 Dyes definition: Dyes are colored organic compounds that are used

to color various substances like fabrics, paper, food, hair drugs, etc.
 Regarding their solubility, organic colorants fall into two classes, viz.
dyes and pigments. The key distinction is that dyes are soluble in
water and/or an organic solvent, while pigments are insoluble in both
types of liquid media. Dyes are used to color substrates to which they
have affinity. Pigments can be used to color any polymeric substrate
but by a mechanism quite different from that of dyes, in that surface,
only coloration is involved unless the pigment is mixed with the
polymer before fiber or molded article formation.
 To be used dye must possess these four properties.
(i) Color
(ii) Solubility in water and/or an organic solvent.
(iii) Ability to be absorbed and retained by fiber (substantivity) or to be
chemically combined with it (reactivity).
(iv) Ability to withstand washing, dry cleaning, and exposure to light.
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 Classification of Dyes
1. based on source:
(a) Natural dyes: Natural dyes are dyes or colorants derived from plants,
invertebrates, or minerals. The majority of natural dyes are vegetable dyes
from plant sources—roots, berries, bark, leaves, and wood—and other
biological sources such as fungi and lichens.
There are two types of natural dyes. Additive dyes (non-substantive) such
as madder must use a mordant (a chemical that fixes a dye) to bond with
fibers. These are the most common type and have been used for at least
2,000 years. Substantive dyes require no pretreatment to the fabric (e.g.,
indigo, orchil, and turmeric) and there are three types: direct dye (for
cotton, e.g., turmeric, safflower); acid dye (for silk and wool, e.g., saffron,
lac) or basic dye (for silk and wool, e.g., berberine). Mordants are
chemical compounds that combine with the fiber and the dye forming a
chemical bridge between the two. Common mordants are weak organic
acids, such as acetic or tannic acid, and metal salts including aluminum
ammonium or potassium sulfate, ferrous sulfate, and copper sulfate.
Usually, the textile to be dyed is simmered in a mordant solution before
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 dyeing (pre-mordanting). Other options include adding the mordant to the
dye bath or treating it with another mordant after dyeing to shift the color.
 Natural mordant dyes are either monogenetic or polygenetic; monogenetic
dyes produce only one colour irrespective of mordant, whereas polygenetic
dyes produce different colours according to the mordant employed (e.g.,
logwood, alizarin, fustic, and cochineal).
 Disadvantages of Natural Dyes
Before the advent of synthetic dyes, natural dyes were widely used, often
together with mordants such as alum, to dye natural fibres including wool,
linen, cotton, and silk, but their use declined after the discovery of synthetic
dyes. However, interest in natural dyes has been revived owing to increasing
demands on manufacturers to produce more environmentally friendly
alternatives to petrochemical-derived dyes. One main issue associated with
the use of natural dyes in the coloring of textiles is their poor to moderate
light-fastness, and despite their long tradition, not all natural dyes are
especially environmentally friendly. Some natural dyes have no or little
affinity for the textile materials and they require heavy-metal salts as
mordants for fixation and color-fastness. Natural dyes may be sustainable
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 but they need water and land to produce and there is insufficient dye yield
per acre of plant material to sustain industrial-scale production.

 Synthetic Dyes: Synthetic dyes are manufactured from organic molecules.

Before synthetic dyes were discovered in 1856, dyestuffs were
manufactured from natural products but batches of natural dye were never
exactly alike in hue and intensity, whereas synthetic dyestuffs can be
manufactured consistently. The use of computers and computer color
matching (CCM) produces color that is identical from batch to batch.
Discovery of the first synthetic dye
William Henry Perkin, an eighteen-year-old English chemist, was searching for
a cure for malaria, a synthetic quinine, and accidentally discovered the first
synthetic dye. He found that the oxidation of aniline could color silk. From a
coal tar derivative, he made a reddish-purple dye. The brilliant purple was
called mauve. The dye was not stable to sunlight or water and faded easily to
the color presently named mauve, a pale purple. This discovery resulted in
additional research with coal tar derivatives and other organic compounds and
an entire new industry of synthetic dyes was born. In the twenty-first century,
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synthetic dyes are less expensive, have better colorfastness, and completely
dominate the industry as compared with natural dyes. Thousands of distinctly
different synthetic dyes are manufactured in the world.
2. based on chromophore: Dyes may be classified according to the
chromophore present in their structures.
(a) Azo dyes: Azo dyes are characterized by the presence in the molecule of
one or more azo groups —N = N—, which form bridges between organic
residues, of which at least one is usually an aromatic nucleus. Many
methods are available for preparing azo compounds, but the manufacture
of azo dyes is always based on the coupling of diazonium compounds
with phenols, naphthols, arylamines, pyrazolones, or other suitable
components to give hydroxyazo or aminoazo compounds or their
tautomeric equivalents. In the resulting dyes, the azo group is the
chromophore, and the hydroxyl or amino group is an auxochrome. The
importance of azo dyes is shown by the fact that they account for over
60% of the total number of dye structures known to be manufactured. A
full range of shades is available, but on hydrophilic fibres, the blues and
greens lack fastness to light unless they are metalized; the metalized
derivatives have dull shades. The chemistry of these dyes ranges from
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 simple mono-azo compounds to complex polyazo structures with a
molecular weight of 1800 or more, and their properties vary accordingly.

(b) Triphenylmethane Dyes: Synthetic organic dyes having molecular

structures based upon that of the hydrocarbon triphenylmethane. They
have poor resistance to light and chemical bleaches and are used chiefly in
copying papers, in hectograph and printing inks, and in textile applications
for which lightfastness is not an important requirement.
The triphenylmethane derivatives are among the oldest man-made dyes, a
practical process for the manufacture of fuchsine having been developed in
1859. Several other members of the class were discovered before their
chemical constitutions were fully understood. Crystal violet, the most
important of the group, was introduced in 1883.
The range of colors is not complete but includes reds, violets, blues, and
greens. They are applied by various techniques, but most belong to the
basic class, which are adsorbed from solution by silk or wool, but have
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 little affinity for cotton unless it has been treated with a mordant such as
tannin.

(c) Phthalein dyes are a class of dyes mainly used as pH indicators, due to
their ability to change colors depending on pH. They are formed by the
reaction of phthalic anhydride with various phenols. They are a subclass
of triarylmethane dyes.
• Common phthalein dyes include:
• o-Cresolphthalein
• Dixylenolphthalein
• Guaiacolphthalein
• α-Naphtholphthalein
• Phenolphthalein
• Tetrabromophenolphthalein

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c. based on the method of application:
Reactive Dyes, Acid Dyes, Premetallized Dyes, Direct Dyes, Azoic (Napthol)
Dyes, Disperse Dyes, Vat Dyes, Sulfur Dyes, Basic Dyes.
(i) Reactive dyes are the most recent of dyes. These are the most popular in
the world among fiber and fabric artists, used at first only by surface designers,
but recently by weavers as well. There are now reactive dyes for a wide range
of fibres, e.g. cotton (PROCYON), silk, and wool (PROCILAN). The dye
reacts with the fibre molecules to form colour and is, as a result, extremely fast
to both light and washing. There are hot and cold water-reactive dyes there is a
dye for almost every need. They can be most successfully used for silk painting,
with a much better colour fastness than the traditional basic dyes, and are
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already used by batik artists. we can identify a reactive dye by the alkali used to
set off the fixation process, which requires time to take place (silk and wool
reactive use acetic acid). Assistants used are salt, soda ash resist salt, and
sometimes bicarbonate of soda and urea. Reactive dyes are equally suited to
screen printing polychromatic printing, fabric painting yarn, and piece dyeing.
(ii) Acid Dyes are acidified basic dyes, intended for use on protein fibres but
can be used on nylon and acrylics. They have a fair light fastness but poor wash
fastness
(iii) Premetallized Dyes are acid dyes with the addition of one or two
molecules of chromium. The dyes give mute tonings, not unlike those of natural
dyes. They are the synthetic dyes mostly used by weavers who dye their yarns.

(iv) Direct Dyes are substantive dyes colour cellulose fibres directly in a hot
dyebath without a mordant, to give bright colours. They are not very fast to
light or washing. Direct dyes are generally any dyes that use salt as their only
fixative, e.g. Dylon dyes (not to be confused with reactive dyes, which use salt
plus other chemicals).

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(v) Azoic Dyes are another sort of direct dye, but ones that are extremely fast
to wash, bleach, and light. They are intended for cellulose fibres and can be
used successfully on protein fibres, although the colours are different. These
dyes are widely used all over Asia and Australia for batik and direct application.
They can be used to give interesting texture colour effects on fabric, thread, or
paper. Their use for straight silk painting is minimal because of the difficulty in
achieving the evenness of the painted colour.

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(vi) Disperse Dyes originally developed for acetate fibres, these are now the
major dyes for synthetics. They are not soluble in water but in the actual fibres
themselves. They require a carrier to swell the fibres so that the finely ground
particles can penetrate. They are dyed hot, like direct dyes, but do not use salt.
Disperse dyes are widely used for heat transfer printing (Polysol). Dye is
printed or painted onto paper and heat-pressed onto fabric. Prints have excellent
light and wash fastness and strong bright colours. Their major disadvantage is
that only synthetic fabrics can be used.
(vii) Vat dyes are water-insoluble dyes Vat dyes are insoluble in water and
have to be dissolved in water by using sodium hydroxide and sodium hydrogen
sulphite usually at 50 degrees for 15 20 minutes. Vat dyes are the most
important dyes for dyeing and printing on cotton and cellulosic fibres They
have excellent all round fastness, which includes washing, light, perspiration,
chlorine, and rubbing fastnesses. VAT dyes are especially fast to light and
washing. Brilliant colors can be obtained in most shades. Originating in
medieval Europe, vat dyes were so named because of the vats used in the
reduction of indigo plants through fermentation.
(viii) Basic Dyes are very bright, but not very fast to light, washing, or
perspiration. Fastness is improved if they are given an after-treatment or
steaming, e.g. French Silk dyes are basic dyes and should be steamed to fix11.
Chemistry of Dyeing
 The process of applying color to fiber stock, yarn, or fabric is called dyeing.
There may or may not be thorough penetration of the colorant into the fibers or
yarns.
The dyeing of a textile fiber is carried out in a solution, generally aqueous,
known as the dye liquor or dye bath.
For true dyeing to have taken place, coloration of fabric and absorption are
important determinants.
Coloration: The coloration must be relatively permanent: that is not readily
removed by rinsing in water or by normal washing procedures. Moreover, the
dyeing must not fade rapidly on exposure to light.
Absorption: The process of attachment of the dye molecule to the fiber is one
of absorption: that is the dye molecules concentrate on the fibre surface. There
are four kinds of forces by which dye molecules are bound to the fiber:

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1) Ionic forces 2) Hydrogen bonding 3) Vander Wals' forces and 4) Covalent
chemical linkages.
 Exhaustion: In any dyeing process, whatever the chemical class of dye being
used, heat must be supplied to the dye bath; energy is used in transferring dye
molecules from the solution to the fiber as well as in swelling the fiber to render
it more receptive. The technical term for this process is exhaustion.
Levelness: An Important Quality, evenness of dyeing, known as levelness is an
important quality in the dyeing of all forms of natural and synthetic fibers.
It may be attained by the control of dyeing conditions.

Conditions to Attain Levelness

By agitation to ensure proper contact between dye liquor and the substance
being dyed and by use of restraining agents to control the rate of dyeing or
strike. Solvent Dyeing Serious consideration has recently been given to the
methods of dyeing in which water as the medium is replaced by solvents such
as the chlorinated hydrocarbons used in dry cleaning. The technological
advantages of solvent dyeing are:
1. Rapid wetting of textiles
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2. Less swelling
3. Increased speed of dyeing per given amount of material
4. Savings in energy, as less heat is required to heat or evaporate per-
chloroethylene. Thus it eliminates the effluent (pollution) problems
associated with the conventional methods of dyeing and finishing.

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Methyl Orange

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 Preparation:
Step-1. The first step is to dissolve sulfanilic acid in a basic solution.

Step-2. Formation of Nitrosonium Ion

Step-3. Formation of Diazotized Sulphanilic Acid

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Step-4. In addition to N, N-Dimethylaniline
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Overall preparation of methyl orange

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 Malachite Green Dye
 It is a type of triphenylmethane dye.
 bright green in color.
 can be applied directly to wool & silk.
 by addition of mordant, can be applied to cotton.
USES:
 As spot test reagent for detection of sulphuric acid and cerium.
 As an antiseptic for bacterial infections.

Special Points
 Triphenylmethane is usually colorless: known as leuco base.
 Upon oxidation, the leuco base forms the carbinol base.
 Reaction with acid, and carbinol base forms colored dyes.
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 leuco base (colorless)→ carbinol base(colorless) → dye(colored)
Preparation

 During the reaction, 2 moles of Dimethylaniline react with 1 mole of

benzaldehyde yielding a leucobase form of malachite green. Upon oxidation
of this leucobase, a carbinol base is formed, whose acidification yields a
quinoid form of malachite green dye which is colored in nature. Whereas
leucobase and carbinol bases are colorless.
 The color of the quinoid form is due to the conjugation present in it.

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 Rosaniline Dye
 Rosaniline hydrochloride is one of the major constituents of Basic
fuchsin, together with pararosanilin, magenta II, and new fuchsin.
 Fuchsine (sometimes spelled fuchsin) or rosaniline hydrochloride is a
magenta dye with the chemical formula C20H19N3·HCl. There are other
similar chemical formulations of products sold as fuchsine, and several
dozen other synonyms of this molecule.
 It becomes magenta when dissolved in water; as a solid, it forms dark green
crystals. As well as dying textiles, fuchsine is used to stain bacteria and
sometimes as a disinfectant. It is well established that the production of
fuchsine results in the development of bladder cancers by production
workers. Production of magenta is listed as a circumstance known to result
in cancer.
 When sulfur dioxide is passed through it we get a colorless solution called
Schiff’s Reagent. Schiff regent is used for testing the presence of
aldehydes.

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 Rosaniline has a green metallic luster in solid form and a red color in
aqueous media.
 It dyes wool, silk, and cotton.

Synthesis of Rosaniline

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 Phenolphthalein Dye
 Phenolphthalein is an organic compound used as a laboratory reagent and
pH indicator.
 Phenolphthalein exerts laxative effects by stimulating the intestinal mucosa
and constricting smooth muscles. However, phenolphthalein is no longer used
as a laxative due to the suspected carcinogenicity of this compound.
 Phenolphthalein is often used as an indicator in acid–base titrations. For
this application, it turns colorless in acidic solutions and pink in basic solutions.
It belongs to the class of dyes known as phthalein dyes.
 Phenolphthalein is slightly soluble in water and usually is dissolved in
alcohol for use in experiments.
 Phenolphthalein adopts four different states in an aqueous solution: Under
very strongly acidic conditions, it exists in a protonated form (HIn +), providing
an orange coloration. Between strongly acidic and slightly basic conditions, the
lactone form (HIn) is colorless. The doubly deprotonated (In 2-) phenolate form
(the anion form of phenol) gives the familiar pink color. In strongly basic
solutions, phenolphthalein is converted to its In(OH)3− form, and its pink color
25
undergoes a rather slow fading reaction and becomes completely colorless
above 13.0 pH.

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27
 Synthesis of Phenolphthalein

Phenolphthalein is synthesized by electrophilic aromatic substitution of phthalic

anhydride and 2 equivalents of phenol in the presence of concentrated
methanesulfonic acid at 90 °C to yield the product, phenolphthalein.

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 ALIZARIN DYE
The compound alizarin comes from the root of the madder plant. It is used as a
dye to color fabrics. Alizarin dates back to the tenth century when it was used in
Asia Minor. Ancient Egyptians, Romans, and Greeks also used the madder root
for its bright red color. It was used by the British Army to make their ''red coats''
red during the American Revolution. Alizarin can also be used as a pH
indicator. Although it had been extracted from the root of the madder plant for
many centuries, chemists Graebe and Liebermann figured out how to make
alizarin in a laboratory in 1868. This eliminated the need for the cultivation of
the madder root plant and wiped out an entire industry dedicated to growing the
plants.

Alizarin is an orangish red powder. It has a melting point of 287 °C and a

sublimation point (when a substance turns directly from solid to gas) of 430 °C.
It will partially dissolve in water. Alizarin will catch fire in the presence of a
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flame. Alizarin can be irritating if it is inhaled, gets into the eyes, or touches
skin.
Structure Elucidation of Alizarin
 Alizarin's chemical formula is C14H8O4.
 From the reactions of the alizarin (conversion to anthracene and
anthraquinone it is clear that it is a derivative of the anthraquinone.

 The formation of disodium salt and a diacetyl derivative suggests the

presence of the 2 hydroxyl groups in alizarin.
 When it undergoes vigorous oxidation, it gives phthalic acid with no
hydroxyl group. This shows that both the hydroxyl groups are present in the

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 Alizarin is obtained when catechol is condensed with the phthalic anhydride
in the presence of the anhydrous Aluminium chloride or conc sulphuric acid

o
at 140-150 C.
 So this shows that the alizarin should have the 2 hydroxyl groups at the
adjacent positions.
 Out of these structures, 2nd & 3rd structures are ruled out. Alizarin is either
structure 1 or 4.

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 Mild oxidation of alizarin gives purpurin, a trihydroxyanthraquinone. 
Structure 1 gives two isomers of trihydroxy anthraquinone whereas Structure
4 gives one trihydroxy derivative.
 Further purpurin on vigorous oxidation gives phthalic acid. That means the 3
hydroxyl groups are present in the same ring.
 Further purpurin on vigorous oxidation gives phthalic acid. That means the 3
hydroxyl groups are present in the same ring.
 When phthalic anhydride is condensed with hydroquinone and the product is
oxidized with the manganese dioxide, we get purpurin. This shows that it is
1,2,4-trihydroxyanthraquinone.

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33
 Alizarin on nitration gives 2 isomeric mono-nitro derivatives.
 The 2 nitro derivatives are only possible from the 1,2

Hence, it follows that alizarin has the structure 1,2-dihydroxy anthraquinone.

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 Synthesis of Alizarin
 The starting material for the alizarin is anthraquinone. Anthraquinone can be
obtained from Phthalic anhydride.

Anthraquinone to Alizarin

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36
 INDIGOTIN DYE

Indigo is an ancient compound and has been known and used as a

distinctive blue dye since prehistoric times. The earliest users were from
India, and the country gained its English name from the ancient Greek word
for indigo. Historically, indigo played an important role in the economies of
many countries because natural blue dyes are rare. Today it will be most
familiar to you as the dye used to color blue jeans.

The chemical in indigo which is responsible for the blue colour is indigotin,
which is a dark blue powder at room temperature and is insoluble in water
and ethanol. It is most soluble in chloroform, nitrobenzene, and sulphuric
acid.
Indigotin White: Indigo in its synthesized or purified form is insoluble in
water and other polar solvents. To overcome this problem, the dye is reduced
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to soluble leucoindigo (known as 'white indigo') and applied to clothes in this
form. When exposed to atmospheric oxygen it re-oxidizes to the insoluble
form and regains its colour. Originally this reduction was done with urine,
although synthetic urea replaced it in the 19th century and later sodium
hydrosulfite was employed as a much more effective reducing agent. Once the
problem of applying the dye to the clothing had been overcome the
insolubility of the dye is of course beneficial - it will not wash out of the

 Extraction: Indigo has been known since ancient times and originally came
from a plant extract. Plants of the Indigofera genus contain a glycoside called
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indican in their leaves and stems, which is extracted, and acidhydrolyzed into
indoxyl. Mild oxidation in atmospheric oxygen will then produce indigo.

Synthesis: Dissolve 1 g. of o-nitrobenzaldehyde in 3 ml of pure acetone, add

about an equal volume of water, which leaves a clear solution, and then, drop
by drop, sodium hydroxide solution. Heat is developed and the solution
becomes dark brown. After a short time, the dye separates into crystalline
flakes. Collect the precipitate at the pump after five minutes and wash it, first
with alcohol and then with ether. Indigo so prepared is especially pure and
has a beautiful violet lustre.
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 Structure Elucidation
1. Empirical and molecular formula: From the elemental analysis, the
empirical formula of indigo is found to be C8H5ON. Vapor density
determination reveals that its molecular formula is C16H10O2N2.
2. Degradation of Indigo:
(i) The vigorous oxidation of indigo with nitric acid forms two molecules of
Isatin.

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(ii)On distillation with zinc dust at high temperatures, indigo gives a new
product known as Indole.

Inference: From these two reactions, it follows that there exists a close
structural similarity among indigo, isatin, and indole. Further indigo and
indoxyl are structurally related to each other because of indoxyl on
oxidation yiels indigo.

3. Structure of Isatin:
(i) Molecular formula: C8H5O2N.
(ii) The action of PCl5: The reaction of Isatin with PCl5 indicates the presence
of a hydroxyl group in isatin.

(ii) Action with hydroxylamine: Isatin forms oxime with hydroxylamine

which indicates the presence of a carbonyl group in isatin.

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(iii) Action with Alkali: On boiling with an alkali like NaOH, it forms
oaminobenzoyl formic acid indicating that isatin has structure I. The
reaction involved is represented as under:

(iv) Synthesis of Isatin: The structure I for isatin has further been confirmed
by its synthesis from o-nitrobenzoyl chloride.

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4. Structure of Indigo: Based on the above structure of Isatin, Indigo may
possess either of the following structures (II, III, and IV). All of them when
oxidized yield two molecules of Isatin.

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Out of these structures, structure II is found to be correct because indigo
when hydrolyzed with dil. NaOH yields anthranilic acid and indoxyl-
2aldehyde.

5. Synthesis: The structure of indigo has been further confirmed by the

various syntheses of indigo.

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45