Org. Synth. 1929, 9, 74
DOI: 10.15227/orgsyn.009.0074
Submitted by H. T. Clarke and W. W. Hartman.
Checked by C. S. Marvel and A. T. Blomquist.
1. Procedure
To 225 g. (0.87 mole) of crude 2,4,6-trinitrobenzoic acid (p. 543) (Note 1) and (Note 2) is added 2100 g. (1800 cc., 19.6 moles) of concentrated hydrochloric acid (sp. gr. 1.17) in a 5-l. flask provided with a reflux condenser, the inner tube of which is 15–20 mm. in diameter. To this mixture is added 830 g. (7.0 atoms) of granulated tin in small portions, the reaction being started after the addition of 30–40 g. of the tin by immersing the flask in a hot-water bath (Note 3). The rest of the tin is added through the condenser at such a rate that the reaction proceeds briskly. It is advisable to carry out this reaction under a hood since hydrogen chloride is likely to escape at the beginning of the reaction. The mixture is finally heated on a steam bath for one hour in order to complete the reaction, and is then filtered hot through glass wool in order to remove unreacted tin.
The filtrate is made up to exactly 2 l., and a 50-cc. sample is titrated with sodium hydroxide solution of known concentration (35–40 per cent in a suitable strength). The volume of alkali necessary to produce a slight permanent precipitate is noted (it usually requires 6–8 cc. of 40 per cent alkali). More alkali is run in until the liquid is neutral to litmus paper: this second value represents the acid combined with tin. To the filtrate in the 5-l. flask are added 1 kg. of cracked ice and then sufficient alkali to neutralize the free acid as well as 60 per cent of that combined with the tin (about 800 cc. of 40 per cent sodium hydroxide is required). The mixture is diluted, without filtering, to 6 l. and boiled in a 12-l. flask under a reflux condenser in an atmosphere of coal gas (Note 4) for twenty hours. The precipitate is then filtered off and washed very thoroughly with boiling water (2–3 l. of water); the combined filtrates are concentrated to 3 l., then rendered slightly acid to litmus with hydrochloric acid, and chilled to 0°, when crystals of phloroglucinol separate. This crop is filtered off; it should weigh 70–80 g. It is then dissolved in 700 cc. of hot water, filtered, and allowed to crystallize at 0°. In this way 65–75 g. (46–53 per cent of the theoretical amount) of almost colorless (Note 5) phloroglucinol dihydrate melting at 217–218° is obtained.
2. Notes
1. It is unnecessary to remove the small amount of trinitrotoluene with which the crude trinitrobenzoic acid is contaminated, since it has been found that no trace of the methyl homologue of phloroglucinol is obtained on treating trinitrotoluene in the above manner.
2. An equivalent quantity of 1,3,5-trinitrobenzene (p. 541) may be employed in place of trinitrobenzoic acid with exactly the same result; but, as the most convenient method of preparing trinitrobenzene in the laboratory is through trinitrobenzoic acid, nothing is gained by the substitution.
3. In carrying out the reduction, care must be taken that not more than 5 per cent of the tin is added before the reaction starts; otherwise the interaction begins with such violence that the product may be lost. The reaction may be readily checked by immersion of the flask in water. The application of wet towels to the upper half of the flask is often of great assistance to condensation though only slightly checking the reaction itself.
4. Coal gas is recommended as being the most convenient inert gas in which to carry out the hydrolysis; carbon dioxide or nitrogen should serve equally well.
5. If it is found impossible to obtain a white product by crystallization, purification may be effected by dissolving the phloroglucinol dihydrate in four times its weight of boiling water and passing in sulfur dioxide until no further bleaching effect is observed (about one-half minute is usually sufficient).
3. Discussion
Phloroglucinol can be prepared by the following fusion reactions: resorcinol with sodium hydroxide;1 1,3,5-benzenetrisulfonic acid with sodium hydroxide;2 and 3,5-dibromophenol with potassium hydroxide.3 Better yields, however, are obtained by boiling a dilute solution of triaminobenzene hydrochloride4 or triaminobenzoic acid hydrochloride5 with water.
This preparation is referenced from:

References and Notes
  1. Barth and Schreder, Ber. 12, 503 (1879).
  2. Barth and Schreder, Ber. 12, 422 (1879).
  3. Blau, Monatsh. 7, 632 (1886).
  4. Weidel and Pollak, ibid. 21, 20 (1900).
  5. Cassella and Co., Ger. pat. 102,358 [Frdl. 5, 156 (1897–1900)].

Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

methyl homologue of phloroglucinol

hydrogen chloride,
hydrochloric acid (7647-01-0)

sodium hydroxide (1310-73-2)

sulfur dioxide (7446-09-5)

nitrogen (7727-37-9)

tin (7440-31-5)

carbon dioxide (124-38-9)

potassium hydroxide (1310-58-3)

1,3,5-Trinitrobenzene (99-35-4)

Phloroglucinol (108-73-6)

2,4,6-Trinitrobenzoic acid (129-66-8)

phloroglucinol dihydrate (6099-90-7)

trinitrotoluene (118-96-7)

trinitrobenzoic acid


resorcinol (108-46-3)

1,3,5-benzenetrisulfonic acid


triaminobenzene hydrochloride

triaminobenzoic acid hydrochloride