Phenol
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 (-C6H5) bonded to a hydroxyl group (-OH). It is mildly acidic, but requires careful handling due to its propensity to cause burns.
Phenol was first extracted from coal tar, but today is produced on a large scale (about 7 billion kg/year) 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 ring such as benzene or other arene ring. Consequently, phenols have greater acidity than alcohols due to stabilization of the conjugate base through resonance in the aromatic ring.
Nomenclature
By definition, phenol is hydroxybenzene. Phenol is a common name for the compound. Its IUPAC name would be benzenol, derived in the same manner as the IUPAC names for aliphatic alcohols.
When a phenol molecule is substituted with additional groups, either the ortho, meta, para system or the numbering system can be employed. In either case, if the parent molecule is referred to as a phenol, the nomenclature being used is the common system.
In IUPAC nomenclature, the parent molecule is called benzenol, and substituents are always numbered with the OH group being given the understood first position. For the compounds below, the first name listed is the common name and the second is the IUPAC name.
Certain phenols are referred to by common names. For example, methyl phenols are called cresols. In the illustrations below, the first name under each compound is its common name, and the bottom name is its IUPAC name.
Similarly, hydroxyphenols have common names, which are listed first under each of the following illustrations, while the IUPAC names are listed last.
Physical properties
Low molecular weight phenols are normally liquids or low melting solids. Due to hydrogen bonding, most low molecular weight phenols are water‐soluble. Phenols tend to have higher boiling points than alcohols of similar molecular weight because they have stronger intermolecular hydrogen bonding.
Acidity
Phenol is weakly acidic and at high pHs gives the phenolate anion C6H5O− (also called phenoxide):
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
Resonance structures of the phenoxide anion
The phenoxide anion has a similar nucleophilicity to free amines, with the further advantage that its conjugate acid (neutral phenol) 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.
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:[15]
C6H5OH + Zn → C6H6 + ZnO
When phenol is reacted with diazomethane in the presence of boron trifluoride (BF3), anisole is obtained as the main product and nitrogen gas is released:
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 (2003), involves the partial oxidation of cumene (isopropylbenzene) via the Hock rearrangement:
C6H5CH(CH3)2 + O2 → C6H5OH + (CH3)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 (Raschig–Hooker process):
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 (in impure form) from coal tar. 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;[23] 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 (1827–1912) 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 (germ-free) 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 prot