Synthesis of indole

1. Extracted from the wash oil fraction in high temperature coal tar, about 0.10%-0.16% of indole. Generally can be extracted from coal tar and wash oil distillate. After alkali washing and pickling, methyl naphthalene fractions were obtained, and then rectified in a high-efficiency tower of 60 theoretical plates, cut out 225-256℃ fractions, and hydrotreated potassium oxide for melting. The reaction is carried out at 170-240℃ for 2-4h, stirring until it stops bubbling. Let it stand, let the lower potassium indole out to cool, break it up, and wash it with benzene at low temperature to remove oil. Then, the crude indole oil was hydrolyzed at 50-70℃ to obtain the crude indole oil, which was distilled in a distillation column of 20 theoretical plates with reflux ratio of 8-10:1, cut the fraction at the top temperature of 170-256℃, cooled, crystallized, and centrifugated to obtain refined indole oil. After pressing, the oil content is less than 3%, recrystallized with ethanol, and purified indole with 99% purity is obtained.
2. Obtained by catalytic dehydrogenation of o-amino-ethylbenzene. Dihydroindole was obtained by vacuum distillation of o-amino-ethylbenzene in a nitrogen stream in the presence of aluminum nitrate (or aluminum oxide) at 550℃. The indole was obtained by dehydrogenation at 640℃. Other methods are the reaction of o-nitrotoluene and oxalate to produce o-nitrophenylpyruvic acid and then α-indole carboxylic acid, and finally dry distillation with lime to obtain the product; Aniline and acetylene were heated at 600-650℃ to synthesize indole. Indole was synthesized from o-carboxyphenylglycine by 3-hydroxy-2-indole carboxylic acid and indole acid. Indole was obtained by oxidizing static blue with concentrated nitric acid or chromic acid, which was then distilled with zinc powder. Indole can also be obtained by mixing nitrocinnamic acid with 10 parts potassium hydroxide powder, adding iron filings and melting the mixture. In the synthesis of indole and its homologues, the Fischer synthesis method is the most common. The aromatic hydrazone of ketones or aldehydes reacts under acidic conditions and undergoes a benzidine rearrangement to form indole.
3. A 2-substituted or 3-substituted indole derivative is obtained by heating rearrangement of phenylhydrazone under acid catalysis to eliminate one molecule of ammonia. In practice, phenylhydrazone can be obtained by heating aldehydes or ketones with equal equivalent phenylhydrazine in acid, which is immediately rearranged under acid catalysis to eliminate ammonia and obtain indole compounds. Commonly used catalysts are zinc chloride, boron trifluoride, polyphosphate, etc., commonly used acids are AcOH, HCl, trifluoroacetic acid and so on.
4. For indole with no substituents at position 2,3, most of the nitrobenzene derivatives are used in the general industry to synthesize the indole, and o-methyl, o-formyl, o-cyanoethyl, o-vinyl, and nitrobenzene derivatives with hydrogen in the orthosite can be obtained by corresponding methods.