Unit 2 Phenol, Aromatic Amines and Aromatic Acids.pdf

1
Phenol, Aromatic Amines
and Aromatic Acids
• Presented by
• Mr. Sandesh R. Sul,
M. Pharm Research Scholar
Department of Pharmaceutical Chemistry
Unit No. 02

Method of Preparation
1. From Chlorobenzene (Dow Process): This involves the hydrolysis of chlorobenzene with
aqueous NaOH at high temperature and pressure followed by treatment with dilute HCL.
Reaction:
This process was first introduced in 1928 by the Dow Chemical Company of USA.
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2. From Cumene: This involves air-oxidation of cumene followed by treatment with
dilute HCL.
Reaction:
The above Cumene Process account for 80% of the total world production of phenol.
The success of this method is due to the availability of benzene and propene from
petroleum and t the formation of acetone, a valuable by- product.
3. From Sodium Benzenesulfonate: This involves fusion of Sodium Benzenesulfonate
with solid NaOH at 300 C followed by treatment with dilute HCL.
Reaction:
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4. From Benzenediazonium Salt: This reaction can be performed easily in the laboratory
and simply requires warming a solution of benzenediazonium chloride, prepared from
aniline, on a water bath at 50 C.
Reaction:
The Phenol is recovered by steam distillation and extracted with diethyl ether.
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5. From Coal Tar: Coal Tar provides a natural source of phenol and cresol, but
nowadays provides less than 10% of the total supply. The middle oil fraction of Coal
Tar contains phenols, cresol and naphthalene. The oil when cooled, deposits solid
naphthene which is removed by centrifuging the mixture. The oil left is agitated with
NaOH solution when phenol and cresols dissolve as sodium salt.
Reaction:
C6H5OH + NaOH -----C6H5O-Na+ + H2O
The phenol are recovered from the above solution by passing carbon dioxide through it.
2C6H5O-Na+ + CO2 + H2O------------ C6H5OH +NaCO3
Phenol is finally isolated from the resulting mixture of phenols by fractional distillation.
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Chemical Reaction of Phenol:
1. Formation of Salt: Phenol is acidic. It reacts with sodium hydroxide or sodium metal
to form salt.
Phenol is weaker acid than carboxylic acids. It does not react with sodium carbonate
or bicarbonate.
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2. Halogenation: Phenol reacts with Bromine water to give precipitate of 2,4,6-
tribromophenol. Chlorine reacts in the same way.
If the reaction is carried in CS2 or CCL4 (Non-polar solvent), a mixture of o- and p-
bromophenol is formed.
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3. Nitration: Phenol reacts with dilute nitric acid to give a mixture of o- and p-
nitrophenol.
With con. Nitric acid to formed Picric acid.
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4. Sulfonation Reaction: When phenol is treated with con. sulfuric acid at 20 C, o-
phenolsulfonic acid is the main product. At 100 C, p- phenolsulfonic acid is the main
product.
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5. Reimer-Tiemann Reaction: This involves the treatment of phenol with chloroform
in aqueous sodium hydroxide solution followed by acid-hydrolysis. Salicylaldehyde is
formed.
If carbon tetrachloride is used in place of chloroform, salicylic acid is formed.
The Reimer-Tiemann reaction is also given by other phenols and introduces –CHO
group in the ortho position.
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6. Kolbe Reaction: This involves the treatment of sodium phenoxide with carbon dioxide
at 125 C under 6 atmosphere of pressure followed by acid-hydrolysis. Salicylic acid is
formed.
7. Reaction with Benzenediazonium Chloride: Phenol reacts Benzenediazonium
Chloride in an alkaline solution to form p- hyroxyazobenzene.
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8. Reaction with Phthalic Anhydride: Phenol reacts with phthalic anhydride in the
presence of sulfuric acid to form phenolphthalein.
9. Reaction with Formaldehyde: When phenol is treated with an alkaline solution of
formaldehyde, a mixture of o- and p- hydroxybenzyl alcohol is formed.
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Acidity of Phenol
• Phenols are more acidic than alcohols but less than carboxylic acids or even carbonic
acid.
• This is indicated in order of acidity constants:
• Phenols are acidic due to the formation of stable phenoxide ions in aqueous solution.
• Phenol itself gives phenoxide ion on dissociation.
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Formula Ka (Approx)
Phenol Ar-OH 10-19
Alcohol R-OH 10-18
Carboxylic acid R-COOH 10-5
Carbonic acid H2CO3 10-7

• Phenoxide ion is stable due to resonance:
• The negative charge is spared through the benzene ring, and thereby effectively
dispersed. This charge delocalization is a stabilizing factor in the phenoxide ion. On
the other hand, no resonance is possible in alkoxide ions (RO-) derived from
alcohols. The negative charge is con. on a single oxygen atom. Consequently, alcohols
are much weaker acids than phenols.
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Effect of Substitutes on Acidity:
1. Effect of Electron- Withdrawing Substituents: An electron-withdrawing group on the
aromatic ring is acid-strengthening.
• Ex: NO2, Cl, CN, CHO, COOH.
It enables the ring to withdraw more electron from the phenoxy oxygen. This stabilize the
phenoxide ions still further and results in a stronger acid.
Ex: p- nitrophenol is more acidic than phenol.
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Resonance forms of p- nitrophenoxide ion

2. Effect of electron- Releasing Substituents: An electron-releasing group on the
aromatic ring is acid-weakening.
• Ex: CH3, OCH3, NH2.
It strengthens the negative charge on phenoxy oxygen and inhibits the charge
delocalization due to resonance. The destabilizes the phenoxide ion and results in a
weaker acid.
Ex: p- cresol is less acidic than phenol.
The relative acid-strengths of some phenols are as follows:
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Qualitative Test
There are various tests performed to identify phenolic group in a compound:
1. LITMUS PAPER TEST: Since phenols are acidic in nature, hence they turn blue
litmus paper into red.
2. SOLUBILITY TEST: Phenols are weakly acidic in nature; hence they are soluble
in sodium hydroxide but insoluble in sodium carbonate or sodium bicarbonate.
3. FERRIC CHLORIDE TEST: When an aqueous solution of phenol reacts with
ferric chloride solution, it gives dark-coloured complexes. The colour may be
violet, red, blue or green.
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4. LIBERMANN'S TEST: Phenol reacts with concentrated sulphuric acid and
sodium nitrite forming a yellow-coloured quinone monoxime complex, which
further reacts with excess phenol and sulphuric acid to give deep blue indophenol
complex. After reacting with sodium hydroxide, it forms sodium salt of indophenol.
5. Bromine Water: When Bromine water is added to the aqueous solution of phenol a
white precipitate of Tribromphenol is formed.
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Uses of Phenol
• Used in the manufacture of drugs (e.g., aspirin, salicylic acid).
• As an antiseptic and disinfectant (in low concentration).
• Used to prepare resins such as Bakelite and phenol-formaldehyde resin.
• Used in the production of dyes, plastics, and explosives (like picric acid).
• Used as a laboratory reagent and chemical intermediate.
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Structure and Uses
Cresol
Disinfectant and Antiseptic:
Used in disinfectants like Lysol and Cresol solution for cleaning surfaces and
instruments.
Preservative:
Used as a wood preservative (in creosote).
Chemical Intermediate:
Used in the manufacture of resins, dyes, and pharmaceuticals.
Antioxidants and Plasticizers:
Used in making antioxidants for rubber and plasticizers for plastics.
Pesticide and Germicide:
Acts as an ingredient in insecticides and fungicides.
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Resorcinol
• Pharmaceuticals:
Used in the preparation of antiseptic, antifungal, and acne-treatment creams (e.g., ointments for skin diseases like
eczema and psoriasis).
• Hair Dyes and Cosmetics:
Used as an ingredient in hair dyes, hair tonics, and skin-lightening creams.
• Adhesives and Resins:
Used in the manufacture of resorcinol–formaldehyde resins, which are strong adhesives used in rubber and wood
bonding.
• Chemical Intermediate:
Used to synthesize dyes, flavors, and UV-absorbing agents.
• Analytical Reagent:
Used in laboratories as a chemical reagent for analytical and research purposes.
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Naphthol
• Dye Industry:
Used in the manufacture of azo dyes and other coloring agents.
• Pharmaceuticals:
Used as intermediates in the preparation of antiseptics, antioxidants, and
medicines.
• Antiseptic:
β-Naphthol is used as a disinfectant and antiseptic agent.
• Rubber Industry:
Used in the manufacture of antioxidants and stabilizers for rubber.
• Laboratory Reagent:
Used in chemical analysis and research work.
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Aromatic Amines
• These are the derivatives of aromatic hydrocarbon in which a hydrogen of the
benzine ring has been replaced by amino group, -NH2.
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Physical Properties
Physical State and Appearance
Lower aromatic amines (like aniline) are colourless oily liquids when pure.
On exposure to air and light, they become brown due to oxidation.
Higher aromatic amines are solid.
Odor: Aromatic amines have a characteristic unpleasant (fishy) Odor.
Solubility
They are sparingly soluble in water due to the presence of a large hydrophobic aromatic ring.
They are readily soluble in organic solvents like alcohol, ether, and benzene.
Boiling Point: Aromatic amines have higher boiling points than hydrocarbons of similar molecular weight
due to hydrogen bonding between amine molecules.
Example: Aniline: b.p. = 184 °C
Colour Change on Storage
They often darken on exposure to air or light due to oxidation forming coloured impurities (like quinone
imines).
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• All such compound in which an amino or substituted amino group is bonded directly to
an aromatic ring are termed Aromatic Amines.
• Types of Aromatic Amines:
1. Primary Aromatic Amines
2. Secondary Aromatic Amines
3. Tertiary Aromatic Amines
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Methods of Preparation
1. Reeducation of Nitro Group: This is a vary convenient and most widely used method
of preparing aromatic amines. The reduction is carried with (1) H2 in the presence of Ni,
Pd, or Pt as catalyst; (2) Sn or Fe and HCL; (3) Lithium aluminum hydride.
Selective reduction with ammonium sulphide is used to prepare an amino compound such
as m-nitoaniline form m-dinitrobenzene when one NO2 group need be reduced.
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2. Ammonolysis of Aryl Chloride: Aniline is prepared by treating chlorobenzene with
ammonia in the presence of copper salts at high temp. and pressure.
3. Hofmann Rearrangement: When treated with bromine or Sodium Hydroxide
aromatic amines.
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4. Action of Hydroxylamine with Hydrocarbon: In this reaction Hydroxylamine react
with Aromatic Hydrocarbon in the presence of FeCl3 (Catalyst) to give Aromatic
Amines.
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Chemical Reaction
1. Salt Formation: Aromatic Amines reacts with acid such as Hydrochloric acids to form
Phenyl Ammonium Chloride.
2. Reaction with Alkylation: Halogenation with bromine or with chlorine is extremely
rapid. When bromine water or chlorine water is added to aniline at room temperature, a
white precipitate of 2,4,6-tribromo-or 24,6-trichloroaniline is immediately formed.
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3. Acylation: Aromatic Amines reacts with Acetyl Chloride to gives Acetanilide.
4. Reaction with Benzaldehyde: Aromatic Amines reacts with benzaldehyde to give
benzaliniline.
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5. Oxidation: Aromatic Amines are readily oxidized to give products depending on
conditions.
Aniline on oxidation with potassium dichromate and sulfuric acid gives p-
benzoquinone.
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Basicity of Amines
• Aromatic amines are basic in nature due to the presence of lone pair of electrons on
nitrogen atom.
• As we know, according to Lewis's theory, Substance which can donte lone pair of
electron are basic in nature that’s why aromatic amines behaver as Lewis Base.
• Also, when Aromatic amines reacts with acids they give salt.
• The basicity of an Aromatic amines is a measure of a compounds ability to accept a
proton (H+). Aniline accepts a proton as follows:
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• In aliphatic amines, the nonbinding electron pair of N atom is localized and fully
available for coordination with a proton. On the other hand, in aromatic amines, the
nonbonding electron pair is delocalized into the benzene ring by resonance.
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Effect of Substituents
1. Electron Withdrawing Group:
2. Electron Releasing Group:
1. Electron Withdrawing Group: Electron withdrawing group decreases the electron
density on aromatic amine, hence they dents to decrease the basicity of Aromatic Amines.
Ex: NO2, NR3, CN etc.
2. Electron Releasing Group: Electron releasing group increasers the election density on
aromatic amines, hence tends to increases the basicity of Aromatic amines.
Ex: CH3, OH etc.
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Uses
1. In Dye Industry
• Aromatic amines like aniline, toluidine, and naphthylamine are key starting materials for azo dyes, indigo dyes, and aniline dyes.
→ Example: Aniline → Aniline yellow, Methyl orange, Congo red.
2. In Pharmaceutical Industry
• Used as intermediates in the synthesis of many drugs such as:
• Sulfa drugs (e.g., sulphanilamide)
• Antipyretics (e.g., acetanilide, paracetamol)
• Local anaesthetics (e.g., benzocaine)
• Antimalarials (e.g., chloroquine)
3. In Rubber Industry
• Aromatic amines (like p-phenylenediamine) are used as antioxidants and vulcanization accelerators in rubber
processing.
4. In Pesticides and Herbicides
• Serve as precursors for the preparation of insecticides, herbicides, and fungicides, e.g., DDT, carbaryl, and 2,4-D.
5. In Polymer and Plastic Industry
• Used in manufacturing polyurethane foams, epoxy resins, and aromatic polyamides (Kevlar).
6. As Analytical Reagents
• Certain aromatic amines (like diphenylamine) are used as oxidation–reduction indicators and for nitrate detection.
7. In Photography and Perfume Industry
• Used in synthesis of colour developers and aromatic compounds for perfumes.
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Aromatic Acids
• Aromatic acids are compounds in which one or more carboxyl group (-COOH) are
attached directly to the aromatic ring.
• They are named by common system or as derivatives of the parent benzoic acid.
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Physical Properties
1. Physical State and Appearance
• Most aromatic carboxylic acids (like benzoic acid)
are colorless crystalline solids.
• Some substituted aromatic acids may be oily
liquids.
2. Odor and Taste
• They generally have a slightly sweet or pungent
odor and acidic taste.
• Some derivatives (like salicylic acid) have a
characteristic medicinal smell.
3. Solubility
• Slightly soluble in water, because of the large
hydrophobic benzene ring.
• Solubility increases in hot water and in organic
solvents such as alcohol, ether, and benzene.
• Their sodium or potassium salts are highly water-
soluble.
4. Melting and Boiling Points
• Aromatic acids have high melting and boiling
points due to strong intermolecular hydrogen
bonding.
• Example: Benzoic acid → M.P. 122 °C, b.p. 249
°C
5. Acidity
• They are acidic in nature due to the –COOH group
attached to the aromatic ring.
• The carboxylic hydrogen is ionizable and forms
carboxylate ions in solution.
• Acidity is influenced by substituents on the ring:
• Electron-withdrawing groups (–NO₂, –Cl) ↑
acidity
• Electron-releasing groups (–OH, –CH₃) ↓
acidity
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Method of Preparation of Aromatic Acids
1. By oxidation of alkylbenzenes with acids potassium permanganate or sodium
dichromate.
2. By acid-hydrolysis of phenyl cyanide (benzonitrile).
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3. By basic-hydrolysis of Benzotrichloride: Benzotrichloride is obtained by chlorination
of toluene.
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Chemical Reaction
1. Salt Formation: Benzoic acid reacts with sodium hydroxide or sodium bicarbonate to from
sodium benzoate.
2. Ester Formation: It reacts with alcohols in the presence of sulfuric acid catalyst to form ester.
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3. Acyl halide Formation: Benzoic acid reacts with phosphorus pentachloride or thionyl
chloride to form benzoyl chloride.
4. Reduction to Benzyl alcohol: Benzoic acid undergoes reduction with lithium
aluminum hydride to give benzyl alcohol.
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5. Decarboxylation: When heated with soda-lime (CaO/NaOH), benzoic acid loses a
molecule of carbon dioxide to yield benzene.
6. Electrophilic Substitution: The benzene ring of benzoic acid undergoes the usual
electrophilic substitution reaction. Recall that the –COOH group is a meta director and
deactivating.
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Uses
1. As a Food Preservative
• Most important use.
• Used as preservative (E210) in foods, beverages, and fruit juices.
• Prevents growth of bacteria, yeast, and fungi.
• Its sodium salt (sodium benzoate) is commonly used for this purpose.
2. In Pharmaceutical Industry
• Used in the manufacture of:
• Benzocaine (local anesthetic)
• Benzyl benzoate (used for scabies and lice treatment)
• Benzoic acid ointments (antifungal and antiseptic preparations)
3. In Perfume and Cosmetic Industry
• Used as an intermediate in production of benzyl alcohol and benzaldehyde, which are used in
perfumes and cosmetics.
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4. In Plastic and Resin Industry
• Used in the synthesis of alkyd resins, plasticizers, and nylon intermediates.
5. As Laboratory Reagent
• Used as a standard for calibration in various chemical and analytical procedures.
6. In Dye and Pesticide Manufacture
• Acts as a starting material for preparation of benzoyl chloride, which is used in dye, pesticide,
and pharmaceutical synthesis.
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Acidity of Aromatic Acids
• Aromatic acids are organic acids in which the carboxyl group (–COOH) is directly
attached to an aromatic ring (usually benzene).
Example: Benzoic acid (C₆H₅COOH).
• The acidity of aromatic acids depends on the stability of the carboxylate ion (–COO⁻)
formed after loss of a proton (H⁺).
Reason for Acidity
• They donate H⁺ from the –COOH group.
• After losing H⁺, the carboxylate ion is resonance-stabilized.
• Aromatic ring can influence acidity through electron-withdrawing or electron-donating
substituents.
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Effect of Substituents on the Ring
• Substituents change acidity by inductive effect (±I) and resonance effect (±R).
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1. Electron-Withdrawing Groups:
Decrease the electron density on aromatic ring that ultimately
increases the stability of ring and hence Increase acidity. Stabilize
carboxylate ion.
Examples: –NO₂, –CN, –F, –Cl, –SO₃H
Order of acidity (stronger → weaker):
p-Nitrobenzoic acid > m-Nitrobenzoic acid > o-Nitrobenzoic acid >
Benzoic acid
Why para > meta > ortho for EWG?
Para and meta positions withdraw inductively.
Para shows additional resonance stabilization.
Ortho is affected by steric hindrance, not pure resonance.
2. Electron-Donating Groups:
Increases the electron density on Aromatic ring that ultimately
decreases the stability of ring and hence Decrease acidity.
Examples: –OH, –OCH₃, –NH₂, –CH₃
Order (weak acids):
Anisic acid (p-OCH₃) < p-Cresol acid < p-Toluidic acid <
Benzoic acid

IMP Question
1. Draw resonance structures of phenol and aniline.
2. Write ring substitution reactions of aromatic amines; effect of substituents on basicity.
3. Give any two methods of preparation and reactions of benzoic acid.
4. Explain effect of substituents on acidity of phenols.
5. What are phenols? Explain acidity; give three preparations and three reactions.
6. Write qualitative tests for phenols.
7. Explain acidity of aromatic acids/benzoic acid and their reactions.
8. What are aryl diazonium salts? Give applications.
9. Write synthetic uses of aryl diazonium salts.
10. Give halogenation and Reimer–Tiemann reaction of phenol.
11. Predict the product when benzoic acid reacts with PCl₅ and SOCl₂.
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Unit 2 Phenol, Aromatic Amines and Aromatic Acids.pdf

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