Phosphorus-oxygen bond, energy

Some quantitative support for the above qualitative arguments comes from average bond energies of phosphorus, bromine, and oxygen (Appendix E)-. 

It is estimated that more than 25 x 106 different potentially toxic OP esters can be made using Schraders classic (27) formula for effective phosphorylating agents, (39), where R and R are short-chain alkyl, alkoxy, alkylthio, or alkylamino groups, and X is a displaceable moiety with a high energy P-bond such as F or acyl anhydride, and the pentavalent phosphorus atom is bonded to oxygen or sulfur. 

PAPH may be summarized as follows. The effect of an app interaction of sp -hybrid nonbonding electrons on equatorial oxygen with an apical P—O bond, in a phosphorus TBP transition state, is to weaken that apical bond and lower the energy of that pentacoordinate structure relative to other conformers. The hypothesis predicts that (1) a reaction involving such an app interaction will occur faster than a similar reaction where no similar stereoelectronic interaction exists, and (2) a phosphorus TBP will breakdown preferentially via cleavage of an apical P—O bond involved in an app interaction. PAPH may be extrapolated to situations in which heteroatoms other than oxygen are attached to phosphorus. 

The allotropy of carbon, oxygen, phosphorus, and sulfur results from the versatility of their covalent bonding. Carbon occurs as diamond and as graphite (Fig. 21.1). Diamond is extremely hard, in consequence of its stable network covalent structure, which is entirely o--bonded. Graphite is relatively soft, in part because of the ease with which its TT-bonded atomic layers can slip past one another. At ordinary temperatures and pressures, both forms are quite unreactive, and graphite is the form with lower free energy (more stable) by about 0.7 kcal/mole. 

Problems still exist with AM 1, freatment of phosphorus-oxygen bonds is inaccurate, nitro compounds are still too positive in energy, and the peroxide bond, for example, is still too short. In many cases, l M3 is an improvement over AM 1,... 

Phosphorus—Carbon Bond. The P—C bond is 0.184—0.194-nm long and has an energy of ca 272 kj/mol (65 kcal/mol). It is one of the more stable bonds formed by phosphoms, resistant to both hydrolysis and oxidation (7,8). Unlike the phosphoms—halogen or phosphoms—oxygen bonds, the P—C linkage is inert to exchange. A phosphoms atom connected to carbon behaves similarly to another carbon atom in a hydrocarbon chain. 

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