Carbolic acid, more commonly called phenol, is a compound of carbon, hydrogen and oxygen with the chemical formula C6H5OH. The six carbon atoms are arranged in a ring, with a hydroxyl (OH) group bonded to one carbon atom and a hydrogen atom bonded to each of the other five. This ring structure is known as a benzene ring — named after the compound benzene, which has a similar structure, but with hydrogen atoms bonded to all six carbon atoms. Pure phenol is a colorless crystalline solid that melts at 107.6°F (42°C) and is fairly soluble in water.
The term carbolic acid can refer either to the pure compound, or to a solution in water. It is a weak acid, dissociating into the phenoxide anion and hydrogen cation only to a limited extent in aqueous solution. Carbolic acid should not be confused with carbonic acid, which is the weak acid formed when carbon dioxide dissolves in water.
Phenol is the basis for a large number of naturally occurring organic compounds, known as phenols. Quinone and related compounds, for example, are involved in electron transport within living cells. Anthocyanidins act as antioxidants in plant tissue and combine with sugars to produce red, blue and purple pigments called anthocyanins, which are partly responsible for colorful autumn leaves. Polyphenols are complex molecules found in plants and contain many phenol units; they include tannins, which are known for their antioxidant properties.
Carbolic acid was first extracted from coal tar, but as of 2011, most is made from cumene, a component of crude petroleum oil. The compound has strong anti-microbial properties and one of its earliest uses was as an antiseptic. This was pioneered in 1867 by the British surgeon Joseph Lister, who used it in dilute solutions to sterilize wounds and surgical instruments, greatly improving the survival rates of patients undergoing surgery. It was also used in “coal tar soap” until the 1970s and is still used in various antiseptic and pharmaceutical products.
Among the biggest carbolic acid uses today is the production of plastics. Bakelite, one of the earliest plastics, was first made from phenol and formaldehyde in 1907. Carbolic acid is now used in the synthesis of many plastics, including polycarbonates, epoxy resins and nylon. Other uses include the manufacture of dyes, disinfectants and antiseptics.
Although many compounds that are essential or beneficial to life are based on phenol groups, carbolic acid itself is toxic and corrosive. It is volatile and is easily absorbed through the skin, so inhalation and skin contact as well as ingestion are possible routes of exposure. The vapor irritates the respiratory tract and high concentrations can cause lung damage.
Skin contact results in white discoloration followed by burns that can be severe — these may not be painful initially, due to the compound’s anesthetic effects. Symptoms of carbolic acid poisoning include nausea, vomiting, and abdominal pain, as well as reduced, dark-coloured urine and thirst. Other symptoms include sweating, rapid pulse, convulsions and coma.