Org. Synth. 1942, 22, 98
DOI: 10.15227/orgsyn.022.0098
2-PHENYLINDOLE
[Indole, 2-phenyl-]
Submitted by R. L. Shriner, W. C. Ashley, and E. Welch.
Checked by Homer Adkins, Alan K. Roebuck, and Harry Coonradt.
1. Procedure
In a tall 1-l. beaker is placed an intimate mixture of 53 g. (0.25 mole) of freshly prepared acetophenone phenylhydrazone (Note 1) and 250 g. of powdered anhydrous zinc chloride (Note 2). The beaker is immersed in an oil bath at 170°, and the mixture is stirred vigorously by hand. The mass becomes liquid after 3–4 minutes, and evolution of white fumes begins. The beaker is removed from the bath, and the mixture is stirred for 5 minutes. In order to prevent solidification to a hard mass, 200 g. of clean sand is thoroughly stirred into the reaction mixture. The zinc chloride is dissolved by digesting the mixture overnight on the steam cone with 800 ml. of water and 25 ml. of concentrated hydrochloric acid (sp. gr. about 1.2). The sand and crude 2-phenylindole are removed by filtration, and the solids are boiled with 600 ml. of 95% ethanol. The hot mixture is decolorized with Norit and filtered through a hot 10-cm. Büchner funnel, and the sand and Norit are washed with 75 ml. of hot ethanol. After the combined filtrates are cooled to room temperature, the 2-phenylindole is collected on a 10-cm. Büchner funnel and washed three times with small amounts (15–20 ml.) of cold ethanol. The first crop is quite pure; after drying in a vacuum desiccator over calcium chloride it weighs 30–33 g. and melts at 188–189° (cor.) (Note 3) and (Note 4).
A little Norit is added to the combined filtrate and washings, which are then concentrated to a volume of 200 ml. and filtered. The filtrate, on cooling, deposits a second crop of 5–6 g. (Note 5) of impure product, which melts at 186–188°. The total yield of 2-phenylindole is 35–39 g. (72–80%).
2. Notes
1.
Acetophenone phenylhydrazone is prepared
1 by warming a mixture of
40 g. (0.33 mole) of acetophenone and
36 g. (0.33 mole) of phenylhydrazine on the steam cone for 1 hour. The hot mixture is dissolved in
80 ml. of 95% ethanol, and crystallization is induced by agitation. The mixture is then cooled in an
ice bath, and the product is removed and washed with
25 ml. of ethanol. There is obtained
54–57 g. of white product. A second crop of
4–10 g. is obtained by concentrating the combined filtrate and washings. The combined solids are dried under reduced pressure, over
calcium chloride, for 30 minutes. The total yield of
acetophenone phenylhydrazone, m.p.
105–106°, is
61–64 g. (
87–91%).
2.
It has been reported
2 that such a large amount of
zinc chloride is not necessary, but the submitters found that equal parts of
acetophenone phenylhydrazone and
zinc chloride gave lower yields.
3.
Using 3.2 times the quantities specified above, except that no sand was added in separating the product, the checkers have obtained yields of
75–80% of
2-phenylindole.
4.
While drying, the surface of the product becomes very light green.
5.
The filtrate still contains some
2-phenylindole, but great difficulty is encountered in trying to purify the crude material.
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3. Discussion
2-Phenylindole has been prepared by heating
benzoyl-o-toluidine in an atmosphere of
hydrogen,
3 the reaction being improved by the use of
sodium amyloxide as the condensing agent; by heating
phenylacetaldehydephenylhydrazone with 5 parts of anhydrous
zinc chloride;
4 by warming
phenacyl bromide5,6 or
N-phenacylaniline with
aniline,
7 aniline hydrochloride, or
zinc chloride;
8 by elimination of water from
o-aminodesoxybenzoin,
9 prepared in turn from
o-nitrodesoxybenzoin; by dehydration of
benzal-o-toluidine;
10 by the action of anhydrous
zinc chloride11 or
boron trifluoride12 upon the
phenylhydrazone of acetophenone; by the action of
potassium amide and
potassium nitrate upon
2-phenyl-4-quinolinecarboxylic acid.
13 N15-2-Phenylindole has been prepared from the
α-N15-phenylhydrazone of acetophenone.
14
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
sodium amyloxide
o-aminodesoxybenzoin
o-nitrodesoxybenzoin
phenylhydrazone of acetophenone
N15-2-Phenylindole
α-N15-phenylhydrazone of acetophenone
ethanol (64-17-5)
calcium chloride (10043-52-4)
hydrochloric acid (7647-01-0)
aniline (62-53-3)
hydrogen (1333-74-0)
aniline hydrochloride (142-04-1)
Phenylhydrazine (100-63-0)
Acetophenone (98-86-2)
Norit (7782-42-5)
zinc chloride (7646-85-7)
potassium nitrate (7757-79-1)
boron trifluoride (7637-07-2)
Phenacyl bromide (70-11-1)
acetophenone phenylhydrazone
potassium amide
2-Phenylindole,
Indole, 2-phenyl- (948-65-2)
phenylacetaldehydephenylhydrazone
N-phenacylaniline
2-phenyl-4-quinolinecarboxylic acid (132-60-5)
benzoyl-o-toluidine
benzal-o-toluidine
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