This article is about the molecule.
For the group of chemicals containing a phenol group, see Phenols.
"Carbolic acid" redirects here.
It is not to be confused with carbonic acid.
|3D model (JSmol)|
|CompTox Dashboard (EPA)|
|Molar mass||94.113 g·mol|
|Appearance||Transparent crystalline solid|
|Odor||Sweet and tarry|
|Melting point||40.5 °C (104.9 °F; 313.6 K)|
|Boiling point||181.7 °C (359.1 °F; 454.8 K)|
|Solubility in water||8.3 g/100 mL (20 °C)|
|Vapor pressure||0.4 mmHg (20 °C)|
|Acidity (pKa)||9.95 (in water),
18.0 (in DMSO),
29.1 (in acetonitrile)
|UV-vis (λmax)||270.75 nm|
|Dipole moment||1.224 D|
|ATC code||C05BB05 () D08AE03 (), N01BX03 (), R02AA19 ()|
|Safety data sheet|
|GHS hazard statements||H301, H311, H314, H331, H341, H373|
|GHS precautionary statements||P261, P280, P301+310, P305+351+338, P310|
|NFPA 704 (fire diamond)||2|
|Flash point||79 °C (174 °F; 352 K)|
|LD50 (median dose)||317 mg/kg (rat, oral)
270 mg/kg (mouse, oral)
|LDLo (lowest published)||420 mg/kg (rabbit, oral)
500 mg/kg (dog, oral) 80 mg/kg (cat, oral)
|LC50 (median concentration)||19 ppm (mammal)
81 ppm (rat) 69 ppm (mouse)
|PEL (Permissible)||TWA 5 ppm (19 mg/m) [skin]|
|REL (Recommended)||TWA 5 ppm (19 mg/m) C 15.6 ppm (60 mg/m) [15-minute] [skin]|
|IDLH (Immediate danger)||250 ppm|
It is primarily used to synthesize plastics and related materials.
Homogeneous mixtures of phenol and water at phenol to water mass ratios of ~2.6 and higher are possible.
The sodium salt of phenol, sodium phenoxide, is far more water-soluble.
Phenol is a weak acid.
In aqueous solution in the pH range ca. 8 - 12 it is in equilibrium with the phenolate anion C6H5O (also called phenoxide):
- C6H5OH ⇌ C6H5O + H
In this way, the negative charge on oxygen is delocalized on to the ortho and para carbon atoms through the pi system.
An alternative explanation involves the sigma framework, postulating that the dominant effect is the induction from the more electronegative sp hybridised carbons; the comparatively more powerful inductive withdrawal of electron density that is provided by the sp system compared to an sp system allows for great stabilization of the oxyanion.
Thus, the greater number of resonance structures available to phenoxide compared to acetone enolate seems to contribute very little to its stabilization.
However, the situation changes when solvation effects are excluded.
A recent in silico comparison of the gas phase acidities of the vinylogues of phenol and cyclohexanol in conformations that allow for or exclude resonance stabilization leads to the inference that about ⁄3 of the increased acidity of phenol is attributable to inductive effects, with resonance accounting for the remaining difference.
The enthalpies of adduct formation and the –OH IR frequency shifts accompanying adduct formation have been studied.
The relative acceptor strength of phenol toward a series of bases, versus other Lewis acids, can be illustrated by C-B plots.
It can react at both its oxygen or carbon sites as an ambident nucleophile (see HSAB theory).
Generally, oxygen attack of phenoxide anions is kinetically favored, while carbon-attack is thermodynamically preferred (see Thermodynamic versus kinetic reaction control).
Mixed oxygen/carbon attack and by this a loss of selectivity is usually observed if the reaction rate reaches diffusion control.
Phenol exhibits keto-enol tautomerism with its unstable keto tautomer cyclohexadienone, but only a tiny fraction of phenol exists as the keto form.
The equilibrium constant for enolisation is approximately 10, which means only one in every ten trillion molecules is in the keto form at any moment.
The small amount of stabilisation gained by exchanging a C=C bond for a C=O bond is more than offset by the large destabilisation resulting from the loss of aromaticity.
Phenol therefore exists essentially entirely in the enol form.
Under normal circumstances, phenoxide is more reactive at the oxygen position, but the oxygen position is a "hard" nucleophile whereas the alpha-carbon positions tend to be "soft".
However, phenol's ring is so strongly activated—second only to aniline—that bromination or chlorination of phenol leads to substitution on all carbon atoms ortho and para to the hydroxy group, not only on one carbon.
Phenol reacts with dilute nitric acid at room temperature to give a mixture of 2-nitrophenol and 4-nitrophenol while with concentrated nitric acid, more nitro groups get substituted on the ring to give 2,4,6-trinitrophenol which is known as picric acid.
Aqueous solutions of phenol are weakly acidic and turn blue litmus slightly to red.
Phenol is neutralized by sodium hydroxide forming sodium phenate or phenolate, but being weaker than carbonic acid, it cannot be neutralized by sodium bicarbonate or sodium carbonate to liberate carbon dioxide.
- C6H5OH + NaOH → C6H5ONa + H2O
This is an example of the Schotten–Baumann reaction:
- C6H5OH + C6H5COCl → C6H5OCOC6H5 + HCl
- C6H5OH + Zn → C6H6 + ZnO
- C6H5OH + CH2N2 → C6H5OCH3 + N2
When phenol reacts with iron(III) chloride solution, an intense violet-purple solution is formed.
Because of phenol's commercial importance, many methods have been developed for its production, but only the cumene process is the dominant technology.
Accounting for 95% of production (2003) is the cumene process, also called Hock process.
It involves the partial oxidation of cumene (isopropylbenzene) via the Hock rearrangement: Compared to most other processes, the cumene process uses relatively mild conditions and relatively inexpensive raw materials.
For the process to be economical, both phenol and the acetone by-product must be in demand.
In 2010, worldwide demand for acetone was approximately 6.7 million tonnes, 83 percent of which was satisfied with acetone produced by the cumene process.
A route analogous to the cumene process begins with cyclohexylbenzene.
Via the Hock rearrangement, cyclohexylbenzene hydroperoxide cleaves to give phenol and cyclohexanone.
Cyclohexanone is an important precursor to some nylons.
Oxidation of benzene and toluene
The direct oxidation of benzene to phenol is theoretically possible and of great interest, but it has not been commercialized:
- C6H6 + O → C6H5OH
Nitrous oxide is a potentially "green" oxidant that is a more potent oxidant than O2.
Routes for the generatation of nitrous oxide however remain uncompetitive.
- C6H5CH3 + 2 O2 → C6H5OH + CO2 + H2O
The reaction is proposed to proceed via formation of benzyoylsalicylate.
Early methods relied on extraction of phenol from coal derivatives or the hydrolysis of benzene derivatives.
Hydrolysis of benzenesulfonate
The conversion is represented by this idealized equation:
- C6H5SO3H + 2 NaOH → C6H5OH + Na2SO3 + H2O
Hydrolysis of chlorobenzene
- C6H5Cl + NaOH → C6H5OH + NaCl
- C6H5Cl + H2O → C6H5OH + HCl
These methods suffer from the cost of the chlorobenzene and the need to dispose of the chloride by product.
Phenol is also a recoverable byproduct of coal pyrolysis.
In the Lummus Process, the oxidation of toluene to benzoic acid is conducted separately.
Phenyldiazonium salts hydrolyze to phenol.
The method is of no commercial interest since the precursor is expensive.
- C6H5NH2 + HCl/NaNO2 → C6H5OH + N2 + H2O + NaCl
Salicylic acid decarboxylates to phenol.
The major uses of phenol, consuming two thirds of its production, involve its conversion to precursors for plastics.
Phenol was once widely used as an antiseptic, its use pioneered by Joseph Lister (see History section).
From the early 1900s to the 1970s it was used in the production of carbolic soap.
Concentrated phenol liquids are commonly used for permanent treatment of ingrown toe and finger nails, a procedure known as a chemical matrixectomy.
The procedure was first described by Otto Boll in 1945.
Since that time it has become the chemical of choice for chemical matrixectomies performed by podiatrists.
Phenol in medicinal formulation is also used as a preservative in some vaccines.
Phenol spray, usually at 1.4% phenol as an active ingredient, is used medically to help sore throat.
Phenol is so inexpensive that it attracts many small-scale uses.
It is a component of industrial paint strippers used in the aviation industry for the removal of epoxy, polyurethane and other chemically resistant coatings.
Runge called phenol "Karbolsäure" (coal-oil-acid, carbolic acid).
Coal tar remained the primary source until the development of the petrochemical industry.
In 1841, the French chemist Auguste Laurent obtained phenol in pure form.
In 1836, Auguste Laurent coined the name "phène" for benzene; this is the root of the word "phenol" and "phenyl".
In 1843, French chemist Charles Gerhardt coined the name "phénol".
Lister decided that the wounds themselves had to be thoroughly cleaned.
He then covered the wounds with a piece of rag or lint covered in phenol, or carbolic acid as he called it.
The skin irritation caused by continual exposure to phenol eventually led to the introduction of aseptic (germ-free) techniques in surgery.
Joseph Lister was a student at University College London under Robert Liston, later rising to the rank of Surgeon at Glasgow Royal Infirmary.
Lister experimented with cloths covered in carbolic acid after studying the works and experiments of his contemporary, Louis Pasteur in sterilizing various biological media.
Lister was inspired to try to find a way to sterilize living wounds, which could not be done with the heat required by Pasteur's experiments.
In examining Pasteur's research, Lister began to piece together his theory: that patients were being killed by germs.
He theorized that if germs could be killed or prevented, no infection would occur.
Lister reasoned that a chemical could be used to destroy the micro-organisms that cause infection.
Meanwhile, in Carlisle, England, officials were experimenting with a sewage treatment, using carbolic acid to reduce the smell of sewage cess pools.
Having heard of these developments and having himself previously experimented with other chemicals for antiseptic purposes without much success, Lister decided to try carbolic acid as a wound antiseptic.
He had his first chance on August 12, 1865, when he received a patient: an eleven-year-old boy with a tibia bone fracture which pierced the skin of his lower leg.
Ordinarily, amputation would be the only solution.
However, Lister decided to try carbolic acid.
After setting the bone and supporting the leg with splints, Lister soaked clean cotton towels in undiluted carbolic acid and applied them to the wound, covered with a layer of tin foil, leaving them for four days.
When he checked the wound, Lister was pleasantly surprised to find no signs of infection, just redness near the edges of the wound from mild burning by the carbolic acid.
Reapplying fresh bandages with diluted carbolic acid, the boy was able to walk home after about six weeks of treatment.
Phenol was the main ingredient of the Carbolic Smoke Ball, an ineffective device marketed in London in the 19th century as protection against influenza and other ailments, and the subject of the famous law case Carlill v Carbolic Smoke Ball Company.
Second World War
The toxic effect of phenol on the central nervous system, discussed below, causes sudden collapse and loss of consciousness in both humans and animals; a state of cramping precedes these symptoms because of the motor activity controlled by the central nervous system.
It was originally used by the Nazis in 1939 as part of the Aktion T4 euthanasia program.
The Germans learned that extermination of smaller groups was more economical by injection of each victim with phenol.
Phenol injections were given to thousands of people.
Approximately one gram is sufficient to cause death.
Phenol is a normal metabolic product, excreted in quantities up to 40 mg/L in human urine.
It is also one of the chemical compounds found in castoreum.
This compound is ingested from the plants the beaver eats.
Occurrence in whisky
This amount is different from and presumably higher than the amount in the distillate.
Rhodococcus phenolicus is a bacterium species able to degrade phenol as sole carbon sources.
Phenol and its vapors are corrosive to the eyes, the skin, and the respiratory tract.
Its corrosive effect on skin and mucous membranes is due to a protein-degenerating effect.
Repeated or prolonged skin contact with phenol may cause dermatitis, or even second and third-degree burns.
Inhalation of phenol vapor may cause lung edema.
The kidneys may be affected as well.
There is no evidence that phenol causes cancer in humans.
Since phenol is absorbed through the skin relatively quickly, systemic poisoning can occur in addition to the local caustic burns.
Resorptive poisoning by a large quantity of phenol can occur even with only a small area of skin, rapidly leading to paralysis of the central nervous system and a severe drop in body temperature.
The LD50 for oral toxicity is less than 500 mg/kg for dogs, rabbits, or mice; the minimum lethal human dose was cited as 140 mg/kg.
The Agency for Toxic Substances and Disease Registry (ATSDR), U.S. Department of Health and Human Services states the fatal dose for ingestion of phenol is from 1 to 32 g.
Removal of contaminated clothing is required, as well as immediate hospital treatment for large splashes.
Phenol is also a reproductive toxin causing increased risk of abortion and low birth weight indicating retarded development in utero.
Main article: Phenols
Thus, phenols are a class of organic compounds of which the phenol discussed in this article is the simplest member.
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Phenol.