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Question: Phthalic anhydride reacts with resorcinol in the presence of concentrated \({{H}_{2}}S{{O}_{4}}\) to...

Phthalic anhydride reacts with resorcinol in the presence of concentrated H2SO4{{H}_{2}}S{{O}_{4}} to give:
(A) Phenolphthalein
(B) alizarin
(C) coumarin
(D) fluorescein

Explanation

Solution

Phthalic acid is a dicarboxylic acid. It reacts with a dehydrating agent i.e. sulphuric acid. Phthalic acid loses water on heating and acts as a dehydrating agent. The reaction of phthalic anhydride with resorcinol results in the formation of organic dye. This dye has a wide application as a fluorescent material in the laser application.

Complete Solution :
We have been provided that phthalic acid with resorcinol in the presence of concentrated sulphuric acid,
Phthalic acid is an aromatic dicarboxylic acid, with formula C6H4(CO2H)2{{C}_{6}}{{H}_{4}}{{(C{{O}_{2}}H)}_{2}} .It is an isomer of isophthalic acid and terephthalic acid. Resorcinol is an organic compound with the formula C6H4(OH)2{{C}_{6}}{{H}_{4}}{{(OH)}_{2}}. It is one of three isomeric benzenediols.
- When these two compounds react together in presence of concentrated sulphuric acid,
Sulphuric acid acts as a dehydrating agent because a substance that absorbs moisture from its surroundings is called a dehydrating agent. Sulphuric acid readily protonated H2O{{H}_{2}}O leading to the formation of hydronium ions.

- Phthalic acid reacts with resorcinol in the presence of concentrated H2SO4{{H}_{2}}S{{O}_{4}} to give fluorescein. Phthalic acid gets dehydrated on treatment with concentrated H2SO4{{H}_{2}}S{{O}_{4}} to form phthalic anhydride. Phthalic anhydride further reacts with resorcinol to form fluorescein.
- So, we can say that Phthalic anhydride reacts with resorcinol in the presence of concentrated H2SO4{{H}_{2}}S{{O}_{4}} to give fluorescein.
So, the correct answer is “Option D”.

Note: Note that Adolf Von Baeyer first synthesized fluorescein in the laboratory. He prepared it from the phallic anhydride and resorcinol in presence of zinc chloride through the Friedel-Crafts reaction. There is one more method for synthesis using methane sulphonic acid . This gives a high yield.
Sulphuric acid is not used as a drying agent H2S{{H}_{2}}S because it reacts with it to form sulphur.
 H2SO4+H2S→2H2O+SO2+S \text{ }{{H}_{2}}S{{O}_{4}}+{{H}_{2}}S\to 2{{H}_{2}}O+S{{O}_{2}}+S\text{ }