4-Hydroxybenzoic acid is a monohydroxybenzoic acid, a phenolic derivative of benzoic acid. It is a white crystalline solid that is slightly soluble in water and chloroform

Chloroform

Chloroform is an organic compound with formula CHCl3. It is one of the four chloromethanes. The colorless, sweet-smelling, dense liquid is a trihalomethane, and is considered somewhat hazardous...

 but more soluble in polar organic solvents such as alcohol

Alcohol

In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

s and acetone

Acetone

Acetone is the organic compound with the formula 2CO, a colorless, mobile, flammable liquid, the simplest example of the ketones.Acetone is miscible with water and serves as an important solvent in its own right, typically as the solvent of choice for cleaning purposes in the laboratory...

. 4-Hydroxybenzoic acid is primarily known as the basis for the preparation of its ester

Ester

Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

s, known as parabens, which are used as preservatives in cosmetics. It is isomer

Isomer

In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...

ic with 2-hydroxybenzoic acid, known as salicylic acid

Salicylic acid

Salicylic acid is a monohydroxybenzoic acid, a type of phenolic acid and a beta hydroxy acid. This colorless crystalline organic acid is widely used in organic synthesis and functions as a plant hormone. It is derived from the metabolism of salicin...

, a precursor to aspirin

Aspirin

Aspirin , also known as acetylsalicylic acid , is a salicylate drug, often used as an analgesic to relieve minor aches and pains, as an antipyretic to reduce fever, and as an anti-inflammatory medication. It was discovered by Arthur Eichengrun, a chemist with the German company Bayer...

.

Natural occurrence

4-Hydroxybenzoic acid can be found naturally in Cocos nucifera. It is one of the main catechins metabolite

Metabolite

Metabolites are the intermediates and products of metabolism. The term metabolite is usually restricted to small molecules. A primary metabolite is directly involved in normal growth, development, and reproduction. Alcohol is an example of a primary metabolite produced in large-scale by industrial...

s found in humans after consumption of green tea

Green tea

Green tea is made solely from the leaves of Camellia sinensis that have undergone minimal oxidation during processing. Green tea originates from China and has become associated with many cultures throughout Asia. It has recently become more widespread in the West, where black tea is traditionally...

 infusions.

Açaí oil

Acai oil

Açaí oil is obtained from the fruit of Euterpe oleracea , which grows in the Amazon Rainforest. The oil is rich in phenolic compounds similar in profile to the pulp itself, such as vanillic acid, syringic acid, p-hydroxybenzoic acid, protocatechuic acid and ferulic acid as well as -catechin and...

, obtained from the fruit of the açaí Palm (Euterpe oleracea), is rich in p-hydroxybenzoic acid (892 ± 52 mg/kg), protocatechuic acid

Protocatechuic acid

Protocatechuic acid is a dihydroxybenzoic acid, a type of phenolic acid. It has mixed effects on normal and cancer cells in in vitro and in vivo studies.-Biological effects:...

 (630 ± 36 mg/kg) and ferulic acid

Ferulic acid

Ferulic acid is a hydroxycinnamic acid, a type of organic compound. It is an abundant phenolic phytochemical found in plant cell wall components such as arabinoxylans as covalent side chains. It is related to trans-cinnamic acid. As a component of lignin, ferulic acid is a precursor in the...

 (101 ± 5.9 mg/kg).

Chemical production

4-Hydroxybenzoic acid is produced commercially from potassium phenoxide

Phenol

Phenol, also known as carbolic acid, phenic acid, is an organic compound with the chemical formula C6H5OH. It is a white crystalline solid. The molecule consists of a phenyl , bonded to a hydroxyl group. It is produced on a large scale as a precursor to many materials and useful compounds...

 and carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 in the Kolbe-Schmitt reaction

Kolbe-Schmitt reaction

The Kolbe–Schmitt reaction/Kolbe process is a carboxylation chemical reaction that proceeds by heating sodium phenolate with carbon dioxide under pressure , then treating the product with sulfuric acid...

.

4-Hydroxybenzoic acid can also be produced in the laboratory by heating potassium salicylate

Salicylic acid

Salicylic acid is a monohydroxybenzoic acid, a type of phenolic acid and a beta hydroxy acid. This colorless crystalline organic acid is widely used in organic synthesis and functions as a plant hormone. It is derived from the metabolism of salicin...

 with potassium carbonate

Potassium carbonate

Potassium carbonate is a white salt, soluble in water , which forms a strongly alkaline solution. It can be made as the product of potassium hydroxide's absorbent reaction with carbon dioxide. It is deliquescent, often appearing a damp or wet solid...

 to 240 °C, followed by treating with acid.

Chemical reactions

It is about 10x less acidic than benzoic acid

Benzoic acid

Benzoic acid , C7H6O2 , is a colorless crystalline solid and the simplest aromatic carboxylic acid. The name derived from gum benzoin, which was for a long time the only source for benzoic acid. Its salts are used as a food preservative and benzoic acid is an important precursor for the synthesis...

, K_a = 3.3 x 10⁻⁵ M at 19 °C:

HOC₆H₄CO₂H HOC₆H₄CO₂⁻ + H⁺

Safety

4-Hydroxybenzoic acid is popular antioxidant in part because of its low toxicity. The is 2200 mg/kg in mice (oral).