Phosphorous pentoxide, dehydration agent

Dehydration of the corresponding acid amides. This process usually requires phosphorus pentoxide (correctly termed phosphoric anhydride) as a dehydrating agent. 

Strong dehydrating agents such as phosphorous pentoxide or sulfur trioxide convert chlorosulfuric acid to its anhydride, pyrosulfuryl chloride [7791-27-7] S20 Cl2. Analogous trisulfuryl compounds have been identified in mixtures with sulfur trioxide (3,19). When boiled in the presence of mercury salts or other catalysts, chlorosulfuric acid decomposes quantitatively to sulfuryl chloride and sulfuric acid. The reverse reaction has been claimed as a preparative method (20), but it appears to proceed only under special conditions. Noncatalytic decomposition at temperatures at and above the boiling point also generates sulfuryl chloride, chlorine, sulfur dioxide, and other compounds. 

Unsubstituted amides can be dehydrated to nitriles.403 Phosphorous pentoxide is the most common dehydrating agent for this reaction, but many others, including POCI3, PCI5, CCI4-... 

Cyclisation of 1-hydroxy-2-napt ho ic acid in dry toluene in the presence of phosphorous pentoxide as a dehydrating agent gives a small yield of a cyclic tetramer, termed tetra-l-napthoid (7.34). Host 7.34 forms a number of 1 2 clathrates with small molecules such as CHC13 and benzene, which are very unstable with respect to desolvation. Complexes of 2-bromobutyric acid and napthalene are much more stable. A charge transfer complex with tetracyanoethylene (TCNE) is also known in which the electron-poor TCNE accepts electron density from the electron-rich aryl rings of the macrocycle. 

The second general method into the thiochrom-4-one ring involves the interaction of a thiophenol with compounds possessing a carbon atom bonded by at least two electron withdrawing groups, usually in the presence of a dehydrating agent (phosphorus pentoxide or poly-phosphoric acid). The first application of this was by Simonis and Elias and is illustrated in Eq. (19). The most successful active methylene substrates have been /3-ketoesters, i )3-cyanoketones, ... 

In 1884, C. Paal and L. Knorr almost simultaneously reported that 1,4-diketones upon treatment with strong mineral acids underwent dehydration to form substituted furans. This transformation soon became widely used and now it is referred to as the Paal-Knorr furan synthesis. The general features of the method are 1) virtually any 1,4-dicarbonyl compound (mainly aldehydes and ketones) or their surrogates are suitable substrates 2) the dehydration is affected by strong mineral acids such as hydrochloric acid or sulfuric acid, but often Lewis acids and dehydrating agents (e.g., phosphorous pentoxide, acetic anhydride, etc.) can be used and 3) the yields are usually moderate to good. The two major drawbacks of the reaction are the relative difficulty to obtain the 1,4-dicarbonyl substrates, and the sensitivity of many functionalities to acidic conditions. 

The dehydration of aqueous acid by contact with dehydrating agents such as concentrated sulfuric acid, phosphorous pentoxide, or acetic anhydride. 

Do not allow perchloric acid to come into contact with strong dehydrating agents (concentrated sulfuric acid, anhydrous phosphorous pentoxide, etc.). 

P2O5 (phosphorus(V) oxide, phosphorus pentoxide) m.p., 580°C under pressure. It sublimes at 360°C and reacts violently with water, forming phosphoric acid. It is used as a drying agent and as a dehydrating agent. 

The synthesis of 5-arylthianthrenium salts, 8, [CRI 02a, CRI 04] can be carried out by the condensation of thianthrene-S-oxide with various aromatic compounds bearing electron-donating substituents, R, as depicted in equation [2.1], A mixture of methanesulfonic acid and phosphorous pentoxide serves as an acid catalyst and dehydrating agent for this reaction. Replacement of the methanesulfonate anion by a metathetical reaction with NaMtXn affords the desired active photoinitiator, 8. Depending on the specific R substituent, 5-arylthianthrenium photoinitiators have UV absorption maxima in the range of 300-400 nm. 

The oxide CI2O7 can be prepared by the action of phosphorus pentoxide, P2O5, on perchloric acid (Figure 6.15). P2O5 is a powerful dehydrating agent, which can often abstract the elements of water from substances by forming phosphoric acid ... 

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