Resorcinol

Resorcinol (or resorcin) is an organic compound with the formula C6H4(OH)2. It is one of three isomeric benzenediols, the 1,3-isomer (or "meta"-isomer). It is a white, water-soluble solid.
Production.
Resorcinol is produced in several steps from benzene, starting with dialkylation with propene to give 1,3-diisopropylbenzene. Oxidation and Hock rearrangement of this disubstituted arene gives acetone and resorcinol.
Resorcinol is an expensive chemical, produced in only a very few locations around the world (to date only three commercial plants are known to be operative: in the United States, Germany, and Japan), and as such it is the determining factor in the cost of PRF adhesives.
Many additional routes exist for resorcinol. It can be produced when any of a large number of resins (such as galbanum and asafoetida) are melted with potassium hydroxide, or by the distillation of Brazilwood extract. It may be prepared synthetically by melting 3-iodophenol, phenol-3-sulfonic acid, or benzene-1,3-disulfonic acid with potassium carbonate; by the action of nitrous acid on 3-aminophenol or on 1,3-diaminobenzene. Many "ortho"- and "para"-compounds of the aromatic series (for example, the bromophenols, benzene-"para"-disulfonic acid) also yield resorcinol on fusion with potassium hydroxide.
Properties.
Resorcinol crystallizes from benzene as colorless needles that are readily soluble in water, alcohol, and ether, but insoluble in chloroform and carbon disulfide. 
Reactions.
It reduces Fehling's solution and ammoniacal silver solutions. It does not form a precipitate with lead acetate solution, as does the isomeric pyrocatechol. Iron(III) chloride colors its aqueous solution a dark-violet, and bromine water precipitates tribromoresorcinol. These properties are what give it its use as a colouring agent for certain chromatography experiments.
Sodium amalgam reduces it to dihydroresorcin, which when heated to 150 to 160 °C with concentrated barium hydroxide solution gives γ-acetylbutyric acid
When fused with potassium hydroxide, resorcinol yields phloroglucin, pyrocatechol, and diresorcin. It condenses with acids or acid chlorides, in the presence of dehydrating agents, to oxyketones, for example, with zinc chloride and glacial acetic acid at 145 °C it yields resacetophenone (HO)2C6H3COCH3. With the anhydrides of dibasic acids, it yields fluoresceins. When heated with calcium chloride—ammonia to 200 °C it yields "meta"-dioxydiphenylamine. 
With sodium nitrite it forms a water-soluble blue dye, which is turned red by acids, and is used as a pH indicator under the name of lacmoid. It condenses readily with aldehydes, yielding with formaldehyde, on the addition of catalytic hydrochloric acid, methylene diresorcin [(HO)C6H3(O)]2CH2. Reaction with chloral hydrate in the presence of potassium bisulfate yields the lactone of tetra-oxydiphenyl methane carboxylic acid. In alcoholic solution it condenses with sodium acetoacetate to form 4-methylumbelliferone.
In addition to electrophilic aromatic addition, resorcinol (and other polyols) undergo nucleophilic substitution via the enone tautomer. 
Nitration with concentrated nitric acid in the presence of cold concentrated sulfuric acid yields trinitroresorcin (styphnic acid), an explosive.
Occurrence.
Resorcinol per se does not appear to occur naturally in the free state. It is found in argan oil.
Parts of a molecule of catechin, another natural compound that is present in tea, has the resorcinol skeleton structure in it.
Alkylresorcinols are a marker of whole grain diet.
Applications.
Resorcinol is mainly used in the production of resins. As a mixture with phenol, it condenses with formaldehyde to afford adhesives.
Medical.
It is present in over-the-counter topical acne treatments at 2% or less concentration, and in prescription treatments at higher concentrations. Monoacetylresorcinol, C6H4(OH)(O–COCH3), is used under the name of euresol. It is used in hidradenitis suppurativa with limited evidence showing it can help with resolution of the lesions. Resorcinol is one of the active ingredients in products such as Resinol, Vagisil, and Clearasil.
In the 1950s and early 1960s the British Army used it, in the form of a paste applied directly to the skin. One such place where this treatment was given to soldiers with chronic acne was the Cambridge Military Hospital, Aldershot, England. It was not always successful. 
4-Hexylresorcinol is an anesthetic found in throat lozenges.
Chemical.
Resorcinol is also used as a chemical intermediate for the synthesis of pharmaceuticals and other organic compounds. It is used in the production of diazo dyes and plasticizers and as a UV absorber in resins.
Resorcinol is an analytical reagent for the qualitative determination of ketoses (Seliwanoff's test).
Resorcinol is the starting material for resorcinarene molecules and the initiating explosive lead styphnate.
Resorcinol reacts with formaldehyde to form a thermoset resin which can form the basis of an aerogel.
Related compounds.
Resazurin, C12H7NO4, obtained by the action of nitrous acid on resorcinol, forms small dark red crystals possessing a greenish metallic glance. When dissolved in concentrated sulfuric acid and warmed to 210 °C, the solution on pouring into water yields a precipitate of resorufin, C12H7NO3, an oxyphenoxazone, which is insoluble in water but is readily soluble in hot concentrated hydrochloric acid, and in solutions of caustic alkalis. The alkaline solutions are of a rose-red color and show a cinnabar-red fluorescence. A tetrabromresorufin is used as a dyestuff under the name of Fluorescent Resorcin Blue.
Thioresorcinol is obtained by the action of zinc and hydrochloric acid on "meta"-benzenedisulfonyl chloride. It melts at 27 °C and boils at 243 °C. Resorcinol disulfonic acid, (HO)2C6H2(HSO3)2, is a deliquescent mass obtained by the action of sulfuric acid on resorcin. It is readily soluble in water and ethanol.
Resorcinol is also a common scaffold that is found in a class of anticancer agents, some of which (luminespib, ganetespib, KW-2478, and onalespib) were in clinical trials . Part of the resorcinol structure binds to inhibits the N-terminal domain of heat shock protein 90, which is a drug target for anticancer treatments.
History, etymology, and nomenclature.
Austrian chemist Heinrich Hlasiwetz (1825–1875) is remembered for his chemical analysis of resorcinol and for his part in the first preparation of resorcinol, along with Ludwig Barth, which was published in 1864.
Benzene-1,3-diol is the name recommended by the International Union of Pure and Applied Chemistry (IUPAC) in its "1993 Recommendations for the Nomenclature of Organic Chemistry".
Resorcinol is so named because of its derivation from ammoniated resin gum, and for its relation to the chemical orcinol.