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Some assorted reactions to show the versatility of phosphorus and to exemplify the points
The first step of the Wittig Reaction - attack of the phosphonium ylid on the carbonyl - can be reversible or irreversible depending on the nature of the reactants, and this can have an important effect on the stereoselectivity of the process (if both reactants are asymmetrical).
The main points about phosphorus i. Has two oxidation states readily available; (+/-) 3 and (+/-) 5.