Phthalic acid, when reacted with resorcinol in the presence of concentrated H2SO4, undergoes a condensation reaction to produce fluorescein.

Here is a detailed explanation of the reaction:

1. Condensation reaction:
The reaction between phthalic acid and resorcinol is a condensation reaction. In this type of reaction, two molecules combine to form a larger molecule while eliminating a smaller molecule, such as water. In this case, the smaller molecule being eliminated is water.

2. Formation of fluorescein:
The reaction between phthalic acid and resorcinol leads to the formation of fluorescein. Fluorescein is a fluorescent dye that is commonly used in biological and chemical applications. It is known for its bright green fluorescence under ultraviolet light.

3. Role of concentrated H2SO4:
Concentrated sulfuric acid (H2SO4) acts as a catalyst in this reaction. A catalyst is a substance that speeds up a chemical reaction without undergoing any permanent change itself. In this case, the concentrated sulfuric acid facilitates the reaction between phthalic acid and resorcinol, leading to the formation of fluorescein.

4. Mechanism of the reaction:
The exact mechanism of the reaction between phthalic acid and resorcinol is complex and involves multiple steps. However, the general mechanism can be summarized as follows:

- The acidic conditions provided by concentrated H2SO4 protonate the carbonyl group in phthalic acid, making it more reactive.
- The protonated carbonyl group then undergoes a nucleophilic attack by the hydroxyl group in resorcinol.
- This leads to the formation of an intermediate compound.
- The intermediate compound further undergoes dehydration, eliminating a water molecule and forming the final product, fluorescein.

In conclusion, the reaction between phthalic acid and resorcinol in the presence of concentrated H2SO4 results in the formation of fluorescein. This reaction is a condensation reaction and involves the elimination of water. The concentrated sulfuric acid acts as a catalyst to facilitate the reaction.

Phthalic acid gets dehydrated on treatment with concentrated H2SO4 to form phthalic anhydride. Phthalic anhydride further reacts with resorsinol to form fluorescin