substitution.

Phenols also undergo many other reactions like esterification, coupling reactions, etc.

Reactions that are used to add substituents to aromatic compounds by an electrophilic substitution process are called Friedel-Crafts reactions.

There are mainly two types- Friedel-Crafts alkylation used to add alkyl groups and Friedel-Crafts acylation to add acyl groups.

In phenols, Friedel-Crafts reactions are favored in ortho and para positions as the alkyl groups and acyl groups are electrophilic.

The reactions are catalyzed by lewis acids like aluminum trichloride.

a. A mixture of nitric acid and sulfuric acid is called a nitrating mixture as it can produce nitronium ions. Nitronium ion is an electrophile.

When phenol is treated with a nitrating mixture, nitronium ions get substituted at ortho and para positions forming ortho nitrophenol and para nitrophenol.

When the products are treated with tin and HCl, reduction occurs to form ortho amino phenol and para amino phenol.

b. The first reaction is a Friedel-Crafts acylation reaction with 2-ethylbutyryl chloride to form ortho and para-substituted products.

The products on reaction with zinc amalgam and HCl undergo reduction to form ortho isobutyl phenol and para isobutyl phenol.

c. The first reaction is a Friedel-Crafts alkylation reaction with ethyl chloride to form ortho and para-substituted products.

The product of the photochemical reaction with bromine undergoes bromination to form ortho (1-bromoethyl) phenol and para (1-bromoethyl) phenol

d. The first reaction is a Friedel-Crafts alkylation reaction with ethyl chloride to form ortho and para-substituted products.

The product undergoes oxidation with potassium permanganate to form ortho hydroxybenzoic acid and para hydroxybenzoic acid.