Salicylic Acid: Definitions, Structure, Chemical and Physical Properties, Uses

Salicylic Acid Structure Overview

Salicylic acid is a mono-hydroxybenzoic acid that is lipophilic. It is a beta-hydroxy acid and a kind of phenolic acid (BHA). Beta hydroxy acid is a naturally occurring chemical in plants. This colorless crystalline organic acid is widely used in organic chemistry. It is produced by the metabolism of salicin. Salicylic acid is an organic carboxylic acid that is crystalline and possesses keratolytic, bacteriostatic, and fungicidal characteristics. When ingested in high quantities, it is toxic to our bodies. When ingested in modest amounts, it can be used as an antiseptic and a food preservative. Salicylic acid contains a carboxyl group, i.e. COOH. It has no odor and no color. It is most likely well-known for being a significant element in topical anti-acne treatments. Salicylates are salicylic acid salts and esters. The World Health Organization has classified salicylic acid as an essential medicine. Salicylic acid is one of the safest and most effective drugs used in healthcare.

Salicylic Acid Structure

Salicylic acid has the chemical formula C₆H₄(OH)COOH. The molecular formula is sometimes written in condensed form asC₇H₆O₃. Salicylic acid's IUPAC name is 2-hydroxybenzoic acid. Salicylic acid has a hydroxyl group, or -OH group, connected at the ortho position to the carboxylic acid. This COOH group may be found on the benzene ring. Salicylic acid has a molecular weight or molar mass of 138.12 g/mol. All carbon atoms in salicylic acid's benzene ring are sp² hybridized. An intramolecular hydrogen bond is formed by salicylic acid. Salicylic acid dissociates in an aqueous solution, releasing a proton from the carboxylic acid. The resultant carboxylate ion, COO, interacts with the hydrogen atom of the hydroxyl group, -OH, via intermolecular interactions. It causes an intramolecular hydrogen bond to develop.

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Salicylic Acid Structure Preparation Methods

The two most often used ways of producing salicylic acid are as follows-

1. From Phenol: When phenol reacts with sodium hydroxide, sodium phenoxide is formed. The sodium phenoxide is then distilled and dehydrated. This is followed by a carboxylation reaction with carbon dioxide, which produces sodium salicylate, a salt of salicylic acid. This salt was then reacted with an acid, hydronium ion, or any species denoting a proton to produce salicylic acid.
2. From Methyl Salicylate: In the pharmaceutical business, methyl salicylate, often known as the oil of wintergreen, is usually referred to as an analgesic. It is used in the production of salicylic acid. This reaction involves the interaction of methyl salicylate with sodium hydroxide (NaOH), which results in the creation of a sodium salt intermediate of salicylic acid. This acid is known as disodium salicylate, and following subsequent interaction with sulphuric acid, it produces salicylic acid.

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Salicylic Acid Structure Properties 

The following are various kinds of properties-

• Salicylic Acid's Physical Properties

1. At normal temperatures, salicylic acid crystals are colorless, odorless, and needle-shaped.
2. Salicylic acid has an unpleasant flavor and has a flashpoint of 157°C.
3. Salicylic acid has 211°C boiling and 315°C melting points, respectively.
4. There are two hydrogen bond donors and three hydrogen bond acceptors in the salicylic acid molecule.
5. Because of its lipophilic nature, it has an extremely low water solubility of 1.8 g/L at 25°C.
6. Organic solvents such as carbon tetrachloride, benzene, propanol, ethanol, and acetone dissolve salicylic acid.
7. At 20°C, the density of salicylic acid is 1.44 and at 25°C, its vapor pressure is 8.2 x 10⁵ mmHg.
8. It has a propensity to discolor when exposed to direct sunshine owing to photochemical deterioration.
9. It releases annoying fumes and acrid-smelling smoke as it degrades.
10. At 25°C, its heat of combustion is 3.026mj/mole.
11. A saturated solution of salicylic acid has a pH of 2.4.
12. It has a pka value of 2.97, indicating a dissociation constant.

• Salicylic Acid's Chemical Properties

1. Formation of Aspirin: The most important reaction related to the usage of salicylic acid in the pharmaceutical business is the synthesis of aspirin, acetylsalicylic acid. It is one of the most widely used analgesics and blood thinners. Salicylic acid is interacting with acetic anhydride in this process. It causes the hydroxyl group in salicylic acid to be acetylated, resulting in the creation of acetylsalicylic acid, or aspirin. As a byproduct of this process, acetic acid is produced. This is also present as an impurity in large-scale aspirin manufacture. Several refining operations are required to eliminate these contaminants from the resultant product combination.
2. Esterification Reaction: Because salicylic acid is an organic acid, it reacts with organic alcohol groups to form a new organic chemical class known as an ester. When salicylic acid reacts with methanol in an acidic media, particularly sulphuric acid, with heat, a dehydration reaction occurs, with the loss of water OH ion. This ion is lost from the carboxylic acid functional group in the salicylic acid molecule, and the H+ ion is lost during methanol deprotonation, resulting in the creation of methyl salicylate (an ester).

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Uses of Salicylic Acid Structure

The following are the uses of salicylic acid structure-

1. The most common and well-known use of salicylic acid is the production of acetylsalicylic acid (aspirin). It's also utilised in the manufacture of methyl salicylate (oil of wintergreen).
2. It also aids in the eradication of whiteheads and blackheads by not allowing pores to clog and opening existing congested pores.
3. It is used to treat Ichthyosis, a relatively uncommon inherited skin condition.
4. It is useful in the treatment of certain ringworm infections as well as the moist type of tinea pedis infection (commonly known as athlete's foot).
5. When used in modest amounts, it can be utilised as a food preservative.
6. Salicylic acid is a keratolytic (peeling agent) that aids in the exfoliation of the skin's outer layer.
7. It is utilized in toothpaste because it is a bacteriostatic agent with a slight antiseptic effect.
8. Helpful in the treatment of acne, dandruff, seborrhea, and psoriasis, as well as the removal of corns, calluses, and warts, as well as many other skin-related issues.