Phenol, also known as carbolic acid, is an aromatic organic compound with the molecular
formula C6H5OH. It is a white crystalline solid that is volatile. The molecule consists
of a phenyl group bonded to a hydroxyl group. It is mildly acidic, but requires careful
handling due to its propensity to cause chemical burns. Phenol was first extracted from coal
tar, but today is produced on a large scale from petroleum. It is an important industrial
commodity as a precursor to many materials and useful compounds. Its major uses involve
its conversion to plastics or related materials. Phenol and its chemical derivatives are key
for building polycarbonates, epoxies, Bakelite, nylon, detergents, herbicides such as phenoxy
herbicides, and numerous pharmaceutical drugs. Although similar to alcohols, phenols have
unique distinguishing properties. Unlike in alcohols where the hydroxyl group is bound
to a saturated carbon atom, in phenols the hydroxyl group is attached to an unsaturated
aromatic hydrocarbon ring such as benzene. Consequently, phenols have greater acidity
than alcohols due to stabilization of the conjugate base through resonance in the aromatic
ring.
Properties Phenol is appreciably soluble in water, with
about 84.2 g dissolving in 1000 mL. Homogeneous mixtures of phenol and water at phenol to
water mass ratios of ~2.6 and higher are also possible. The sodium salt of phenol, sodium
phenoxide, is far more water soluble. Acidity
Phenol is weakly acidic and at high pHs gives the phenolate anion C6H5O−:
PhOH ⇌ PhO− + H+ (K = 10−10)
Compared to aliphatic alcohols, phenol is about 1 million times more acidic, although
it is still considered a weak acid. It reacts completely with aqueous NaOH to lose H+, whereas
most alcohols react only partially. Phenols are less acidic than carboxylic acids, and
even carbonic acid. One explanation for the increased acidity
over alcohols is resonance stabilization of the phenoxide anion by the aromatic ring.
In this way, the negative charge on oxygen is delocalized on to the ortho and para carbon
atoms. In another explanation, increased acidity is the result of orbital overlap between the
oxygen's lone pairs and the aromatic system. In a third, the dominant effect is the induction
from the sp2 hybridised carbons; the comparatively more powerful inductive withdrawal of electron
density that is provided by the sp2 system compared to an sp3 system allows for great
stabilization of the oxyanion. The pKa of the enol of acetone is 10.9, comparable
to that for phenol. The acidities of phenol and acetone enol diverge in the gas phase
owing to the effects of solvation. About 1/3 of the increased acidity of phenol is attributable
to inductive effects, with resonance accounting for the remaining difference.
Phenoxide anion
The phenoxide anion has a similar nucleophilicity to free amines, with the further advantage
that its conjugate acid does not become entirely deactivated as a nucleophile even in moderately
acidic conditions. Phenols are sometimes used in peptide synthesis to "activate" carboxylic
acids or esters to form activated esters. Phenolate esters are more stable toward hydrolysis
than acid anhydrides and acyl halides but are sufficiently reactive under mild conditions
to facilitate the formation of amide bonds. Tautomerism
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 10−13, meaning that 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 entirely in the enol form.
Phenoxides are enolates stabilised by aromaticity. 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". Reactions
Phenol is highly reactive toward electrophilic aromatic substitution as the oxygen atom's
pi electrons donate electron density into the ring. By this general approach, many groups
can be appended to the ring, via halogenation, acylation, sulfonation, and other processes.
However, phenol's ring is so strongly activated—second only to aniline—that bromination or chlorination
of phenol leads to substitution on all carbons ortho and para to the hydroxy group, not only
on one carbon. Aqueous solution of phenol is weakly acidic
and turns blue litmus slightly to red. Phenol is easily neutralized by sodium hydroxide
forming sodium phenate or phenolate, but it being weaker than carbonic acid cannot be
neutralized by sodium bicarbonate or sodium carbonate to liberate carbon dioxide
C6H5OH + NaOH → C6H5ONa + H2O When a mixture of phenol and benzoyl chloride
when shaken in presence of dilute sodium hydroxide solution, phenyl benzoate is formed. This
is an example of Schotten-Baumann reaction: C6H5OH + C6H5COCl → C6H5OCOC6H5 + HCl
Phenol is reduced to benzene when it is distilled with zinc dust or its vapour is passed over
granules of zinc at 400 °C: C6H5OH + Zn → C6H6 + ZnO
When phenol is reacted with diazomethane in the presence of boron trifluoride, anisole
is obtained as the main product and nitrogen gas
C6H5OH + CH2N2 → C6H5OCH3 + N2 Production
Because of phenol's commercial importance, many methods have been developed for its production.
The dominant current route, accounting for 95% of production, involves the partial oxidation
of cumene via the Hock rearrangement: C6H5CH(CH3)2 + O2 → C6H5OH +2CO
Compared to most other processes, the cumene-hydroperoxide process uses relatively mild synthesis conditions,
and relatively inexpensive raw materials. However, to operate economically, there must
be demand for both phenol, and the acetone by-product.
An early commercial route, developed by Bayer and Monsanto in the early 1900s, begins with
the reaction of a strong base with benzenesulfonate: C6H5SO3H + 2 NaOH → C6H5OH + Na2SO3 + H2O
Other methods under consideration involve: hydrolysis of chlorobenzene, using base or
steam: C6H5Cl + H2O → C6H5OH + HCl
direct oxidation of benzene with nitrous oxide, a potentially "green" process:
C6H6 + N2O → C6H5OH + N2 oxidation of toluene, as developed by Dow
Chemical: C6H5CH3 + 2 O2 → C6H5OH + CO2 + H2O
In the Lummus Process, the oxidation of toluene to benzoic acid is conducted separately.
Phenol is also a recoverable byproduct of coal pyrolysis.
Uses The major uses of phenol, consuming two thirds
of its production, involve its conversion to precursors to plastics. Condensation with
acetone gives bisphenol-A, a key precursor to polycarbonates and epoxide resins. Condensation
of phenol, alkylphenols, or diphenols with formaldehyde gives phenolic resins, a famous
example of which is Bakelite. Partial hydrogenation of phenol gives cyclohexanone, a precursor
to nylon. Nonionic detergents are produced by alkylation of phenol to give the alkylphenols,
e.g., nonylphenol, which are then subjected to ethoxylation.
Phenol is also a versatile precursor to a large collection of drugs, most notably aspirin
but also many herbicides and pharmaceutical drugs. Phenol is also used as an oral anesthetic/analgesic
in products such as Chloraseptic or other brand name and generic equivalents, commonly
used to temporarily treat pharyngitis. Niche uses
Phenol is so inexpensive that it attracts many small-scale uses. It once was widely
used as an antiseptic, especially as carbolic soap, from the early 1900s to the 1970s. It
is a component of industrial paint strippers used in the aviation industry for the removal
of epoxy, polyurethane and other chemically resistant coatings.
Phenol derivatives are also used in the preparation of cosmetics including sunscreens, hair colorings,
and skin lightening preparations. Concentrated phenol liquids are commonly used
in the surgical treatment of ingrown toenails to prevent a section of the toenail from growing
back. This process is called phenolization. History
Phenol was discovered in 1834 by Friedlieb Ferdinand Runge who extracted it from coal
tar. Runge called phenol "Karbolsäure". 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". The antiseptic properties of phenol were used
by Sir Joseph Lister in his pioneering technique of antiseptic surgery. 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 substitution of aseptic techniques
in surgery. Phenol is the active ingredient in some oral
analgesics such as Chloraseptic spray and Carmex.
Phenol was the main ingredient of the Carbolic Smoke Ball, an ineffective device marketed
in London in the 19th century as protecting against influenza and other ailments, and
the subject of the famous law case Carlill v Carbolic Smoke Ball Company.
Second World War Injections of phenol were used as a means
of individual execution by the Nazis during the Second World War. It was originally used
by the Nazis in 1939 as part of Action T4. Although Zyklon-B pellets were used in the
gas chambers to exterminate large groups of people, the Nazis learned that extermination
of smaller groups was more economical via injection of each victim with phenol. Phenol
injections were given to thousands of people, especially at Auschwitz-Birkenau. Approximately
one gram is sufficient to cause death. One of the best known inmates to be executed with
a phenol injection in Auschwitz was St. Maximilian Kolbe, a Catholic priest who volunteered to
undergo two weeks of starvation and dehydration in the place of another inmate.
Natural occurrences Temporal glands secretion examination showed
the presence of phenol and 4-methylphenol during musth in male elephants.
It is also one of the chemical compounds found in castoreum. This compound is gathered from
the plants the beaver eats. Occurrence in whisky
Phenol is a measurable component in the aroma and taste of the distinctive Islay scotch
whisky, generally ~30 ppm, but can be over 150 ppm in the malted barley used to produce
whisky. Biodegradation
Cryptanaerobacter phenolicus is a bacterium species that produces benzoate from phenol
via 4-hydroxybenzoate. Rhodococcus phenolicus is a bacterium species able to degrade phenol
as sole carbon sources. Toxicity
Phenol and its vapors are corrosive to the eyes, the skin, and the respiratory tract.
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 substance may cause harmful
effects on the central nervous system and heart, resulting in dysrhythmia, seizures,
and coma. The kidneys may be affected as well. Long-term or repeated exposure of the substance
may have harmful effects on the liver and kidneys. There is no evidence that phenol
causes cancer in humans. Besides its hydrophobic effects, another mechanism for the toxicity
of phenol may be the formation of phenoxyl radicals.
Chemical burns from skin exposures can be decontaminated by washing with polyethylene
glycol, isopropyl alcohol, or perhaps even copious amounts of water. Removal of contaminated
clothing is required, as well as immediate hospital treatment for large splashes. This
is particularly important if the phenol is mixed with chloroform. Phenol is also a reproductive
toxin causing increased risk of abortion and low birth weight indicating retarded development
in utero. Phenols
The word phenol is also used to refer to any compound that contains a six-membered aromatic
ring, bonded directly to a hydroxyl group. Thus, phenols are a class of organic compounds
of which the phenol discussed in this article is the simplest member.
See also Bamberger rearrangement
Claisen rearrangement Cresols
Fries rearrangement Polyphenol
References
External links International Chemical Safety Card 0070
Phenol Material Safety Data Sheet National Pollutant Inventory: Phenol Fact
Sheet NIOSH Pocket Guide to Chemical Hazards
CDC - Phenol - NIOSH Workplace Safety and Health Topic
IARC Monograph: "Phenol" Arcane Radio Trivia outlines competing uses
for Phenol circa 1915