Formation of Osazones:

Osazones are crystalline compounds that are formed by the condensation reaction between carbohydrates containing a free aldehyde or keto group and phenylhydrazine. This reaction is known as the osazone formation reaction and is widely used for the identification and characterization of carbohydrates.

Reaction Mechanism:

The reaction between a carbohydrate and phenylhydrazine involves the following steps:

1. Formation of Schiff's base: The first step is the formation of a Schiff's base between the carbonyl group of the carbohydrate and the phenylhydrazine. This is a nucleophilic addition reaction where the lone pair of electrons on the nitrogen atom of phenylhydrazine attacks the carbonyl carbon of the carbohydrate. This results in the formation of an imine or Schiff's base.

2. Conversion to osazone: The imine formed in the previous step undergoes further reaction with another molecule of phenylhydrazine. This leads to the formation of a hydrazone. The hydrazone then undergoes a condensation reaction with the elimination of water molecule to form the osazone.

Role of Phenylhydrazine:

Phenylhydrazine is a strong nucleophile due to the presence of the lone pair of electrons on the nitrogen atom. It can attack the carbonyl carbon of the carbohydrate, leading to the formation of a Schiff's base. Phenylhydrazine also acts as a reducing agent, converting the aldehyde or keto group of the carbohydrate into an imine or hydrazone, respectively.

Role of Sulphuric Acid, Mercaptoethanol, and Hydroxylamine:

Sulphuric acid, mercaptoethanol, and hydroxylamine are not involved in the formation of osazones. They may have other roles in different chemical reactions or processes. However, in the context of osazone formation, the correct answer is option 'A' - Phenylhydrazine.

Overall, osazones are formed by the chemical reaction between carbohydrates containing a free aldehyde or keto group and phenylhydrazine. Phenylhydrazine acts as a nucleophile and reducing agent, leading to the formation of Schiff's base and subsequent condensation to form the osazone.