2. INTRODUCTION
Contains โOH group that directly attached to an aryl carbon atom(s).
Aromatic compounds in which -OH group is not directly attached to benzene
ring are not phenols but are called aromatic alcohols such as benzyl alcohol.
Phenols are similar to alcohols but form stronger hydrogen bonds
Phenols are more soluble in water than are alcohols and have higher boiling
points.
Phenols occur either as colourless liquids or white solids at room temperature
and may be highly toxic and caustic.
3. NOMENCLATURE OF PHENOLS
Many phenolic compounds were discovered and used long before chemists
were able to determine their structures.
Trivial names (i.e., vanillin, salicylic acid, pyrocatechol, resorcinol, cresol,
hydroquinone, and eugenol) are often used for the most common phenolic
compounds.
4. Phenols with only one other substituent can be named using either the
appropriate numbers or the ortho (1,2), meta (1,3), and para (1,4) system.
Compounds with other principal functional groups can be named with the
hydroxyl group as a hydroxy substituent.
For example, the systematic name for vanillin is 4-hydroxy-3-
methoxybenzaldehyde.
5. Systematic names are more useful, because a systematic name specifies the
actual structure of the compound.
If the hydroxyl group is the principal functional group of a phenol, the
compound can be named as a substituted phenol, with carbon atom 1 bearing
the hydroxyl group.
For example, the systematic name for thymol is 5-methyl-2-isopropylphenol.
7. Hydrolysis of chlorobenzene (the Dow Process)
Benzene is easily converted to chlorobenzene by the Dow Process.
Chlorobenzene is hydrolyzed by a strong base at high temperatures to give a
phenoxide salt, which is acidified to phenol.
8. Oxidation of isopropylbenzene
Benzene is converted to isopropylbenzene (cumene) by treatment with
propylene and an acidic catalyst.
Oxidation yields a hydroperoxide (cumene hydroperoxide), which undergoes
acid-catalyzed rearrangement to phenol and acetone.
Although this process seems more complicated than the Dow process, it is
advantageous because it produces two valuable industrial products: phenol
and acetone.
1. 2.
9. Diazotization of an arylamine
A milder, more general reaction is the diazotization of an arylamine (a
derivative of aniline, C6H5NH2) to give a diazonium salt, which hydrolyzes to a
phenol.
Most functional groups can survive this technique, as long as they are stable in
the presence of dilute acid.
11. Oxidation
Phenols give different types of products from those seen with aliphatic
alcohols.
For example, chromic acid oxidizes most phenols to conjugated 1,4-diketones
called quinones.
In the presence of oxygen in the air, many phenols slowly oxidize to give dark
mixtures containing quinones.
12. Electrophilic aromatic substitution
Phenols are highly reactive toward electrophilic aromatic substitution,
because the nonbonding electrons on oxygen stabilize the intermediate cation.
This stabilization is most effective for attack at the ortho or para position of
the ring; therefore, the hydroxyl group of a phenol is considered to be
activating (i.e., its presence causes the aromatic ring to be more reactive than
benzene) and ortho- or para-directing.
13. Phenoxide ions, generated by treating a phenol with sodium hydroxide, are so
strongly activated that they undergo electrophilic aromatic substitution even
with very weak electrophiles such as carbon dioxide (CO2).
This reaction is used commercially to make salicylic acid for conversion to
aspirin and methyl salicylate.
14. Formation of phenol-formaldehyde resins
Phenol-formaldehyde resins are inexpensive, heat-resistant, and waterproof,
though somewhat brittle.
The polymerization of phenol with formaldehyde involves electrophilic
aromatic substitution at the ortho and para positions of phenol (probably
somewhat randomly), followed by cross-linking of the polymeric chains.
15. Safety Measures
Hazards with Acute Exposure
Contact with eyes may cause severe damage and blindness.
Contact with skin may cause severe burns or systemic poisoning as phenol is readily
absorbed through the skin. Skin exposure may not cause immediate pain as phenol
has a local anesthetic effect.
Systemic effects may occur from any route of exposure, especially after skin
absorption.
Hazards with Chronic Exposure
Repeated or prolonged skin exposure to phenol or vapors from heated phenol may
cause headache, nausea, dizziness, muscle ache, difficulty swallowing, diarrhea,
vomiting, shock, convulsions, and death.
Phenol affects the central nervous system, liver, and kidneys.
Phenol can cause sensitization reactions with repeated exposure.
16. Special Safety Precautions
Phenol should be used with adequate ventilation to minimize inhalation. When
heating phenol, use a water bath inside a chemical fume hood. Never heat or melt
phenol in an incubator, microwave, drying oven, or similar appliance.
Prevent contact with skin by wearing neoprene gloves, lab coat, and chemical
resistant apron.
Wear chemical splash goggles if splashing may occur, such as during dispensing,
heating or transferring phenol.
Store phenol in a cool, dry, well-ventilated area, away from heated surfaces or
ignition sources. Do not store with acids, strong oxidizers, calcium hypochlorite,
and aluminum chloride.
Skin contact requires immediate washing of the affected area with water for at
least 30 minutes. Do not rub or wipe the affected area. Remove contaminated
clothing and dispose.
Spills of undiluted phenol should be considered serious and cleaned up
immediately. Small liquid spills of 50 ml or less may be absorbed using paper
towels, or commercially available absorbent and placed in a sealed container or
double plastic bags. If the spill is larger than 50 ml, remove ignition sources,
provide adequate ventilation, evacuate the laboratory, close the doors, and call the
UC Davis Fire Department at 911.
17. Waste Treatment
DISTILLATION
ADSORPTION AND EXTRACTION
PHENOL REMOVAL BY MEMBRANE PROCESSES
CHEMICAL OXIDATION
ELECTROCHEMICAL OXIDATION
BIOLOGICAL TREATMENT
ENZYMATIC TREATMENT
18. PHENOL REMOVAL BY MEMBRANE PROCESSES
Membrane technologies are reliable and economically feasible to treat phenol
and have many advantages such as low power consumption, high quality
effluent, small footprint, and easy scaling up with membrane modules.
However, consideration must be given to membrane fouling which can occur
due to particles and colloids present in the feed streams.
The most important membrane technologies used to remove phenols from
wastewater are:
Extractive membrane bioreactors and hollow fiber membranes
i.
Photocatalytic membrane reactors
ii.
High-pressure membrane processes such as nanofiltration, reverse osmosis, and
pervaporation
iii.
Membrane distillation.
iv.
19. CHEMICAL OXIDATION
Chemical oxidants provide destructive treatments of aqueous phenols.
The processes have low reagent and energy costs, operating under mild
conditions (temperature and pH) most commonly in the parts per million range
and higher.
Ozone, chlorine, chlorine dioxide, chloramines, ferrate [Fe (VI)], and
permanganate [Mn (VII)] are the most common chemicals applied in oxidative
treatment of wastewater.
Ferrate has the ability to oxidize various contaminants over a wide range of
pH. It reduces to ferric hydroxide which has coagulation/flocculation
properties, thus providing better efficiency.
On the other hand, permanganate is relatively cheap, easy to handle
(including the manganese dioxide sludge produced), stable, and does not form
chlorinated or brominated by-products.
20. An investigation conducted by Peings about the oxidation mechanism of phenol
by sulfatoferrate (VI) and compared it with permanganate and hypochlorite.
The experiment was carried out with 30 mg/L of phenol in alkaline solution
(pH 9) to avoid ferrate degradation. Complete transformation of phenol was
achieved only with excess oxidant.
The reaction of ferrate (VI) and phenol followed first-order kinetics with
respect to both reactants and was not influenced by the presence of
impurities.
Data from a spin-trapping study confirmed formation of phenoxy radical and
hence a radical reaction pathway.
In the case of hypochlorite, various amounts of chlorophenolic by-products
were observed depending on reactant molar ratio.
21. ELECTROCHEMICAL OXIDATION
Electrochemical oxidation is an alternative destructive treatment of aqueous
phenols which has no reagent requirement and cost, but does incur equipment
and energy costs.
As previously reviewed by MartinezโHuitle and Tasic, electrochemical
oxidation techniques are divided into direct and indirect oxidation.
Direct or anodic treatment happens through adsorption of the pollutants onto
the anode surface.
Various anode materials are used with Pt, PbO2, SnO2, IrO2, and BDD (boron-
doped diamond) being the most investigated ones. Parameters such as current
density, pH, anode material, and electrolytes used influence treatment
efficiency.
Degradation of phenol follows pseudo first-order kinetics and the
effectiveness of the process is indicated by apparent current efficiency,
electrochemical oxidation index, or instantaneous current efficiency.
22. Indirect oxidation takes advantage of intermediary redox reagents to effect
electron transfer between electrode and pollutant, thereby preventing
electrode fouling by contaminants.
The presence of chloride ions enhances removal of phenolic compounds
through the formation of Cl2 or ClOโ in a process called electrochemical
oxidation of active chlorine.
23. PHENOL REMOVAL BY UV/H2O2 TREATMENT
Microwave (MW) irradiation is a useful adjunct for UV/H2O2 wastewater
treatment, leading to shorter reaction time, reduction in the activation
energy, smaller equipment size, greater ease of operation, and high product
yield.
For example, a MW irradiation at 2.5 GHz combined with a UV/H2O2 system
improved the oxidative decomposition of phenol. Although a greater amount
of H2O2 was needed for mineralization in aqueous solution, conversion of
phenol and the TOC removal efficiency increased up to 50 %.
