Phosphorus-oxygen double bond

What is special about double bonds between oxygen and sulfur or phosphorus ... 

With phosphorus ylides as used for the Wittig Reaction, the phosphorus atom forms a strong double bond with oxygen. This leads the mechanism in a different direction, to effect olefination instead of epoxidation through intermediate oxaphosphetanes. 

The initial step of olefin formation is a nucleophilic addition of the negatively polarized ylide carbon center (see the resonance structure 1 above) to the carbonyl carbon center of an aldehyde or ketone. A betain 8 is thus formed, which can cyclize to give the oxaphosphetane 9 as an intermediate. The latter decomposes to yield a trisubstituted phosphine oxide 4—e.g. triphenylphosphine oxide (with R = Ph) and an alkene 3. The driving force for that reaction is the formation of the strong double bond between phosphorus and oxygen ... 

Phosphorus(V) oxide, P4 Oio, contains the same triangular arrangement of phosphorus atoms as in P4, but an oxygen atom is inserted into each P—P bond. An additional terminal O atom is double-bonded to each P atom. 

The monomeric metaphosphate ion itself commands a fair amount of attention in discussions of metaphosphates. It is postulated as an intermediate of numerous hydrolysis reactions of phosphoric esters 52 S4,S5) and also of phosphorylation reactions S6> kinetic and mechanistic studies demonstrate the plausibility of such an assumption. In addition, the transient formation of ester derivatives of meta-phosphoric acid — in which the double-bonded oxygen can also be replaced by thio and imino — has also been observed they were detected mainly on the basis of the electrophilic nature of the phosphorus. 

One of the hydrogen atoms is bound directly to the phosphorus atom and is not normally acidic. Note that in this case, the distance between the phosphorus atom and the oxygen atom without a hydrogen atom attached is only 147 pm, indicating more double bond character than there is in the corresponding bond in the H3P04 molecule. 

Nerve agents are organophosphonate compounds. They contain phosphorus double-bonded to an oxygen atom and single-bonded to a carbon atom. GB is O-isopropyl methylphosphonofluoridate. VX is O-ethyl-S [2-(diisopropyl amino) ethyl]-methylphospho-nothiolate. 

Molecular orbital theory may provide an explanation for stereochemical differences between carboxylate-metal ion and phosphate-metal ion interactions. Detailed ab initio calculations demonstrate that the semipo-lar 1 0 double bond of RsP=0 is electronically different from the C=0 double bond, for example, as found in H2C=0 (Kutzelnigg, 1977 Wallmeier and Kutzelnigg, 1979). The P=0 double bond is best described as a partial triple bond, that is, as one full a bond and two mutually perpendicular half-7r bonds (formed by backbonding between the electrons of oxygen and the empty d orbitals of phosphorus). Given this situation, a lone electron pair should be oriented on oxygen nearly opposite the P=0 bond, and these molecular orbital considerations for P=0 may extend to the phosphinyl monoanion 0-P=0. If this extension is valid, then the electronic structure of 0-P=0 should not favor bidentate metal complexation by phosphate this is in accord with the results by Alexander et al. (1990). 

To explain the existence of a conventional double bond in the phosphate ion requires five electrons to be supplied by the central phosphorus atom, which may enter into MO formation with suitable orbitals from the ligand oxygen atoms. To arrange this requires the use of one of the 3d orbitals of the phosphorus atom. This cannot occur in compounds of elements of the second period, where hypervalence is not observed. The availability of accessible d-orbitals has, in the past, been used as the justification of hypervalence in compounds of the subsequent periods. An alternative method of representing the structure of the PO4 ion is shown in Figure 6.6. 

Figure 2.23 Examples of a special double bond between phosphorus and oxygen indicated by the arrow between 0 and P.

Phosphorus can form five covalent bonds. The conventional representation of Pj (Pig. 10a), with three P—0 bonds and one P=0 bond, is not an accurate picture. In Pi four equivalent phosphorus-oxygen bonds share some double-bond character, and the anion has a tetrahedral structure (Fig. 10b). As oxygen is more electronegative than phosphorus, the sharing of electrons is unequal the central phosphorus bears a partial positive... 

Selective oxidation of methyl ethyl ketone to diacetyl has been studied by passing a mixture of the ketone in artificial air over vanadium phosphorus oxide catalysts in the temperature range 200-350 C. Products observed included diacetyl, methyl vinyl ketone, acetaldehyde, acetic acid and carbon dioxide. C4 products were favoured at low temperatures and at low or zero oxygen partial pressures. These results are rationalised in terms of two pathways for C2 products, namely oxidation of the double bond in the enol form of methyl ethyl ketone to yield acetic acid and acetaldehyde, and acid catalysed hydration of the keto form to yield acetaldehyde only. The C4 products are envisaged to go through a common intermediate, namely, CH3COCHOHCH3, formed by interaction between methyl ethyl ketone and lattice oxygen. 

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