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MECHANISMS: The reactions involving Phosphorus are:

  1. Substitution reactions: one group on phosphorus is replaced by another group; has two possiblities.
  2. Oxidation reactions: oxidation state of phosphorus increases after the reaction; the reverse is Reduction reaction.
  3. Addition reactions: another molecule adds to phosphorus; the reverse is dissociation/elimination reaction.
  4. Elimination reactions: one molecule leads to two molecules.
  5. Oxidative addition: the reaction combines both 2 and 3.

The following are the changes at phosphorus: (CN = Coordination Number; OS - Oxidation State)
  1. Substitution reactions: CN - same; OS - same.
  2. Oxidation reactions: CN - changes; OS - changes.
  3. Addition reactions: CN - changes; OS - same.
  4. Elimination reactions: CN - changes; OS - same.
  5. Oxidative addition: CN - changes; OS - changes.

  1. Ph2PCl + MeONa ==> Ph2POMe + NaCl
  2. Ph3P + Me3NO ==> Ph3PO + Me3N
  3. Dimerisation of P=N compounds to form four membered P2N2 rings
  4. (EtO)5P ==> (EtO)3PO + EtOEt
  5. Ph3P + O ==> Ph3PO

Electrophilic Substitution Reactions: In nature electrophilic reactions at phosphorus do not occur. However, in labs and industries it occurs when a metal-phosphide is treated with any halogen compound or with compounds containing OH or SH group. For example: [ Ph2PLi + MeBr ==> Ph2PMe + LiBr ] or [ Ph2PLi + H2O ==> Ph2PH + LiOH ]

NUCLEOPHILIC SUBSTITUTION REACTIONS: Most of the reactions at phosphorus proceed by this mechanism. The group coming toward the phosphorus is an electron rich group and is ready to donate the electrons to the electrophilic phosphorus. Phosphorus wants more electrons and so it is electrophilic. The donor wants an electron acceptor (similar to a nucleus which has a positive charge) so it is called nucleophilic (that is positive charge liking). The name of the mechanism is defined by the group which is attacking the phosphorus atom. The nucleophile is any group with a lone pair of electrons (with or without a negative charge). Alcohols/phenols (and their sulfur analogs) and amines are nucleophiles; all of their metal salts are stronger nucleophiles. Water is also a good nucleophile and so, many times phosphates react with water to undergo "hydrolysis".
All the biological reactions involve this mechanism only. This is the extensively studied mechanism and still not known in completeness. There are many possiblities in this mechanism itself. They are: The last two are the most important pathways by which vast majority of phosphorus compounds react, including in biological systems.

FIGURES for these Mechanisms:

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Created by Dr. A. Chandrasekaran
Department of Chemistry, University of Massachusetts, AMHERST, MA 01003, USA.

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