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). Resorcinol crystallizes from benzene as colorless needles that are readily soluble in water, alcohol, and ether, but insoluble in chloroform and carbon disulfide. Production Resorcinol is produced in several steps from benzene, starting with dialkylation with propylene 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 four commercial plants are known to be operative: in the United States, Germany,China and Japan), and as such it is the determining factor in the cost of PRF adhesives. Many additional routes exist for resorcinol. It was formerly produced by disulfonation of benzene followed by hydrolysis of the 1,3-disulfonate. This method has been discarded because it cogenerates so much sulfur-containing waste. Resorcinol can also 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 synthesized by melting 3-iodophenol, phenol-3-sulfonic acid with potassium carbonate. Diazotization of 3-aminophenol or on 1,3-diaminobenzene followed by hydrolysis provides yet another route. 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. Reactions Partial hydrogenation of resorcinol gives dihydroresorcinol, also known as 1,3-cyclohexanedione. 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 diresorcinol. 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