P-O-C bonds

Phosphorus The main role of phosphorus in carbon materials is as an oxidation protector and a fire retardant [136-144]. Its source can be in phosphoric acid, which is used in some technologies of carbon activation [143,144] or in the cross-linking precursor. The phosphorus present in the carbon matrix is stable between 773 and 1273 K. It can be fixed as red phosphorus and/or in chemically bonded forms, such as -C-P-bonds or -C-O-P-bonds [143-145]. During the carbonization process at low temperatures, phosphocarbonaceous species are created. Their content decreases by scission of the P-O-C bonds with an increase in the temperature, due to the growth of aromatic structures [143]. Possible phosphorus-containing functionalities are presented in Figure 2.4. 

The introduction of phosphorus into the carbon matrix is usually achieved by polymerization of a carbon precursor in the presence of phosphorus compounds that provide multi-functionality for cross linking the precursor. The phosphorous thereby introduced to the carbon matrix has been reported to be stable between 773-1273 K, as content decreases rapidly with an increase in temperature above 1273K. Some evidences have been provided indicating that the phosphorus is fixed in the carbon matrix as red phosphorus and as some chemically bonded forms, such as -C-P- bonds or -C-O-P bonds [118-120]. During... 

In most soils, phosphorus associated with carhon (i.e. organic phosphorus) accounts for between 30% and 65% of total phosphorus, although some soils contain up to 90% organic phosphorus (Harrison, 1987). The majority of the organic phosphorus is present as phosphate esters (C-O-P bonds) including inositol phosphates, nucleic acids and phospholipids, together with small quantities of phosphonates (C-P bonds) (Magid et ah, 1996 Condron et ah,... 

In the glycerate to pyruvate transition the free energy of hydrolysis of the C—O— P bond increases from 3 kcal. to ca. 12 kcal. per mole in consequence of a redistribution of energy within the molecule. The phosphate group of enolphosphopyruvate can now interact with ADP to form ATP in presence of potassium ions ... 

Phosphonates are organic phosphates characterized by a C-P-O bond, which is much more resistant to hydrolysis than the polyphosphate bond (O-P-O) or the phosphate ester bond (C-O-P), making them suitable for many types of BW treatment formulation. Phosphonates were originally developed for the industrial and institutional (I I) cleaning market in the 1960s, but are commonly employed in a myriad of applications. 

The phosphate group is derived from phosphoric acid (H3 PO4) by replacing an O—H bond by an O—C or O— P bond. Phosphate is an important functional group in biochemistry, being involved in cellular energy production as well as acting as an important monomer in biopolymers, particularly in DNA. Bonds to phosphate groups form or break in the course of a number of important biochemical reactions. 

The only element that was discovered in body fluids (urine). This is plausible, as P plays a main role in all life processes. It is one of the five elements that make up DNA (besides C, H, N, and 0 evolution did not require anything else to code all life). The P-O-P bond, phosphoric acid anhydride, is the universal energy currency in cells. The skeletons of mammals consists of Ca phosphate (hydroxylapatite). The element is encountered in several allotropic modifications white phosphorus (soft, pyrophoric P4, very toxic), red phosphorus (nontoxic, used to make the striking surface of matchboxes), black phosphorus (formed under high pressures). Phosphates are indispensable as fertilizer, but less desirable in washing agents as the waste water is too concentrated with this substance (eutrophication). It has a rich chemistry, is the basis for powerful insecticides, but also for warfare agents. A versatile element. 

In contrast to the other X-H additions surveyed in this chapter, there have been only a limited number of studies on the addition of P H linkages across C C (and C—O) multiple bonds. At the same time, there has been significant progress made in the development of catalytic systems for performing hydrophosphination and hydrophosphorylation compared to what was known pre-1982. 

The experimental weight loss in the first and second step (4.0 and 4.5% respectively) is in agreement with that corresponding to condensation to pyrophosphate (4.0%) and polyphosphate (4.2X, n 1). Furthermore, the IR spectra of melamine phosphate and of the residues at 300 and 330 C (Figure 14 spectra A, B and C respectively) show that besides the typical bands of phosphate salts (950-1300 cm-1) which are present in the three spectra, a new absorption due to P-O-P bonds (ca. 890 cm-1) appears in the spectra of the residues. The absorptions due to melamine salt structures (e.g. 780-790 and 1450-1750 cm ) are closely similar in the three spectra of Figure 14. Fire retardants based on melamine pyrophosphate and polyphosphate are reported in the literature 51 as well as methods for preparation of these salts (25-... 

The X—O distances in both the nitrate and phosphite sets (Tables 4-8) are on the long side compared to the other X—O bonds reviewed here. The differences are smallest for X = C in the nitrate. The X... Op interaction should tend to lengthen X—O. The C—O(N) bond should be better described than C—O(P) because of the latter s higher O—P ionic character. For the same reason, the other X—O(P) bonds should be shorter in the phosphite compounds compared to the nitrates. These expectations are realized in the calculated relative X—O bond lengths (Tables 4-9) except for the lead compounds. 

The conversion of 8 to 9 uses PPI13 and I2. The former is a nucleophile, the latter is an electrophile, so they react to give PI13P-L The P is attacked by the alcohol to give an O-P bond, and the I- then displaces PI13PC) from C to give the alkyl iodide. 

The bisphosphonates are analogs of pyrophosphate in which the P-O-P bond has been replaced with a nonhydrolyzable P-C-P bond (Figure 42-4). Etidronate, pamidronate, and alendronate have now been joined by risedronate, tiludronate, ibandronate, and zoledronate for clinical use. The bisphosphonates owe at least part of their clinical usefulness and toxicity to their ability to retard formation and dissolution of hydroxyapatite crystals within and outside the skeletal system. They localize to regions of bone resorption and so exert their greatest effects on osteoclasts. However, the exact mechanism by which they selectively inhibit bone resorption is not clear. 

Of particular importance is the conversion of much of the energy that results from photosynthesis, or from the oxidation of fats, carbohydrates, and proteins in cells into formation of phosphate ester bonds (C—O—P) or phosphate anhydride bonds (P—O—P). The energy so stored is used in other reactions, the net result of which is hydrolysis ... 

Because carbon stands at the head of its group, we expect it to be different from the other members of the group. In fact, the differences are more pronounced in Group 14 than anywhere else in the periodic table. Some of the differences between carbon and silicon stem from the smaller atomic radius of carbon, which explains the wide occurrence of C=C and C O double bonds, compared with the rarity of Si=Si and Si=0 double bonds. Silicon atoms are too large for the side-to-side overlap of p-orbitals necessary for -ir-bonds to form between them. Carbon dioxide, which consists of discrete 0=C=0 molecules, is a gas that we breathe. 

The basic structural unit needed for activity is R-pdT-R. R may be a free 3 -OH but a phosphate here markedly increases substrate binding. R may be a p-nitrophenyl group. The bond that is broken is the P-0 of the 5 -C—O-P ester linkage (60). 

In sum, a new olefinic link is produced, but by an addition-elimination sequence. In this reaction a stronger C-O double bond in the starting material is replaced by a weaker C-C double bond in the product. The thermodynamic driving force for die reaction is the formation of the P-O bond, which is very strong. 

---