INDOLE

Indole is precursor to tryptophan synthesis; substrate in analytical reagents like Kovac’s (Ehrlich’s) for detecting indoles
Pharmaceuticals & Biochemistry uses of Indole: Core structure in neurotransmitters (e.g., serotonin), plant growth hormones (auxins), and many alkaloids; essential scaffold in medicinal chemistry

CAS Number: 120‑72‑9 
EC Number: 204‑420‑7 
Molecular Formula: C₈H₇N
Molar Mass: 117.15 g/mol 

SYNONYMS:
Indole, 2,3-Benzopyrrole, 1H‑Indole, 2,3‑Benzopyrrole, Benzopyrrole, Ketole, 1‑Benzazole, 1H‑Indol, Indol, 1‑Benzo[b]pyrrole, 1H-Indole, 2,3-Benzopyrrole, ketole, 1-benzazole, indole, 1H-Indole, 120-72-9, 2,3-Benzopyrrole, Indol, 1-Benzazole, Ketole, 1-Azaindene, 2,3-Benzopyrole, Indole (natural), Caswell No. 498B, 1-Benzo(b)pyrrole, FEMA No. 2593, CCRIS 4421, HSDB 599, EPA Pesticide Chemical Code 025000, AI3-01540, NSC 1964, EINECS 204-420-7, UNII-8724FJW4M5, INDOOLUM, DTXSID0020737, CHEBI:16881, 8724FJW4M5, NSC-1964, DTXCID40737, INDOLE (USP-RS), INDOLE [USP-RS], Indole, 3-Benzopyrrole, 1-benzazole, 204-420-7, Benzopyrrole, Indol [German], 1H-Benzo[b]pyrrole, MFCD00005607, Benzo[b]pyrrole, CHEMBL15844, Indole 100 microg/mL in Acetonitrile, NCGC00167539-01, IND, benzazole, CAS-120-72-9, mono-indole, 1-H-indole, Indole, 7, Indole (8CI), 1H-Indole (9CI), INDOLUM [HPUS], INDOLE [FHFI], INDOLE [HSDB], INDOLE [FCC], INDOLE [MI], Indole, >=99%, SCHEMBL698, bmse000097, Indole, analytical standard, SCHEMBL1188, SCHEMBL1538, Indole, >=99%, FG, SCHEMBL20275, SCHEMBL23200, SCHEMBL23314, SCHEMBL23566, SCHEMBL25894, WLN: T56 BMJ, BIDD:GT0304, SCHEMBL448936, SCHEMBL449240, SCHEMBL449275, SCHEMBL449872, SCHEMBL449943, SCHEMBL450257, SCHEMBL450679, SCHEMBL451288, SCHEMBL451350, SCHEMBL451368, SCHEMBL451603, SCHEMBL451836, SCHEMBL451879, SCHEMBL451985, SCHEMBL452761, SCHEMBL453718, SCHEMBL453859, SCHEMBL454496, SCHEMBL454533, SCHEMBL454580, SCHEMBL454701, INDOLE BENZO-PYRROLE, SCHEMBL8316256, SCHEMBL16861354, NSC1964, 185l, HMS5086O16, BCP27232, STR01201, Tox21_112536, Tox21_201677, Tox21_302937, BBL011739, BDBM50094702, s6358, SBB055980, STL163380, AKOS000119629, Tox21_112536_1, AT36838, CG-0501, CS-W001132, DB04532, HY-W001132, Indole, puriss., >=98.5% (GC), NCGC00167539-02, NCGC00167539-03, NCGC00256348-01, NCGC00259226-01, BP-10563, DS-011308, I0021, NS00010849, ST51046571, EN300-18285, C00463, SBI-0653862.0001, Q319541, SR-01000944736, SR-01000944736-1, Z57833933, F2190-0647, InChI=1/C8H7N/c1-2-4-8-7(3-1)5-6-9-8/h1-6,9, 82451-55-6, 1H-Indole, Ketole, 1-Azaindene, 1-Benzazole, 2,3-Benzopyrrole, Benzopyrrole, Indol, 1-H-indol

Indole (C₈H₇N, MW 117.15 g/mol) is a foundational aromatic compound used extensively in perfumery, biochemical synthesis, and industrial chemistry.
While naturally occurring and valuable in complex molecules, Indole requires cautious handling due to toxicological and environmental considerations.

Indole is an organic compound with the formula C6H4CCNH3.
Indole is classified as an aromatic heterocycle.
Indole has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered pyrrole ring.

Indoles are derivatives of indole where one or more of the hydrogen atoms have been replaced by substituent groups.
Indoles are widely distributed in nature, most notably as amino acid tryptophan and neurotransmitter serotonin.
When indole is a substituent on a larger molecule, it is called an indolyl group by systematic nomenclature.

Indole undergoes electrophilic substitution, mainly at position 3.
Substituted indoles are structural elements of (and for some compounds, the synthetic precursors for) the tryptophan-derived tryptamine alkaloids, which includes the neurotransmitter serotonin and the hormone melatonin, as well as the naturally occurring psychedelic drugs dimethyltryptamine and psilocybin.

Other indolic compounds include the plant hormone auxin (indolyl-3-acetic acid, IAA), tryptophol, the anti-inflammatory drug indomethacin, and the betablocker pindolol.
The name indole is a portmanteau of the words indigo and oleum, since indole was first isolated by treatment of the indigo dye with oleum.
indole, a heterocyclic organic compound occurring in some flower oils, such as jasmine and orange blossom, in coal tar, and in fecal matter.

Indole is a naturally occurring compound composed of a six-membered benzene ring fused to a five-membered pyrrole ring.
This structure makes indole an important building block for many other compounds.
Its molecular formula is C8H7N, which signifies that Indole is made up of eight carbon atoms, seven hydrogen atoms, and one nitrogen atom.

This unique molecular structure allows indole to exhibit both aromatic and basic properties, making it highly reactive and versatile in chemical reactions.
Indole is found in many natural sources, including plant and animal products.

Indole is a critical intermediate in the synthesis of tryptophan, an essential amino acid found in most organisms.
Indole also serves as a precursor for the biosynthesis of indole-3-acetic acid (IAA), which is a major plant hormone responsible for regulating growth and development.

USES and APPLICATIONS of INDOLE:
Fragrances & Flavors uses of Indole: Key component in jasmine absolutes and musks; imparts a rich floral aroma
Biological & Chemical Reagent: Indole is precursor to tryptophan synthesis; substrate in analytical reagents like Kovac’s (Ehrlich’s) for detecting indoles

Pharmaceuticals & Biochemistry uses of Indole: Core structure in neurotransmitters (e.g., serotonin), plant growth hormones (auxins), and many alkaloids; essential scaffold in medicinal chemistry
Industrial Synthesis: Indole is produced industrially via Fischer or Madelung syntheses from substituted phenylhydrazines or N-acyl-o-toluidines

Indole is used in perfumery and in making tryptophan, an essential amino acid, and indoleacetic acid (heteroauxin), a hormone that promotes the development of roots in plant cuttings.

-Indole is used in Microorganisms:
Many microorganisms, particularly bacteria, produce indole as a secondary metabolite.
Indole plays a role in cell-to-cell communication, a process known as quorum sensing.

Through quorum sensing, bacteria can coordinate behaviors such as biofilm formation and antibiotic resistance in response to environmental changes or the presence of other microorganisms.

-Pharmaceutical Industry uses of Indole:
One of the primary uses of indole in industry is in the production of pharmaceuticals.
Indole's structure serves as the backbone for a variety of biologically active compounds, including serotonergic drugs.

Serotonin, as mentioned, is derived from indole, and many drugs used to treat depression, anxiety, and other mood disorders work by targeting the serotonin system.
These include selective serotonin reuptake inhibitors (SSRIs), such as fluoxetine (Prozac), which are widely prescribed in psychiatric medicine.

Indole derivatives are also used in the development of antibiotics, anticancer agents, and anti-inflammatory drugs.
Researchers are continuously exploring new ways to modify the indole structure to create novel compounds that could treat a wide range of diseases, from infections to chronic conditions.

-Fragrance and Flavor Industry uses of Indole:
Indole's distinct musky odor has long made it a desirable compound in the fragrance industry.
Despite its somewhat unpleasant smell at high concentrations, Indole has a valuable role in creating complex fragrance profiles.
At lower concentrations, indole is used in perfumes and scents to add depth and richness, often contributing to floral and woody notes.

In addition to fragrances, indole derivatives are also used in the food industry to produce certain flavor compounds.
These compounds, derived from the basic indole structure, help create rich, savory, and unique flavors in processed foods.

-Agriculture and Plant Science uses of Indole:
Indole-based compounds have wide applications in agriculture.
For instance, indole-3-acetic acid, as a plant growth regulator, is used to stimulate root development, especially in agricultural practices such as plant propagation and hydroponics.

The synthetic application of indole derivatives enhances plant growth, yields, and resistance to diseases, providing an essential tool for modern farming.

Researchers are also looking into indole's potential to promote sustainable agriculture.
By improving the efficiency of water and nutrient uptake in crops, indole derivatives may help reduce the need for chemical fertilizers and pesticides, supporting eco-friendly farming practices.

-Environmental and Industrial Applications of Indole:
Indole and its derivatives are also explored for their potential use in biodegradable plastics and environmental cleanup technologies.
As part of the growing emphasis on green chemistry, indole-based compounds offer a way to develop eco-friendly materials that break down more easily in nature.
This has significant implications for reducing plastic waste and improving sustainability across industries.

-Medical applications of Indole:
Indoles and their derivatives are promising against tuberculosis, malaria, diabetes, cancer, migraines, convulsions, hypertension, bacterial infections of methicillin-resistant Staphylococcus aureus (MRSA) and even viruses.

SYNTHETIC APPLICATION OF GRAMINE OF INDOLE:
Gramine and, especially, Indole's quaternary salts are useful synthetic intermediates as they are easily prepared and the dimethylamino group is easily displaced by nucleophiles for example – reactions with cyanide and acetamidomalonate anions.
The electron donating power of is never better demonstrated than in case of mannich bases.

For the removal of dimethylamino group, normal mannich bases require alkylation to convert them to its quaternary salts before elimination by heating.

However, in case of indole derived mannich bases no alkylation is needed and nitrogen of indole itself can expel the Me2N group in presence of CN ions.
The reaction is slow but gives high yield.

*Reaction with carbene:
Halocarbenes react with indole to add onto 2,3 C-C double bond to give a mixture of two products as given below.

*Oxidation Reaction of Indole:
C2-C3 double bond of indole is oxidatively cleaved by the use of reagents such as ozone, sodium periodate, potassium superoxide or CuCl2 in O2 atmosphere.

*Reduction Reaction of Indole:
The indole ring can be reduced selectively in carbocyclic or the heterocyclic ring.
Nucleophilic reducing agents such as LiAlH4 or NaBH4 does not affect indole nucleus.

However, lithium/liquid ammonia reduces the benzene ring to 4,7-dihydroindole as major product.
The heterocyclic ring can be reduced in acidic reagents such as Zn/HCl or NaCNBH3/CH3COOH to give indoline.

*Reaction of N-metallated Indoles:
In presence of very strong bases, indoles behave as weak acid, thus; it can be deprotonated with strong bases to obtain its N-metallated derivatives.

N-metallated indoles are nucleophiles that can react with suitable electrophiles either at N or at C-3.
These N-sodio derivatives can be prepared by reaction with sodamide in liquid ammonia, or by the use of sodium hydride in organic solvent.

Salts of other metals can be prepared by using various bases such as potassium t-butoxide, Grignard reagent or butyl lithium.
The more ionic sodium and potassium salts tend to react at N, especially with hard electrophiles.
In contrast to 1-indolyl magnesium halides are alkylated and acylated at C-3.

*Reaction of C-metallated Indoles:
C-Metallation of indoles can be brought about only in absence of the much more acidic N-hydrogen.
N-hydrogen can be removed by the N-substitution of indole with methyl, ethyl etc. or by the use of a protecting group such as phenylsulfonyl, lithium carboxylate and tbutoxycarbonyl, that can be easily removed after the desired product formation.

Each of these removable substituents assists lithiation at C-2 position of indole by intramolecular chelation.
This way obtained lithiated indoles can be converted into 2-substituted indoles by reaction with appropriate electrophiles.

CO2 is one of the most convenient N-protecting groups in indole α-lithiations because the N-protecting group is installed in situ and, further, falls off during normal work-up.
This technique has been used to prepared 2-halo-indoles and to introduce a variety of substituents by reaction with appropriate electrophiles – aldehydes, ketones, chloroformates, etc.

BIOSYNTHESIS AND FUNCTION OF INDOLE:
Indole is biosynthesized in the shikimate pathway via anthranilate.
Indole is an intermediate in the biosynthesis of tryptophan, where it stays inside the tryptophan synthase molecule between the removal of 3-phospho-glyceraldehyde and the condensation with serine.

When indole is needed in the cell, it is usually produced from tryptophan by tryptophanase.
As an intercellular signal molecule, indole regulates various aspects of bacterial physiology, including spore formation, plasmid stability, resistance to drugs, biofilm formation, and virulence.
A number of indole derivatives have important cellular functions, including neurotransmitters such as serotonin.

CHEMICAL REACTIONS OF INDOLE:
*Basicity
Unlike most amines, indole is not basic: just like pyrrole, the aromatic character of the ring means that the lone pair of electrons on the nitrogen atom is not available for protonation.

Strong acids such as hydrochloric acid can, however, protonate indole.
Indole is primarily protonated at the C3, rather than N1, owing to the enamine-like reactivity of the portion of the molecule located outside of the benzene ring.

The protonated form has a pKa of −3.6.
The sensitivity of many indolic compounds (e.g., tryptamines) under acidic conditions is caused by this protonation.

*Electrophilic substitution
The most reactive position on indole for electrophilic aromatic substitution is C3, which is 10¹³ times more reactive than benzene.
For example, it is alkylated by phosphorylated serine in the biosynthesis of the amino acid tryptophan.
Vilsmeier–Haack formylation of indole will take place at room temperature exclusively at C3.

*The Vilsmeyer–Haack formylation of indole
Since the pyrrolic ring is the most reactive portion of indole, electrophilic substitution of the carbocyclic (benzene) ring generally takes place only after N1, C2, and C3 are substituted.

A noteworthy exception occurs when electrophilic substitution is carried out in conditions sufficiently acidic to exhaustively protonate C3.
In this case, C5 is the most common site of electrophilic attack.

Gramine, a useful synthetic intermediate, is produced via a Mannich reaction of indole with dimethylamine and formaldehyde.
It is the precursor to indole-3-acetic acid and synthetic tryptophan.

SYNTHESIS OF GRAMINE FROM INDOLE:
N–H acidity and organometallic indole anion complexes
The N–H center has a pKa of 21 in DMSO, so that very strong bases such as sodium hydride or n-butyl lithium and water-free conditions are required for complete deprotonation.

The resulting organometalic derivatives can react in two ways.
The more ionic salts such as the sodium or potassium compounds tend to react with electrophiles at nitrogen-1, whereas the more covalent magnesium compounds (indole Grignard reagents) and (especially) zinc complexes tend to react at carbon 3.

In analogous fashion, polar aprotic solvents such as DMF and DMSO tend to favour attack at the nitrogen, whereas nonpolar solvents such as toluene favour C3 attack.

*Carbon acidity and C2 lithiation
After the N–H proton, the hydrogen at C2 is the next most acidic proton on indole.
Reaction of N-protected indoles with butyl lithium or lithium diisopropylamide results in lithiation exclusively at the C2 position.
This strong nucleophile can then be used as such with other electrophiles.

*2-position lithiation of indole
Bergman and Venemalm developed a technique for lithiating the 2-position of unsubstituted indole, as did Katritzky.

*Oxidation of indole
Due to the electron-rich nature of indole, it is easily oxidized.
Simple oxidants such as N-bromosuccinimide will selectively oxidize indole 1 to oxindole (4 and 5).

*Oxidation of indole by N-bromosuccinimide
Cycloadditions of indole
Only the C2–C3 pi bond of indole is capable of cycloaddition reactions.
Intramolecular variants are often higher-yielding than intermolecular cycloadditions.

For example, Padwa et al. have developed this Diels-Alder reaction to form advanced strychnine intermediates.
In this case, the 2-aminofuran is the diene, whereas the indole is the dienophile.
Indoles also undergo intramolecular [2+3] and [2+2] cycloadditions.

Example of a cycloaddition of indole
Despite mediocre yields, intermolecular cycloadditions of indole derivatives have been well documented.

One example is the Pictet-Spengler reaction between tryptophan derivatives and aldehydes, which produces a mixture of diastereomers, leading to reduced yield of the desired product.

*Hydrogenation
Indoles are susceptible to hydrogenation of the imine subunit to give indolines.
1H-indole is an indole and a polycyclic heteroarene.

It has a role as an Escherichia coli metabolite.
It is a tautomer of a 3H-indole.

Indole is a metabolite found in or produced by Escherichia coli.
Indole has been reported in Tetrastigma hemsleyanum, Daphne odora, and other organisms with data available.

RELATED COMPOUNDS OF INDOLE:
-Other cations    
*Indolium

-Related aromatic
*compounds    
*benzene, benzofuran,
*carbazole, carboline,
*indene, benzothiophene,
*indoline,
*isatin, methylindole,
*oxindole, pyrrole,
*skatole, benzophosphole

GENERAL PROPERTIES AND OCCURRENCE OF INDOLE:
Indole is a solid at room temperature.
Indole occurs naturally in human feces and has an intense fecal odor.
At very low concentrations, however, Indole  has a flowery smell, and is a constituent of many perfumes.

Indole also occurs in coal tar.
Indole has been identified in cannabis.
Indole is the main volatile compound in stinky tofu.

KEY CHARACTERISTICS & BENEFITS OF INDOLE:
Indole is a bicyclic aromatic heterocycle combining benzene and pyrrole moieties; foundational in natural products chemistry
Indole is volatile and odor-active, critical for fragrance and flavor industries
Indole is versatile chemical scaffold in drug discovery, agrochemicals, and biochemical pathways
Indole exhibits moderate toxicity and irritating properties; safe handling protocols are essential

HISTORY OF INDOLE:
Indole chemistry began to develop with the study of the dye indigo.
Indigo can be converted to isatin and then to oxindole.

Then, in 1866, Adolf von Baeyer reduced oxindole to indole using zinc dust.
In 1869, he proposed a formula for indole.

Certain indole derivatives were important dyestuffs until the end of the 19th century.
In the 1930s, interest in indole intensified when it became known that the indole substituent is present in many important alkaloids, known as indole alkaloids (e.g., tryptophan and auxins), and it remains an active area of research today.

SYNTHESIS AND CHEMISTRY OF INDOLE:
➢ Indole is a benzo[b]pyrrole formed by the fusion of benzene ring to the 2,3 positions of pyrrole nucleus.
➢ The word “Indole” is derived from the word India, as the heterocycle was first isolated from a blue dye “Indigo” produced in India during sixteenth century.
➢ In 1886, Adolf Baeyer isolated Indole by the pyrolysis of oxindole with Zn dust.
➢ Oxindole was originally obtained by the reduction of isatin, which in turn was isolated by the oxidation of Indigo.
➢ Commercially indole is produced from coal tar.
➢ Indole is the most widely distributed heterocycle.
➢ Indole nucleus is an integral part of thousands of naturally occurring alkaloids, drugs and other compounds.

SYNTHESIS OF INDOLE:
-Fischer Indole Synthesis:
➢ This reaction was discovered in 1983 by Emil Fischer and so far remained the most extensively used method of preparation of indoles.
➢ The synthesis involves cyclization of arylhydrazones under heating conditions in presence of protic acid or Lewis acids such as ZnCl2, PCl3, FeCl3, TsOH, HCl, H2SO4, PPA etc.
➢ The starting material arylhydrazoles can be obtained from aldehydes, ketones, keto acids, keto esters and diketones etc.
➢ Reaction produces 2,3-disubstituted products.
➢ Unsymmetrical ketones can give a mixture of indoles.

-Madelung Synthesis:
➢ Base catalyzed cyclization of 2-(acylamino)-toluenes under very harsh conditions (typically sodium amide or potassium t-butoxide at 250-300°C).
➢ Limited to the synthesis of simple indoles such as 2-methyl indoles without having any sensitive groups.
➢ A modern variant of madelung reaction is performed under milder conditions by the use of alkyllithiums as bases.
➢ 2-Substituted indoles bearing sensitive groups can be synthesized using this method.

-Reissert Synthesis:
Reissert indole synthesis is a multistep reaction:
Step 1: Base catalysed condensation of o-nitrotoluene with oxalic ester (methyl oxalate) to give o-nitrophenylpyruvic ester.
Step 2: Reduction of the nitro group to an amino group.
Step 3: Cyclization to indole-2-carboxylic acid.
Step 4: Decarboxylation.

-Bartoli Indole Synthesis:
➢ Efficient and extremely practical approach for indole synthesis.
➢ Ortho-substituted nitrobenzenes react with three mole equivalents of vinyl magnesium bromide (Grignard reagent) to give 7-substituted indoles.

-Nenitzescu Synthesis:
➢ Reaction provides direct route for the synthesis of 5-hydroxyindoles.
➢ Condensation of substituted 1,4-benzoquinone with a β-amino-substituted -α, β-unsaturated carbonyl compounds with ring closure to a 5-hydroxyindole.

-Bischler Indole Synthesis:
➢ Reaction involves acidic treatment of 2-arylamino-ketones (produced from a 2–halo-ketone and an arylamine) to bring about electrophilic cyclisation onto the aromatic ring.
➢ Often results in mixtures of products via rearrangements.

REACTIONS OF INDOLE:
➢ The fusion of a benzene ring to the 2,3-positions of a pyrrole generates one of the most important heterocyclic ring systems – indole.
➢ Aromatic: Indole is a planar molecule and follows Huckel’s rule [(4n+2) π electrons].
All atoms in indole are sp2 hybridized and each of them possesses one unhybridized p-orbital.
These p-orbitals overlap to generate π-molecular orbital containing 10 electrons (eight electrons from eight carbons and two lone pair electrons from N).

➢ It is resonance hybrid of several canonical structures.
➢ Indole is a π-excessive (electron rich) heterocycle, so their chemistry is mostly dominated by electrophilic substitution reactions.
➢ Indole is markedly less reactive than corresponding monocyclic heterocycles.

-Electrophilic Substitution Reactions:
➢ The pyrrole ring in indole is very electron rich, in comparison to the benzene ring, therefore, electrophile’s attack always takes place in the five-membered ring, except in special circumstances.

The preferred site of electrophilic substitution is C-3, because the cation formed by the C-3 attack of electrophile is more stable than that of the C-2 attack.
In case of C-3 attack, transition intermediate formed has positive charge adjacent to N atom that can be stabilized by the delocalization of lone pair of electrons of nitrogen.

Whereas, the positive charge of transition intermediate formed by the C-2 attack, cannot be stabilized without disturbing aromaticity of benzene ring.
If C-3 position is occupied, then electrophilic substitution takes place at C-2 and if both of them are occupied then electrophile attacks at C-6 position.

**Protonation:
Indole is a very weak base pKa -3.5.
The nitrogen atom of indole gets easily protonated even in water (at pH = 7) giving 1H-indolium cation.
However, thermodynamically most stable cation is formed by the protonation of C-3 rather than N.
3-protonated cation (3 H-indolium cation) retains full benzene aromaticity (in contrast to the 2-protonated cation) with delocalisation of charge over the nitrogen and α carbon.

**Nitration:
➢ Common nitrating reagent, mixture of acids (H2SO4 + HNO3) leads to acid-catalysed polymerisation of indole.
➢ Therefore, nitration of indole is carried out using non-acidic nitrating agent such as benzoyl nitrate and ethyl nitrate.
Nitration of 2-Methylindole with benzoyl nitrate gives a 3-nitro derivative however, nitration under acidic conditions using nitric/sulfuric acids gives C-5 –NO2 substituted product.

➢ The absence of attack on the heterocyclic ring can be explained: 2-methylindole undergoes protonation at C-3 under strongly acidic condition and this leads to deactivation of pyrrole ring for further electrophilic attack.
The regioselectivity of attack, para to the nitrogen, may mean that the actual moiety attacked is a hydrogen sulfate adduct of the initial 3H-indolium cation.

**Sulphonation:
Sulfonation of indole, at C-3, is performed using the pyridine–sulfur trioxide complex in hot pyridine.

**Halogenation:
3-Halo- and 2-halo-indoles are unstable therefore, must be utilised immediately after their preparation.

**Acylation:
➢ Indoles react with acetic anhydride in acetic acid above 140°C to afford 1,3-diacetylindole predominantly.
➢ On the other hand, acetylation in the presence of sodium acetate or 4-dimethylaminopyridine, affords exclusively 1–acetylindole.
➢ Acylation occurs at C-3 position before N in 1,3-diacetylindole as, N-acylated product showed resistance to conversion to 1,3-diacetylindole but 3-acetylindole showed easy conversion to 1,3-diacetylindole.

**Alkylation:
Indoles do not react with alkyl halides at room temperature.
Indole itself begins to react with methyliodide in dimethylformamide at about 80°C, to give main product 3-methyl indole (skatole).
As the temperature is increased, further methylation takes place resulting in 1,2,3,3-tetramethyl-3H-indolium iodide.

**The Vilsmeier Haack reaction:
The Vilsmeier reaction is a very efficient method for the formylation of electron rich aromatic rings by the use of acid chloride (POCl3) and DMF.

Indoles can be readily formylated to 3-formyl-indoles via Vilsmeier reaction.
Even indoles carrying an electron-withdrawing group at the 2-position, for example ethyl indole-2-carboxylate, undergo smooth Vilsmeier 3-formulation.

**Mannich reaction (reaction with iminium ions):
Indole reacts with a mixture of formaldehyde and dimethylamine at 0°C in neutral conditions to give N-substituted dimethylaminomethyl indole.
Under neutral conditions at higher temperature or in acidic medium acetic acid, N-substituted dimethylaminomethyl indole gets transformed into thermodynamically more stable 3-(dimethylaminomethyl)indole, gramine.

Gramine can be directly prepared in high yield, by reaction of indole with formaldehyde and dimethylamine in acetic acid.
The Mannich reaction is useful because the product gramine can be used as intermediates to access various 3-substituted indoles.

SYNTHETIC ROUTES OF INDOLE:
Indole and its derivatives can also be synthesized by a variety of methods.
According to a 2011 review, all known syntheses fall into 9 categories.

The main industrial routes start from aniline via vapor-phase reaction with ethylene glycol in the presence of catalysts:
Reaction of aniline and ethylene glycol to give indole.
In general, reactions are conducted between 200 and 500 °C.

Yields can be as high as 60%.
Other precursors to indole include formyltoluidine, 2-ethylaniline, and 2-(2-nitrophenyl)ethanol, all of which undergo cyclizations.

*Leimgruber–Batcho indole synthesis
The Leimgruber–Batcho indole synthesis is an efficient method of synthesizing indole and substituted indoles.
Originally disclosed in a patent in 1976, this method is high-yielding and can generate substituted indoles.
This method is especially popular in the pharmaceutical industry, where many pharmaceutical drugs are made up of specifically substituted indoles.

*Fischer indole synthesis
One of the oldest and most reliable methods for synthesizing substituted indoles is the Fischer indole synthesis, developed in 1883 by Emil Fischer.
Although the synthesis of indole itself is problematic using the Fischer indole synthesis, it is often used to generate indoles substituted in the 2- and/or 3-positions.

Indole can still be synthesized, however, using the Fischer indole synthesis by reacting phenylhydrazine with pyruvic acid followed by decarboxylation of the formed indole-2-carboxylic acid.
This has also been accomplished in a one-pot synthesis using microwave irradiation.

*Other indole-forming reactions
*Bartoli indole synthesis
*Bischler–Möhlau indole synthesis
*Cadogan-Sundberg indole synthesis
*Fukuyama indole synthesis
*Gassman indole synthesis
*Hemetsberger indole synthesis
*Larock indole synthesis
*Madelung synthesis
*Nenitzescu indole synthesis
*Reissert indole synthesis
*Baeyer–Emmerling indole synthesis

In the Diels–Reese reaction dimethyl acetylenedicarboxylate reacts with 1,2-diphenylhydrazine to an adduct, which in xylene gives dimethyl indole-2,3-dicarboxylate and aniline.
With other solvents, other products are formed: with glacial acetic acid a pyrazolone, and with pyridine a quinoline.

DETECTION METHODS OF INDOLE:
Common classical methods applied for the detection of extracellular and environmental indoles, are Salkowski, Kovács, Ehrlich’s reagent assays and HPLC.
For intracellular indole detection and measurement genetically encoded indole-responsive biosensor is applicable.

HISTORY & EVOLUTION OF INDOLE:
1866: discovery and synthesis of indole | 1869: chemical structure identified

Indole was first discovered during the process of making dyes with indigo in the mid-nineteenth century.
German chemist Adolf von Baeyer first isolated indole through a reaction of indigo, sulfuric acid and sulfuric anhydride (Gribble 2016).
The metabolite gets its name from the words “indigo” and “oleum,” because it was the product of treating indigo dye with oleum.

Indole is the main metabolite produced by gut bacteria during tryptophan metabolism, and is a precursor of metabolites that can be both beneficial and harmful for the host.

Indoles, also known as benzopyrroles, are heterocyclic compounds found widely in nature.
They have a double-ring structure, with one benzene and one pyrrole ring.

This structure is of great interest in medicinal chemistry, as it can be used as a scaffold for new drugs, including treatments for cancer, liver disease, diabetes, hypertension, inflammation, and depression.

Solid at room temperature, indole has a pungent odor, commonly associated with the smell of feces and coal tar.
Interestingly, it has a pleasant floral odor in very small concentrations, which makes it a perfect ingredient for perfumes and even a flavor enhancer in chocolate.
Indole is also found in plant-based psychedelic drugs including psilocybin and dimethyltryptamine.

BIOSYNTHESIS VS. DIETARY UPTAKE OF INDOLE:
In humans, indole is synthesized in the intestine as a product of tryptophan metabolism.
Tryptophan, one of the nine essential amino acids, cannot be synthesized by humans, and is therefore primarily obtained through dietary sources, particularly cruciferous vegetables, dairy and meat products.

Tryptophan is mostly absorbed in the small intestine and liver, but any that reaches the colon can be catabolized to indole derivatives, such as indoleamine 2,3-dioxygenase 1 (IDO1), which generates indole metabolites and alters gut function.

INDOLE AND THE MICROBIOME:
Gut bacteria activate ingested tryptophan to trigger three main catabolic pathways, including the indole pathway, the kynurenine pathway and the serotonin pathway.
Due to the different catabolic enzymes present in different bacteria, a variety of tryptophan catabolites are produced, including indole.

For example, Clostridium sporogenes metabolizes tryptophan to release the enzyme tryptophanase (Roager et al. 2018).
This produces 3-indolepropionic acid (3-IPA), which binds to the pregnane X receptor (PXR) in intestinal cells.
Once absorbed into the bloodstream, IPA is distributed around the body.

Another pathway involves Lactobacillus, which metabolizes tryptophan into indole-3-aldehyde (I3A).
This activates the aryl hydrocarbon receptor (AhR) in intestinal immune cells, which in turn triggers the production of the immuno-protective cytokine, interleukin-22 (IL-22).

CHROMATOGRAPHIC TECHNIQUES OF INDOLE:
One of the most common methods for analyzing indole is chromatography, particularly high-performance liquid chromatography (HPLC) and gas chromatography (GC).
These techniques separate indole from other compounds present in a mixture, allowing for precise identification and quantification.

HPLC:
HPLC is widely used in pharmaceutical research and environmental science to measure indole in biological fluids, plant extracts, or environmental samples.
By using a stationary phase and a mobile phase, HPLC can provide detailed profiles of indole concentration and its derivatives in complex matrices.

GC:
GC is especially useful when indole is present in volatile compounds.
It is often employed in fragrance analysis and to measure indole in air or water samples.
When coupled with mass spectrometry (MS), GC-MS becomes a powerful tool for highly sensitive and selective indole detection.

Spectroscopic Methods
Spectroscopic techniques, such as UV-Vis spectroscopy and fluorescence spectroscopy, are also employed to analyze indole.
These methods are based on the principle that indole absorbs light at specific wavelengths, allowing for quick and non-destructive analysis.

UV-Vis Spectroscopy:
Indole has characteristic absorbance peaks in the ultraviolet region, which can be used for both qualitative and quantitative analysis.
This method is ideal for determining the presence of indole in laboratory settings and can be easily adapted for high-throughput screening.

Fluorescence Spectroscopy:
Indole exhibits natural fluorescence under UV light, which makes it a useful technique for detecting indole in biological samples, such as urine or serum, where traditional chromatography methods might be less efficient.

Mass Spectrometry (MS)
Mass spectrometry (MS) is another powerful tool for the analysis of indole and its derivatives.
This method provides detailed information about the molecular structure of indole, its fragmentation pattern, and its exact molecular weight.

It is often coupled with chromatographic techniques (GC-MS or HPLC-MS) to enhance detection sensitivity and accuracy, particularly in complex biological or environmental samples.

Indole derivatives are important building blocks that play a crucial role in medicinal chemistry and drug discovery.
With diverse pharmacological activities and potential therapeutic applications such as antifungal, antiprotozoal, anti-inflammatory, anticancer, or antimicrobial, these versatile pharmacophores serve as scaffolds for the synthesis and development of novel pharmaceuticals.

HOW TO ANALYZE INDOLE?
Indole analysis is an essential process in both research and industrial applications, allowing scientists to study the presence, concentration, and behavior of indole in various systems.

Accurate analysis of indole is critical for fields such as pharmaceutical development, biotechnology, and environmental monitoring.
Several techniques are used to detect and quantify indole, each tailored to specific needs depending on the sample type and required sensitivity.

THE ROLE OF INDOLE IN BIOLOGY:
*In Plants
In plants, indole is primarily known for its role in the synthesis of indole-3-acetic acid, one of the most important plant hormones.
IAA regulates several critical plant functions, including cell elongation, root development, and response to environmental stimuli.

Indole, as the precursor to IAA, directly influences plant growth, especially in response to light, gravity, and other external factors.
It is also involved in phototropism (growth towards light) and gravitropism (growth in response to gravity).

Indole derivatives also play a role in the stress response of plants.
Under adverse environmental conditions, the synthesis of these compounds helps the plant adapt, promoting resistance to drought or pathogens.

*In Animals and Humans
In animals, indole is primarily produced in the gut microbiota, where it is generated as a byproduct of the breakdown of tryptophan, an essential amino acid.

This indole is involved in the regulation of intestinal health and contributes to the overall balance of the gut microbiome.
Indole also has a role in the immune system and may influence the body's inflammatory response.

In humans, indole and its derivatives have a direct impact on neurotransmitter production, particularly serotonin.
Serotonin, a chemical vital for regulating mood, sleep, and appetite, is synthesized from indole.
As such, indole indirectly affects mental health, with disruptions in serotonin levels being linked to depression, anxiety, and mood disorders.

SYNTHETIC METHODS FOR PRODUCING INDOLE:
Although indole can be isolated from natural sources, most commercial production relies on synthetic methods.
These chemical processes enable the large-scale production of indole, particularly for use in pharmaceuticals, fragrances, and agriculture.
Some of the most common synthetic routes include:

*Fischer Indole Synthesis
The Fischer indole synthesis, discovered by Hermann Fischer in 1883, is one of the most widely used methods for producing indole.
This reaction involves the condensation of phenylhydrazine with aldehydes or ketones under acidic conditions.

The process results in the formation of the indole ring, which can be further modified to create a variety of indole derivatives.
This method is still a key route for creating indole and its derivatives in the laboratory and industrial settings.

*Bischler–Napieralski Reaction
Another important method for synthesizing indole involves the Bischler–Napieralski reaction.
In this process, β-phenylethylamine undergoes cyclization in the presence of acidic catalysts (such as aluminum chloride) to form indole derivatives.

This reaction is commonly used in organic chemistry to create indole-based compounds, which can be applied in the development of pharmaceuticals, especially in the design of serotonin-related drugs.

*Leimgruber-Batcho Method
The Leimgruber-Batcho method is a widely used approach for large-scale indole synthesis.
In this process, a nucleophilic aromatic substitution is carried out with 1,2-dihydroquinoline to form indole.

This method is particularly useful for producing indole for applications in the fragrance and pharmaceutical industries due to its efficiency and scalability.

*Biotechnological Production of Indole
Beyond traditional chemical methods, there is increasing interest in the biotechnological production of indole.
Certain microorganisms, including bacteria and fungi, naturally produce indole as part of their metabolic pathways.

Researchers are exploring ways to leverage these natural processes for sustainable, eco-friendly production of indole.
This biotechnological approach offers a greener alternative to synthetic routes, particularly for industries seeking more environmentally responsible methods of production.

THE HISTORY OF INDOLE:
The history of indole traces back to the mid-19th century when it was first isolated.
In 1866, the German chemist Kuno Fritz extracted indole from the distillation of tryptophan, an essential amino acid.
This marked the beginning of indole's journey from being a simple organic compound to a key player in both natural and synthetic chemistry.

In the early years after its discovery, indole was mostly studied for its presence in biological systems, particularly in the decomposition of proteins.

It wasn't until the late 19th century that the chemical structure of indole was fully understood.
Indole's aromatic structure, with a fused benzene and pyrrole ring, was a subject of much interest in organic chemistry due to its unique properties.

The 20th century saw a rapid expansion in the understanding and utilization of indole.
As research progressed, Indole was found to be not just a simple molecule, but a precursor for a wide variety of biologically active compounds.

The most significant of these were indole derivatives, which were discovered to have applications in pharmaceuticals, agriculture, and fragrances.

Indole's role as a building block in the synthesis of bioactive compounds contributed significantly to the development of several drugs, including those used to treat neurological disorders.

Indole's involvement in serotonin synthesis, for example, highlighted its importance in the pharmaceutical industry, as serotonin is crucial for regulating mood, sleep, and appetite.

In addition to its pharmaceutical applications, indole's fragrance properties became well-known in the early 20th century.
Its musky odor made Indole a valuable component in the perfume industry, where it was used to create a variety of scent profiles.

CHEMICAL PROPERTIES OF INDOLE:
Indole's chemical properties stem from its unique structure, combining an aromatic benzene ring with a pyrrole ring.
This fusion gives indole several interesting characteristics.
Indole is stable yet reactive, thanks to its aromaticity and the delocalized electrons in the rings, which allow it to undergo a variety of reactions.

Indole is particularly reactive in electrophilic aromatic substitution reactions.
This makes Indole an ideal intermediate for synthesizing a wide range of derivatives used in pharmaceuticals, agriculture, and materials science.
Its nitrogen atom in the pyrrole ring can engage in hydrogen bonding, which enhances the solubility and bioavailability of indole derivatives in biological systems.

Due to these properties, indole derivatives have a broad range of applications in drug development, including the creation of antidepressants, antimicrobial agents, and anticancer drugs.

HOW IS INDOLE MADE?
NATURAL OCCURRENCE OF INDOLE
In nature, indole is commonly produced as a metabolic byproduct in animals and plants.
Indole is particularly abundant in the decomposition of the amino acid tryptophan, which is found in many organisms.

In plants, indole is a key precursor to IAA, a plant hormone involved in growth and development.
Indole is also present in the gut microbiota of humans and animals, where it is produced by bacteria as part of the digestion process.
Additionally, indole is found in small amounts in foods like cruciferous vegetables (e.g., broccoli and cabbage) and coffee.

PHYSICAL and CHEMICAL PROPERTIES of INDOLE:
CAS Number: 120-72-9
EC Number: 204-420-7
Hill Formula: C₈H₇N
Molar Mass: 117.15 g/mol
MDL Number: MFCD00005607
UNSPSC Code: 12352005
HS Code: 2933 99 20

Appearance: White to pale-yellow crystalline solid or flakes
Physical State: Crystalline, solid
Color: White to beige
Odor: Characteristic fecal at high concentrations; floral (jasmine-like) at trace levels
Melting Point: 52–54 °C
Boiling Point: 253–254 °C (1.013 hPa)
Flash Point: 121 °C (closed cup)
Density: 1.22 g/cm³ at 20 °C
Bulk Density: ~230 kg/m³

Water Solubility: 0.19 g/100 mL at 20 °C; soluble in hot water
pKa: 16.2 (in water), ~21 in DMSO
Magnetic Susceptibility: –85.0 × 10⁻⁶ cm³/mol
Dipole Moment: 2.11 D (in benzene)
Crystal Structure: Orthorhombic Pna2₁, planar aromatic heterocycle
Vapor Pressure: 0.016 hPa at 25 °C
Log P (n-octanol/water): 2.14 (literature)

Partition Coefficient: Bioaccumulation is not expected
Decomposition Temperature: > 253 °C
pH (1000 g/L in H₂O, 20 °C): 5.9
Molecular Properties:
XLogP3: 2.1
Hydrogen Bond Donor Count: 1
Hydrogen Bond Acceptor Count: 0
Rotatable Bond Count: 0
Exact Mass: 117.057849228 Da
Monoisotopic Mass: 117.057849228 Da

Topological Polar Surface Area: 15.8 Ų
Heavy Atom Count: 9
Formal Charge: 0
Complexity: 101
Isotope Atom Count: 0
Defined Atom Stereocenter Count: 0
Undefined Atom Stereocenter Count: 0
Defined Bond Stereocenter Count: 0
Undefined Bond Stereocenter Count: 0
Covalently-Bonded Unit Count: 1
Compound Is Canonicalized: Yes

FIRST AID MEASURES of INDOLE:
-Description of first-aid measures
*General advice:
Show this material safety data sheet to the doctor in attendance.
*If inhaled:
After inhalation: 
Fresh air.
*In case of skin contact: 
Take off immediately all contaminated clothing. 
Rinse skin with
water/ shower.
*In case of eye contact:
After eye contact: 
Rinse out with plenty of water. 
Call in ophthalmologist. 
Remove contact lenses.
*If swallowed:
After swallowing: 
Immediately make victim drink water (two glasses at most). 
Consult a physician.
-Indication of any immediate medical attention and special treatment needed.
No data available

ACCIDENTAL RELEASE MEASURES of INDOLE:
-Environmental precautions:
Do not let product enter drains.
-Methods and materials for containment and cleaning up:
Cover drains. 
Collect, bind, and pump off spills. 
Observe possible material restrictions. 
Take up dry. 
Dispose of properly. 
Clean up affected area.

FIRE FIGHTING MEASURES of INDOLE:
-Extinguishing media:
*Suitable extinguishing media:
Carbon dioxide (CO2) 
Foam 
Dry powder
*Unsuitable extinguishing media:
For this substance/mixture no limitations of extinguishing agents are given.
-Further information:
Prevent fire extinguishing water from contaminating surface water or the ground water system.

EXPOSURE CONTROLS/PERSONAL PROTECTION of INDOLE:
-Control parameters:
--Ingredients with workplace control parameters:
-Exposure controls:
--Personal protective equipment:
*Eye/face protection:
Use equipment for eye protection. 
Safety glasses
*Body Protection:
protective clothing
*Respiratory protection:
Recommended Filter type: Filter A 
-Control of environmental exposure:
Do not let product enter drains.

HANDLING and STORAGE of INDOLE:
-Conditions for safe storage, including any incompatibilities:
*Storage conditions:
Tightly closed. 
Dry.

STABILITY and REACTIVITY of INDOLE:
-Chemical stability:
The product is chemically stable under standard ambient conditions (room temperature).
-Possibility of hazardous reactions:
No data available