Phloroglucinol is an organic compound with the formula C6H3(OH)3. It is a colorless solid. It is used in the synthesis of pharmaceuticals and explosives. Phloroglucinol is one of three isomeric benzenetriols. The other two isomers are hydroxyquinol (1,2,4-benzenetriol) and pyrogallol (1,2,3-benzenetriol). Phloroglucinol, and its benzenetriol isomers, are still defined as "phenols" according to the IUPAC official nomenclature rules of chemical compounds. Many such monophenolics are often termed "polyphenols" by the cosmetic and parapharmaceutical industries, which does not match the scientifically accepted definition.
A modern synthesis of involves hydrolysis of benzene-1,3,5-triamine and its derivatives. Representative is the following route from trinitrobenzene.
The synthesis is noteworthy because ordinary aniline derivatives are unreactive toward hydroxide. Because the triaminobenzene also exists as its imine tautomer, it is susceptible to hydrolysis.
Tautomerism and acid-base behavior
Phloroglucinol is a weak triprotic acid. The first two pKa's are 8.5 and 8.9.
As an enol, phloroglucinol in principle exists in equilibrium with keto tautomers. Evidence for this equilibrium is provided by the formation of the oxime:
C6H3(OH)3 + 3 NH2OH → (CH2)3(C=NOH)3 + 3 H2O
But it behaves also like a benzenetriol as the three hydroxyl groups can be methylated to give 1,3,5-trimethoxybenzene.
For the neutral compound, the keto tautomers are undetectable spectroscopically. Upon deprotonation, the keto tautomer predominates.
From water, phloroglucinol crystallizes as the dihydrate, which has a melting point of 116–117 °C, but the anhydrous form melts at a much higher temperature, at 218–220 °C. It does not boil intact, but it does sublime.
Phloroglucinol is used as a treatment for gallstones, spasmodic pain and other related gastrointestinal disorders.
A 2020 review found insufficient evidence that phloroglucinol was effective for treating pain caused by obstetric and gynecological conditions.
It has the A03AX12 code in the A03AX Other drugs for functional bowel disorders section of the ATC code A03 Drugs for functional gastrointestinal disorders subgroup of the Anatomical Therapeutic Chemical Classification System. It also has the D02.755.684 code in the D02Organic chemicals section of the Medical Subject Headings (MeSH) codes by the United States National Library of Medicine.
Phloroglucinol is mainly used as a coupling agent in printing. It links diazo dyes to give a fast black.
Phloroglucinolysis is an analytical technique to study condensed tannins by means of depolymerisation. The reaction makes use of phloroglucinol as nucleophile. Phlobaphenes formation (tannins condensation and precipitation) can be minimized in using strong nucleophiles, such as phloroglucinol, during pine tannins extraction.
Phloroglucinol is used in plant culture media. It demonstrates both cytokinin-like and auxin-like activity. Phloroglucinol increases shoot formation and somatic embryogenesis in several horticultural and grain crops. When added to rooting media together with auxin, phloroglucinol further stimulates rooting.
Use in tests
Phloroglucinol is a reagent of the Tollens' test for pentoses. This test relies on reaction of the furfural with phloroglucinol to produce a colored compound with high molar absorptivity.
A solution of hydrochloric acid and phloroglucinol is also used for the detection of lignin (Wiesner test). A brilliant red color develops, owing to the presence of coniferaldehyde groups in the lignin. A similar test can be performed with tolonium chloride.
It is also part of Gunzburg reagent, an alcoholic solution of phloroglucinol and vanillin, for the qualitative detection of free hydrochloric acid in gastric juice.
Hlasiwetz, Heinrich (1855). "Ueber das Phloretin" [On phloretin]. Annalen der Chemie und Pharmacie. 96 (1): 118–123. doi:10.1002/jlac.18550960115. On p. 120, Hlasiwetz named phloroglucin: "Die auffallendste Eigenschaft dieses Körpers ist, daſs er überaus süſs schmeckt, weſshalb er bis auf weiteres Phloroglucin genannt sein mag." (The most striking property of this substance is that it tastes extremely sweet, for which reason it may be named "phloroglucin" until further [information emerges].)
Thorpe, Edward, ed., A Dictionary of Applied Chemistry (London, England: Longmans, Green, and Co., 1913), vol. 4, 183.
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