[Chemical Knowledge]：Production process and preparation method of salicylic acid

Salicylic acid is an organic compound widely used in medicine, cosmetics, food and other fields. It can be used to treat various skin diseases, such as acne, psoriasis and eczema. In addition, salicylic acid is also used as preservatives, food additives and spices. In this paper, the production process and preparation method of salicylic acid will be discussed in depth.

Structure and properties of 1. salicylic acid
salicylic acid has a chemical formula of C7H6O3 and a molecular weight of 138.12. It is a white to light yellow needle-like crystal with a special smell of phenol. Salicylic acid is an organic acid with certain acidity. It is soluble in ethanol, ether, acetone and other organic solvents, almost insoluble in water.

Production process of 2. salicylic acid
the production process of salicylic acid is mainly divided into natural method, artificial synthesis method and microbial method.

natural law
natural method refers to the extraction of salicylic acid from the bark of poplar. The bark of poplar contains 1% ~ 6% salicin, and salicylic acid can be obtained after hydrolysis reaction. The advantage of this method is that the source of raw materials is widely available, but the yield is low, and the extraction cost is high, which is not conducive to large-scale production.

artificial synthesis method
artificial synthesis is the most common method in the production of salicylic acid. The core reaction is phenol and carbon dioxide in the catalyst under the action of salicylic acid.
The chemical equation for the reaction of phenol and carbon dioxide is:
HOC6H5 + CO2 → HOOC6H5
this reaction usually requires the use of a catalyst, and sodium hydroxide, sodium carbonate, potassium carbonate and the like are commonly used. During the reaction, phenol and carbon dioxide are mixed together and heated to a suitable temperature. After the catalyst is added, the reaction product is separated from the mother liquor. After cooling, salicylic acid crystals are precipitated.

microbiological method
microbial method refers to the use of microbial enzymatic production of salicylic acid. First of all, it is necessary to screen the strain with salicylic acid synthase from the appropriate microbial strains, and to optimize the culture conditions of the strain. This method has certain advantages, such as making full use of cheap waste materials and realizing process optimization by changing microbial strains, but the process control requirements of this method are high.

Preparation method of 3. salicylic acid
there are many preparation methods of salicylic acid, and several commonly used preparation methods are described below.

1. Phenol method
the condensation reaction of phenol and sodium bicarbonate under the action of sodium hydroxide to obtain mercury phenol, and then the mercury phenol is boiled under the catalysis of hydrochloric acid to obtain salicylic acid. The reaction equation is:
C6H5OH + NaHCO3 → C6H5ONa + CO2 + H2O
C6H5ONa + HgCl2 → C6H5OHgCl + NaCl
C6H5OHgCl + HCl → HOOC6H5 + HgCl2
the advantage of this method is that the reaction conditions are simple, and it is more practical in small-scale production, but it is not conducive to the recovery and reuse of the catalyst in large-scale production.

2. Sodium hydroxide method
salicylic acid can be obtained by reacting phenol with sodium hydroxide to obtain mercury phenol, which is then hydrolyzed with acetic acid. The reaction equation is:
C6H5OH + NaOH → C6H5ONa + H2O
C6H5OHg + CH3COOH + H2O → HOOC6H5 + Hg + CH3COOH
this method has the advantages of weak acidic conditions in the reaction process and easy to realize continuous production.

3. Benzoic anhydride method
the benzoic anhydride method is an addition reaction of benzoic anhydride and sodium acid under the catalysis of nitric acid tungstic acid to generate sodium salicylate, and then sodium salicylate is acidified with acid to obtain salicylic acid. The reaction equation is:
C6H5CO2O + NaOH → C6H5CO2Na + H2O
C6H5CO2Na + HOOC6H5 +12WO4+4NaCl → 2Na2WO4+2CO2 ↑ + 2H2O +2NaCl +2HOOC6H5
2HOOC6H5 + Na2WO4 + HCl → 2HOOC6H5 + 12WO3↑ + NaCl
the advantage of this method is that the process is simple and the catalyst is easy to recover in the reaction process.

Salicylic acid is an important organic compound, which is widely used in medicine, cosmetics, food and other fields. Its production process is mainly divided into natural method, synthetic method and microbial method. In the artificial synthesis method, phenol method and other methods can be used for the preparation of salicylic acid. When selecting the appropriate preparation method, it should be considered according to the actual production needs and cost. Of course, in order to ensure product quality and stability, strict quality control and optimization of process parameters are required.