Salicylic acid is lipophilic monohydroxybenzoic acid. Salicylic acid a type of phenolic acid and a beta-hydroxy acid (BHA). Beta hydroxy acid is found as a natural compound in plants. This colourless crystalline organic acid is broadly in use in organic synthesis. It is derived from the metabolism of salicin. Salicylic acid is a crystalline organic carboxylic acid and has keratolytic, bacteriostatic and fungicidal properties. It is poisonous to our body when consumed in large. It can be in use as an antiseptic and as a food preservative when consumed in small quantities. Salicylic acid has a carboxyl group attached to it i.e., COOH. It is odourless and is colourless.
It is probably known for its use as an important ingredient in topical anti-acne products. The salts and esters of salicylic acid are salicylates. Salicylic acid is on the World Health Organization’s List of Essential Medicines. Salicylic acid is the safest and most effective medicines needed in a health system.
Structure of Salicylic Acid
The structural formula of salicylic acid is C6H4(OH)COOH. The chemical formula can also be written as C7H6O3 in the condensed form. The IUPAC name of salicylic acid is 2-hydroxybenzoic acid. Salicylic acid has a hydroxyl group i.e., -OH group attached at the ortho position with respect to the carboxylic acid. This COOH group is present on the benzene ring. The molecular weight or molar mass of salicylic acid is 138.12 g/mol.
All carbon atoms present in the benzene ring of salicylic acid are sp2 hybridized. Salicylic Acid forms an intramolecular hydrogen bond. In an aqueous solution, salicylic acid dissociates to lose a proton from the carboxylic acid. The resulting carboxylate ion i.e., −COO− undergoes intermolecular interaction with the hydrogen atom of the hydroxyl group i.e., -OH. It leads to the formation of an intramolecular hydrogen bond.
Methods of Preparation of Salicylic Acid
There are two most common methods in use for the preparation of salicylic acid are as follows –
From Phenol: When phenol is reacted with sodium hydroxide, it forms sodium phenoxide. Sodium phenoxide then undergoes distillation and dehydration. This process is followed by a carboxylation reaction with carbon dioxide, which results in the formation of sodium salicylate i.e., salt of salicylic acid. This salt then further reacted with an acid or hydronium ion or any species that denotes a proton to obtain the salicylic acid.
From Methyl Salicylate: Methyl salicylate also known as oil of wintergreen is commonly called analgesic in the pharmaceutical industry. It is in use for the preparation of salicylic acid. In this reaction, methyl salicylate is reacted with sodium hydroxide (NaOH) to lead to the formation of a sodium salt intermediate of salicylic acid. This acid is named disodium salicylate, which upon undergoing further reaction with sulphuric acid leads to the formation of salicylic acid.
Physical Properties of Salicylic Acid
Salicylic acid is a colourless, odourless and needle-shaped crystals at room temperature.
The taste of salicylic acid is acrid.
The boiling point and melting point of salicylic acid are 211oC and 315oC respectively.
The salicylic acid molecule has two hydrogen bond donors and three hydrogen bond acceptors.
The flashpoint of salicylic acid is 157oC.
Due to its lipophilic nature, its solubility in water is very poor i.e., 1.8 g/L at 25oC.
Salicylic acid is soluble in organic solvents like carbon tetrachloride, benzene, propanol, ethanol and acetone.
The density of salicylic acid is 1.44 at 20oC.
Its vapour pressure is 8.2×105mmHg at 25oC.
It is a tendency to undergo discolouration when exposed to direct sunlight due to its photochemical degradation.
Upon degradation, it emits irritating fumes and acrid smelling smoke.
Its heat of combustion is3.026mj/mole at 25oC.
The pH of a saturated solution of salicylic acid is 2.4.
Its pka value i.e., dissociation constant is 2.97.
Chemical Properties of Salicylic Acid
Formation of aspirin: In the pharmaceutical industry, the most important reaction associated with the use of salicylic acid is the production of aspirin i.e., acetylsalicylic acid. It is one of the most commonly used analgesics and blood-thinning agent. In this reaction, salicylic acid is reacting with acetic anhydride. It leads to the acetylation of the hydroxyl group present in the salicylic acid, thereby resulting in the production of acetylsalicylic acid i.e., aspirin. Acetic acid is manufactured as a byproduct of this reaction. This is also present as one of the impurities during large scale production of aspirin. These impurities must be removed from the resulting product mixture by several refining processes.
Esterification Reaction: Since salicylic acid is an organic acid, it undergoes a reaction with organic alcohol groups to produce a new organic chemical class alike ester. When salicylic acid is reacting with methanol in an acidic medium preferably sulphuric acid in the presence of heat, a dehydration reaction occurs with the loss of water −OH− ion. This ion is lost from the carboxylic acid functional group present in the salicylic acid molecule and the H+ ion is lost from the deprotonation of the methanol molecule, resulting in the formation of methyl salicylate (an ester).
Uses of Salicylic Acid
Salicylic acid is particularly in use in the pharmaceutical industry. The most common use of salicylic acid is in the preparation of an analgesic, aspirin, which is an acetylated derivative of salicylic acid. Another analgesic formed from salicylic acid is methyl salicylate, an esterified product of salicylic acid. Both of these analgesics are in use to treat headache and other body aches.
Salicylic acid is in use in the treatment of wart infections. The mechanism by which professionals treats warts infection is similar to its keratolytic action. Firstly, it dehydrates the skin cells that are affected by warts and thereby it gradually leads to its shedding off from the body. It also activates the immune reaction of the body towards the viral wart infection by initiating a mild inflammatory reaction.
Salicylic acid is one of the major components of anti-dandruff shampoos. This acid also helps in clearing away the dead and flaky skin cells from your scalp.
Salicylic acid also in use as a mild antiseptic effect known as a bacteriostatic agent. It does not kill the existing bacteria and hence not an antibacterial agent but prevent the growth of bacteria wherever applied.