By Mrs. V.L. Semalty P.G.T. Chemistry K.V. No1 H.B.K. Dehradun PHENOLS By Mrs. V.L. Semalty P.G.T. Chemistry K.V. No1 H.B.K. Dehradun

Phenols (Ar-OH) Phenols are compounds with an –OH group attached to an aromatic carbon. Although they share the same functional group with alcohols, where the –OH group is attached to an aliphatic carbon, the chemistry of phenols is very different from that of alcohols.

Any simple radical that has а common name may be used in the alkyl alcohol system, with one important exception. The grouping С6Н5 - has the special name phenyl, but the compound C6H5OH is phenol, not phenyl alcohol. phenol

By defination,phenol is hydroxybenzene By defination,phenol is hydroxybenzene.Phenol is common name for the compound. Its IUPAC name would be benzenol.Substituted phenols are named as derivatives of the parent compound phenol. However, phenyl substituted alkyl alcohols are normal alcohols and often have common names. Certain phenols are referred to by common names. For example, methyl phenols are called cresols Examples are:

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 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 (1827–1912) in his pioneering technique of antiseptic surgery. The skin irritation caused by continual exposure to phenol eventually led to the introduction of aseptic (anti-septic, germ-free) techniques in surgery.

Nomenclature Phenols are usually named as substituted phenols. The methylphenols are given the special name, cresols. Some other phenols are named as hydroxy compounds.

Naming Phenols Name substituent's on aromatic ring by their position from -OH

Questions on Nomenclature: Q. Name the simplest hydroxyl derivative of benzene. Ans: Phenol. Q. What is the IUPAC name of anisole? Ans: Methoxybenzene. Q. Write the chemical name of cumene. Ans: Isopropyl benzene. Q. Name the compound which is also known as carbolic acid. Ans: Phenol

Naturally Occurring Phenols. Phenols are common in nature. α-tocopherol (vitamin E) resveratrol

Preparation of Phenols

Physical properties of phenols All phenols have peculiar smell. They are colorless compounds but oxygen from the air can cause brown colour of phenols (oxidation). Most low molecular weight phenols are soluble in water. Reason-Due to hydrogen bonding, soluble in a polar solvent such as water. The physical properties of phenols are strongly influenced by the hydroxyl group, which permits phenols to form hydrogen bonds with other phenol molecules and with water . Electron delocalization in phenoxide is represented by resonance among the structures:

Boiling & Melting Point of Phenols Phenols have higher melting points and boiling points and are more soluble in water than arenes and aryl halides of comparable molecular weight. Some ortho-substituted phenols, such as o-nitrophenol, have significantly lower boiling points than those of the meta and para-isomers. This is because the intramolecular hydrogen bond that forms between the hydroxyl group and the substituent partially compensates for the energy required to go from the liquid state to the vapor. In p-nitrophenol,molecular association takes place due to extensive hyrogen bonding between the molecues.

Intramolecular hydrogen bonding It is possible in some ortho-substituted phenols. This intramolecular hydrogen bonding reduces water solubility and increases volatility. Thus, o-nitrophenol is steam distillable while the isomeric p-nitrophenol is not.

QUESTIONS on ACIDIC NATURE Ques. Which one of the following has the lowest pKa? A) B) C) D) Ans- D

Comparison between Phenol and Alcohol’s Chemical Properties Alcohols Phenols HX NR PX3 NR dehydration NR as acids phenols are more acidic ester formation similar oxidation NR

Solubility of phenols Solubility of phenol in water is governed by the hydroxyl group present. The hydroxyl group in phenol is involved in the formation of intermolecular hydrogen bonding. Thus, hydrogen bonds are formed between water and phenol molecules which make phenol soluble in water. However the aryl group attached to the hydroxyl group is hydrophobic in nature. Thus, the solubility of phenol decreases with the increase in size of aryl group.

Solubility Reactions:

Acidic Nature of Phenols: Phenols react with active metals such as sodium, potassium etc. to form corresponding phenoxide. These reactions of phenols indicate its acidic nature. In phenol, the sp2 hybridized carbon of benzene ring attached directly to the hydroxyl group acts as an electron withdrawing group. Thus, it decreases the electron density on oxygen. Due to the delocalization of negative charge in the benzene ring, phenoxide ions are more stable than alkoxide ions. As a result phenols are more acidic than alcohols. In case of substituted phenols the acidity decreases if an electron donating group is attached to the ring while the acidity increases in case of electron withdrawing group. CH4 < NH3 < HCCH < ROH < H2O < phenols < H2CO3 < RCOOH < HF

Explanation :- (1) In phenol, the -OH group is attached to sp2 hybridised carbon which is more electronegative, hence the -OH bond becomes more polar. (2) Due to resonance is phenol, oxygen gets a positive charge and this increases the polarity of the -OH bond. (3) Delocalisation of negative charge in phenoxide ion makes phenoxide ion more stable than phenol favouring the ionization of phenol.

Effect of substituent groups on acid strength Electron withdrawing groups will decrease the negative charge in the phenoxide, lowering the PE, decreasing the ΔH, shifting the equil farther to the right, stronger acid. Electron donating groups will increase the negative charge in the phenoxide, increasing the PE, increasing the ΔH, shifting the equilibrium to the left, weaker acid.

Ques. Number the following acids in decreasing order of acid strength (let # 1 = most acidic, etc.) Ans. 3 5 1 4 2

(b) Ans. 1 2 3 4

Chemical Reactions of Phenols As acids Ester formation Ether formation Electrophilic Aromatic Substitution reactions

4. Electrophilic Aromatic Substitution reactions a) Nitration b) Sulfonation c) Halogenation d) Friedel-Crafts alkylation e) Friedel-Crafts acylation f) Nitrosation g) Coupling with diazonium salts h) Kolbe’s Reaction i) Reimer-Tiemann Reaction

1. Reaction of Phenols as Acids with active metals: with bases: CH4 < NH3 < HCCH < ROH < H2O < phenols < H2CO3 < RCOOH < HF

Ester formation (similar to alcohols)

NOTE:

Ether formation (Williamson Synthesis) Ar-O-Na+ + R-X  Ar-O-R + NaX note: R-X must be 1o or CH3 Because phenols are more acidic than water, it is possible to generate the phenoxide in situ using NaOH.

Electrophilic Aromatic Substitution The –OH group is a powerful activating group in EAS and an ortho/para director. a) Nitration

b) Halogenation

c) Sulfonation At low temperature the reaction is non-reversible and the lower Eact ortho-product is formed (rate control). At high temperature the reaction is reversible and the more stable para-product is formed (kinetic control).

d) Friedel-Crafts alkylation.

e) Friedel-Crafts acylation

NOTE:

Fries rearrangement of phenolic esters.

f) Nitrosation

g) Coupling with diazonium salts (EAS with the weak electrophile diazonium)

h) Kolbe reaction (carbonation)

i) Reimer-Tiemann reaction

Q. Explain the bromination of phenol forming ortho and para bromophenols with equation. Ans: Phenol reacts with bromine in CS2 at 273 K forming ortho – and para bromophenols respectively.

1) Carboxylation of Phenols: MORE REACTIONS 1) Carboxylation of Phenols: Aspirin and the Kolbe-Schmitt Reaction. Synthesis of salicylic acid (o-hydroxybenzoic acid) from phenol.

2) Preparation of Aryl Ethers. The phenoxide ion is a good nucleophile and reacts with 1° and 2° alkyl halides and tosylates afford aryl ethers (Williamson ether synthesis)

3) Cleavage of Aryl Ethers by Hydrogen Halides. Aryl alkyl ethers can be cleaved by HX to give phenols. 4) Claisen Rearrangement. Thermal rearrangement of an aryl allyl ether to an o-allyl phenol. The Claisen rearrangement involves a concerted, pericyclic mechanism, which is related to the Diels-Alder reaction

5) Oxidation of Phenols: Quinones

Spectroscopy of phenols: Spectroscopy is the study of the interaction between matter and electromagnetic radiation. Infrared: O—H stretching, strong, broad 3200-3600 cm-1 C—O stretch, strong, broad ~1230 cm-1 (alcohols ~ 1050 – 1200) NMR (Nuclear Magnetic resonance): O—H 4-7 ppm (6-12 ppm if intramolecular hydrogen bonding)

USES OF PHENOLS Phenol is used to prepare reagents used in plastic manufacturing industries. The condensation reaction of phenol with acetone produces bisphenol A which is extensively used in polymer industries to synthesis various epoxide resins and polycarbonates. The intermediate produced during the polymerization reaction is called novolac, this is a resin and is used as a binding agent or adhesive in many industries. Novolac is also used for protective coating purposes. Phenol is also used in the study and extraction of bio-molecules. Phenol is also used in cosmetic industry in the manufacturing of sunscreens, skin lightening creams and hair coloring solutions.

Working Safely with Phenol is poisonous, corrosive, and flammable. affects the central nervous system and targets the liver and kidneys. is mutagenic and possibly teratogenic. Routes of exposure include: Inhalation Absorption Ingestion Eyes Use of products containing phenol

Specific Lab Safety Practices Use caution when centrifuging phenol. Centrifugation produces aerosols enhancing exposure via inhalation. If you suspect a tube has broken or a rotor has failed, wait 10 minutes prior to opening the centrifuge and/or rotor lid. This allows aerosolized phenol to settle out. Open them in the fume hood if at all possible. After centrifuging, always open bottles or tubes in a fume hood to prevent exposure to aerosols. Never heat or melt phenol in an incubator, microwave, drying oven or similar device. Have a phenol spill/exposure kit available (List at end of training module)

Storage of Phenol Keep in a tightly closed container Store in a cool, dry, ventilated area away from sources of heat or ignition Store separately from reactive chemicals, combustible materials, and bases. Keep out of direct sunlight Avoid dust formation and control ignition sources Store containers on shelves below eye level

Phenol Waste Disposal Pipet tips, gloves and other contaminated debris should be collected as hazardous waste. Bags are ok for dry solids, as long as the bags are sealed closed and labeled properly and there are no free-flowing liquids. Sharps (needles) must go in puncture-resistant containers. Do not place dry solids cont. with chemicals in red or orange biohaz bags. If the waste is both chemically and biologically contaminated, please contact EHS or Office of Biosafety with questions.

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