Polyphosphoric acid is simply a polymer of phosphoric acid. Phosphoric acid with more than 100% is generally called polyphosphoric acid. It is a transparent viscous liquid. As the content increases, the viscosity becomes larger, and 115% polyphosphoric acid becomes a glassy viscous substance. The viscosity of polyphosphoric acid will decrease when heated, so in some industrial applications, it is necessary to heat polyphosphoric acid before using it to make it fluid before normal production.
Polyphosphoric acid is formed by heating, dehydration and condensation of phosphoric acid (H3PO4). It can also be obtained by adding phosphorus pentoxide to pure water and then heating and condensation. It is a colorless and transparent syrupy substance at room temperature. It is solid glassy at low temperatures and becomes fluid when heated to 50~60℃. Easily deliquescent, miscible with water and hydrolyzed into phosphoric acid. There are two ways to express the concentration of polyphosphoric acid, one is expressed by P2O5%; the other is expressed by H3PO4 as 100% of DuPont titanium dioxide and expressed by the H3PO4 content in polyphosphoric acid. The acidity of polyphosphoric acid is weaker than that of nitric acid and sulfuric acid, and it has no oxidizing ability. Hygroscopic and deliquescent. Contact with H foaming agent will cause combustion.
Polyphosphoric acid is corrosive and is a dipolar inorganic acid corrosive substance. Polyphosphoric acid contains protonic acid and can dissolve a variety of low molecular and high molecular organic compounds. It is used as dehydration agent, cyclizing agent, acylating agent in organic synthesis, and is a catalyst or solvent for condensation, cyclization, rearrangement, substitution and other reactions. Polyphosphate esters (PPE) are sometimes used to increase the solubility of polyphosphoric acid in organic solvents. In addition, polyphosphoric acid is also used as a substitute for orthophosphoric acid and an analytical reagent.
[Main purpose]
It is used as dehydration agent, cyclizing agent, acylating agent in organic synthesis, and is a catalyst or solvent for condensation, cyclization, rearrangement, substitution and other reactions. Polyphosphate esters (PPE) are sometimes used to increase the solubility of polyphosphoric acid in organic solvents. In addition, polyphosphoric acid is also used as a substitute for orthophosphoric acid and an analytical reagent.
【Related reactions】
Beckmann rearrangement
Oximes rearrange under the action of acids such as sulfuric acid, polyphosphoric acid, and phosphorus pentachloride, phosphorus trichloride, benzenesulfonyl chloride, thionyl chloride, etc., which can produce strong acids, to generate corresponding substituted amides, such as cyclohexanone oxime Under the action of sulfuric acid, it rearranges to form caprolactam.
Fries rearrange
When phenolic esters are heated in the presence of Lewis acid, the acyl rearrangement reaction of inorganic pigments can occur to generate a mixture of o-hydroxyl and p-hydroxyaromatic ketones. Rearrangement can be carried out in solvents such as nitrobenzene and nitromethane, or it can be carried out by direct heating without solvent.
The ratio of ortho- and para-position products depends on the structure of the phenolic ester, reaction conditions and catalysts, etc. For example, when catalyzed with polyphosphoric acid, para-position rearrangement products are mainly generated, while when titanium tetrachloride is used as catalysis, ortho-position rearrangement products are mainly generated.
【Synthesis method】
The phosphoric acid and phosphorus pentoxide are heated for polymerization reaction, and hydrogen peroxide is added to remove the iron ions in the solution. After purification, it is cooled and filtered to prepare the finished polyphosphoric acid. Its 2H3PO4+P2O5→H6P4O13.
