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Resorcinol is one of the significant organic compounds having a molecular formula C6H6O2. Resorcinol is otherwise known as m-dihydroxybenzene and resorcin. It is a white-colored crystalline solid and is a meta-isomer of benzenediol. This article explains various aspects of resorcinol such as structure, synthesis, physical properties, chemical properties, and applications.

Structure of resorcinol

The molecular formula of resorcinol is C6H6O2. Chemically, resorcinol is the meta-isomer of benzenediol. Benzene consisting of two hydroxyl ions at 1 and 3 positions is known as resorcinol. Resorcinol, 1,3-isomer of benzenediol, primarily acts as a sensitizer and erythropoietin inhibitor.

Synthesis of resorcinol

Resorcinol can be synthesized or produced in numerous ways using different reactants. Two popular methods for the synthesis of resorcinol are using benzene and meta-phenylenediamine (MPDA). Let us have a look at these two methods of producing resorcinol at an industrial scale.

1. Synthesis of resorcinol from benzene
Benzene, the raw material, undergoes several changes, ultimately resulting in the formation of 1,3-dihydroxybenzene.

The synthesis of resorcinol from benzene involves a series of steps that begins with the dialkylation reaction of propylene—the dialkylation reaction of propylene results in the formation of 1,3-diisopropylbenzene. Next, 1,3-diisopropylbenzene, which is a substituted arene, undergoes a rearrangement reaction resulting in the formation of the end product. In addition to the end product (resorcinol), acetone is also formed as a result. Resorcinol can be separated from the mixture of resorcinol and acetone with the help of any separation technique.

2. Synthesis of resorcinol from meta-phenylenediamine
Resorcinol can also be produced from meta-phenylenediamine in higher yields. This reaction requires a homogenous mineral acid and zeolites. In this process, MPDA undergoes a hydrolysis reaction and gives rise to a product called MAP. MAP, in the presence of ammonia and hydrogen ions, results in the formation of resorcinol.

The most commonly used zeolites for the synthesis of resorcinol are alumino-silicates. The reaction temperature is around 200–275°C. At this temperature, alumino-silicates offer the advantage of minimizing the corrosive properties exhibited by mineral acids. As a result, the yield of resorcinol obtained by the hydrolysis of meta-phenylenediamine is greater than the other methods.

Physical properties of resorcinol

Resorcinol is an organic compound and exhibits specific physical properties. Resorcinol is a white-colored crystalline solid at room temperature. The impure form of resorcinol, when exposed to light, turns pink. It has a faint odor and a sweet taste followed by a sort of bitter taste. Resorcinol is readily soluble in solvents like water, ether, and alcohol. However, it is insoluble in carbon disulfide and chloroform. Resorcinol burns but does not ignite.
The physical properties of resorcinol are tabulated as follows-

Molecular formula C6H6O2
Molecular mass 110.1 g/mol
Physical state Solid
Color Resorcinol is generally white. However, when exposed to light or iron, it turns pink.
Odor Faint and characteristic
Taste A sweet taste is followed by a sort of bitter taste.
Density 1.28 g/cm3
Boiling point 277 °C
Melting point 110 °C
Solubility Resorcinol is soluble in water, alcohol, and ether. It is insoluble in carbon disulphide and chloroform.

Chemical reactions of resorcinol

Resorcinol is widely used in many titrations and chemical reactions. The characteristic chemical reactions of resorcinol are as follows.

- The reaction of resorcinol with phthalic anhydride - Resorcinol reacts with phthalic anhydride and gives rise to a Fluorescein product. This reaction takes place exclusively in the presence of zinc chloride.

- Pechmann condensation reaction - Resorcinol undergoes a Pechmann condensation reaction where it reacts with ethyl acetoacetate resulting in the formation of 7-hydroxy 4-methyl coumarin. Pechmann condensation reaction takes place in the presence of sulphuric acid and at room temperature. The yield of 7-hydroxy 4-methyl coumarin obtained from the reaction of resorcinol with ethyl acetoacetate is 85%.

- The reaction of resorcinol with aldehyde - Resorcinol readily reacts with organic compounds containing the aldehydic functional group. Resorcinol reacts with aldehyde and produces calyx resorcinarene.

1. Resorcinol + vanillin → C-4-hydroxy-3-methoxyphenyl calix (4) resorcinarene
2. Resorcinol + cinnamaldehyde → C-2phenylethenil calix (4) resorcinarene

- Facile synthesis of fluorinated resorcinol formaldehyde aerogels - Resorcinol reacts with formaldehyde in the presence of CF3OCF3 to yield fluorinated resorcinol formaldehyde aerogel. Resorcinol produces aerogels at the temperature of 20 degrees celsius only. A reaction temperature of 50 degrees celsius yields a product with high fluorine content. This method of synthesizing fluorinated resorcinol formaldehyde aerogels from resorcinol is called facile synthesis.

Applications of resorcinol

Resorcinol has a vast range of applications, from treating acne to being used as an aerogel. Let us have a look at a few significant applications of resorcinol.

- Resorcinol is primarily used as an antiseptic or disinfectant in various pharmaceutical products.
- Resorcinol is widely used to treat various skin diseases such as acne, psoriasis, warts, dermatitis, calluses, and eczema.
- Resorcinol is a crucial raw material in the synthesis of resins and aerogels.
- Resorcinol is additionally used in the manufacturing of plasticizers and diazo dyes.
- Resorcinol is capable of withstanding wide ranges of temperatures. Hence, it is utilized in the automotive industry as well.

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