PHOSPHORUS CHEMISTRY: MECHANISMS
MECHANISMS: The reactions involving Phosphorus are:
- Substitution reactions: one group on phosphorus is replaced by another group; has two possiblities.
- Oxidation reactions: oxidation state of phosphorus increases after the reaction; the reverse is Reduction reaction.
- Addition reactions: another molecule adds to phosphorus; the reverse is dissociation/elimination reaction.
- Elimination reactions: one molecule leads to two molecules.
- Oxidative addition: the reaction combines both 2 and 3.
The following are the changes at phosphorus: (CN = Coordination Number; OS - Oxidation State)
- Substitution reactions: CN - same; OS - same.
- Oxidation reactions: CN - changes; OS - changes.
- Addition reactions: CN - changes; OS - same.
- Elimination reactions: CN - changes; OS - same.
- Oxidative addition: CN - changes; OS - changes.
- Ph2PCl + MeONa ==> Ph2POMe + NaCl
- Ph3P + Me3NO ==> Ph3PO + Me3N
- Dimerisation of P=N compounds to form four membered P2N2 rings
- (EtO)5P ==> (EtO)3PO + EtOEt
- 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 Nucleophilic mechanism only. This is the extensively
studied mechanism and still not known in completeness. There are many possiblities
in this mechanism itself. They are:
- Dissociation Mechanism: the CN reduces temporarily and reaches the old value soon.
- Elimination Mechanism: same as above, but one molecule goes out instead of a group/ion.
- Concerted Mechanism: the groups coming and leaving are weakly linked to phosphorus for little time.
- Association Mechanism: same as above, but the linked compound with CN=5 is longer lived.
- Double Association: this is proposed recently; here the incoming group and another group (catalyst?)
add to phosphorus and increase the CN to six (or pseudo-six when there is a lone-pair).
FIGURES for these Mechanisms: