Physical and Chemical Properties of Phenols - Practice Questions & MCQ

1. Phenols are either colourless liquids or white crystalline solids.
2. Phenols are soluble in hot water.
3. The boiling points of phenols are much higher than the corresponding aromatic hydrocarbons and the haloarenes.
4. Phenols have anti-fungal and anti-bacterial properties. Thus used as disinfectants and antiseptics.
5. Phenols are poisonous in nature but act as antiseptic and disinfectant.

Reaction with zinc dust: When phenol is distilled with zinc dust, benzene is obtained. The reaction occurs as follows:

Reaction with NH₃: Phenol reacts with ammonia in presence of anhydrous zinc chloride at 573K or (NH₄)₂SO₃-NH₃ at 423K to form aniline. This conversion of phenol into aniline is called Bucherer reaction.

Reaction with FeCl₃: Phenol gives violet colouration with ferric chloride solution sue to the formation of a coloured iron complex, which is a characteristic to the existence of keto-enol tautomerism in phenols.

Riemer-Tiemann Reaction
Phenols for aromatic compounds containing EDG when refluxed with CHCl₃ and alkali yield o- and p- hydroxybenzaldehyde. The ortho product is the predominant product. It is an electrophilic substitution on PhO^- ion. The electrophile is dichlorocarbene (:CCl₂) which contains a C with only six electrons.

Mechanism
The mechanism of this reaction follows the following steps.

For example,

Koble-Schmitt Reaction
Phenol when heated at(390-410K) under pressure with CO₂ and alkali gives salicylic acid after acidification in addition to some amount of p-isomer.

Mechanism
 

For example,

On heating aryl allyl or alkyl allyl ethers in inert solvent, the allyl group migrates from the ortho position to the ring, preferable at ortho position, but para if the ortho position is blocked.

Mechanism
The mechanism of this reaction occurs in three following steps:

1. Enolate formation
   
2. Nucleophilic attack
   
    
3. Removing of leaving group
   

Nitration
Anisole reacts with a mixture of concentrated sulphuric and nitric acids to yield a mixture of ortho and para nitroanisole. The reaction occurs as follows.

Phthalic Anhydride
Phenol couples with phthalic anhydride in presence of conc. H₂SO₄ to form phenolphthalein which is used as an indicator. The reaction occurs as follows.

With dilute nitric acid at low temperature (298 K), phenol yields a mixture of ortho and para nitrophenols. The reaction occurs as follows.


The ortho and para isomers can be separated by steam distillation. o-Nitrophenol is steam volatile due to intramolecular hydrogen bonding while p-nitrophenol is less volatile due to intermolecular hydrogen bonding which causes the association of molecules.