Phenol — also known as carbolic acid — is an aromatic organic compound with t he molecular formula C6H5OH.
It is a white c rystalline 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.[4] Its major uses involve its conversion to plastics or relate d 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,[5] in phenols the hydroxyl group is attached to an unsaturated ring such as benzene or other arene ring.[6] Consequently, phenols have greater acidity than alcohols due to stabilization of the conjugate base thr ough resonance in the aromatic ring. Properties:
Phenol is appreciably soluble in water, with about 8.42 g dissolving in 100 mL (0.88 M). 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. Properties
Molecular formula
C6H6O
Molar mass
94.11 g mol
Appearance
Transparent crystalline solid
Odor
Sweet and tarry
Density
1.07 g/cm
Melting point
40.5 °C, 314 K, 105 °F
Boiling point
181.7 °C, 455 K, 359 °F
Solubility in water
8.3 g/100 mL (20 °C)
−1
3
Acidity (pK a)
9.95 (in water), 29.1 (in acetonitrile)
[2]
[1]
λmax
270.75 nm
Dipole moment
1.7 D
Acidity: Phenol is weakly acidic and at high pH's gives t he phenolate anion C6H5O− (also called phenoxide):*7]
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 car boxylic 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.[8] In another explanation, increased acidity is the result of orbital overlap between the oxygen's lone pairs and the aromatic system.[9] 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.[10] The acidities of phenol and acetone enol diverge in the gas phase owing to the effects o f solvation. About 1/3 of the increased acidity of phenol is attributable to inductive effects, with re sonance accounting for the remaining difference. Reactions:
Neutral phenol substructure "shape". An image of a computed electrostatic surface of neutral phenol, showing neutral regions in green, elect ronegative areas in orange-red, and the electropositive phenolic proton in blue. Phenol is highly reactive toward electrophilic aromatic substitution as the o xygen atom's pi electrons donate electron density into the ring. By t his 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 e asily 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 whe n 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
Uses: The major uses of phenol, consuming two t hirds 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.[4] 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[edit] Phenol is so inexpensive that it attracts many sma ll-scale uses. It once was widely used as an antiseptic, especially as carbolic soap, from the early 1900s through 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.[18] Phenol derivatives are also used in the preparation of co smetics including sunscreens,[19] hair colorings, and skin lightening preparations.[20]
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. Production[edit]
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:[4] 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 ec onomically, 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, be gins with the reaction of a strong base with benzenesulfonate:[16] C6H5SO3H + 2 NaOH → C6H5OH + Na2SO3 + H2O
Other methods under consideration involve: hydrolysis of chlorobenzene, using base or steam (Raschig –Hooker process):[17] 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 to luene to benzoic acid is conducted separately. Phenol is also a recoverable byproduct of coal pyrolysis
Phenol can be manufactured from Benzene using several ways • Benzene hydrochlorination to form Benzyl chloride followed by hydrolysis of benzyl chloride to form
phenol. • Benzene chlorination to form benzyl chloride which is tr ansformed to sodium benzoate and eventually
to phenol using NaOH and HCl • Benzene sulfonate process: In this proc ess, benzene is convered to benzene sulfonate using sulphuric
acid and eventually through neutralization, fusion and acidification, the benzene sulfonate is gradually transformed to phenol. • In this lecture, we restrict our discussion to the manufacture of phenol from • Benzene hydrochlorination route • Benzene from chlorobenzene route
Phenol using Hydro chlorination route Reactions First reactio • Benzene + HCl + Oxygen → Benzyl chloride + Water • Catalyst: FeCl3 + CuCl2 • Operating conditions: 240°C and atmospheric pressure
Second reaction • Benzyl chloride + water → Phenol + HCl • Catalyst: SiO2 • Here, HCl is regenerated and will be recycled. • Operating conditions: 350°C and atmospheric pressure • In this process, Benzene is used to extract phenol from phenol +water mixture. This unit is termed as
an extraction unit (liquid liquid extraction principle). Therefore, this unit takes up fresh benzene and phenol + water mixture and produces two streams namely water stream (bottom product) and benzene + phenol stream (top product). The water stream is fed to a scrubber unit (i.e., Unit B that will be described later).
• Then onwards, the organic mixture is fe d to a distillation column that produces purer benzene as the
top product. The bottom product is phenol with other impurities. • The bottom phenol rich product is sent t o the phenol fractionator to obtain waste product as to p
product and pure phenol as bottom product. • The purer benzene then enters the hydrochlorination reactor in which a mixture of HCl and O2 is fed
at 220°C. Under these conditions, Benzene will be also in vapour state. • Therefore, the reactor is a gas solid reactor. • The conversions are pretty low and not more than 20% of the benzene is converted to benzyl chloride. • Eventually, the products are sent to two fractionators that separate unreacted benzene, cr ude benzyl
chloride and poly benzyl chlorides as various products. The unreacted benzene is sent back to the hydrochlorination reactor as a recycle stream. • The crude benzyl chlo ride then enters an absorber unit A where phenol is used to purify the benzyl
chloride from other organic compounds (such as benzene and polybenzyl chlorides). • The purified benzyl chloride stream then enters the hydrolysis reactor in which water is passed along
with benzyl chloride over the silica catalyst. The re actor itself is a furnace with catalyst loaded in t he tubes and hot fuel gases are circulated in the shell to obtain the desired higher temperature. • Under these conditions, both reactants are in vapour state (with the benzyl chloride boiling point of
179°C) and therefore, the reaction is also a gas solid reaction. • After hydrolysis reaction, the product vapors are sent to a partial condenser that separates the HCl
from the organic phase. • The HCl is recycled to the hydrochlorination reactor.
• The phenol rich product stream is sent as a solvent for the scrubber (unit A) that purifies crude benzyl
chloride to purer benzyl chloride. The bottom product from the scrubber (i.e., unit A) enters another scrubber (unit B) that receives water from the extractor. • The unit B enables washing of the phenol to remove any water soluble impurities. The water from the
unit B enters the hydrolysis reactor.
