Synthesis

The phosphoric acid esters used in the synthesis of the organophosphorus insecticides are prepared industrially by the reaction of phosphoryl chloride with alcohols:

Cl—P —Cl + 2ROH + 2R3N-► RO—P — Cl + 2R3N- HCl (8-59)

Cl OR

Under suitable conditions, diester chlorides (dialkyl phosphorochloridates) may be prepared. The same reaction pathway may be used for the preparation of corresponding sulfur analogues (dialkyl phosphorothiochloridates):

Cl—P — Cl + 2ROH + 2R3N-- RO—P — Cl + 2R3N HCl (8-60)

Cl OR

Both the phosphoric acid and thiophosphoric acid derivatives may also be prepared by the action of chlorine on the corresponding derivative of phosphorous acid:

RO OR

Most nucleophiles attack phosphorochloridates at the phosphorus atom and displace the chloro group. This reaction [i.e., reaction (8-62) ] is analogous to the sn2 displacement on carbon (Section 8.3.2).

OR RO W

Transition state

The transition state for the reaction is shown in reaction (8-62).

Most of the organophosphorus insecticides are triesters of phosphoric or thiophosphoric acid that are prepared by the attack of a nucleophile on the phosphorochloridates or phosphorothiochloridates, as shown in the examples that follow. Since the ester group is more difficult to displace than the chloro group, the triesters exhibit greater stability than the phosphorochloridates. As a consequence of the slow displacement of the ester, a reaction pathway different from that just illustrated is sometimes observed. For example, in neutral and acid solutions water can attack the a-carbon atom of the phosphate ester grouping with resultant cleavage of the carbon-oxygen bond:

O ii

H2O + R-^O—P — OH -- H2OR + O—P — OR (8-63)

OR OR

It has been shown by isotope tracer studies using 18O that the P—O bond is not broken. Reaction (8-63) is more likely under environmental conditions than the direct displacement on phosphorus shown in reaction (8-62).

The synthesis of parathion and methyl parathion also involves an SN2-like displacement:

(RO)2P—Cl + }—NO2 -- (RO)2P—O—(f J—NO2 + Cl-

R = CH3CH2— , parathion R = CH3— , methyl parathion

The synthesis of malathion involves a nucleophilic addition of a thioacid to a double bond that is conjugated to a carbonyl group as in reaction (8-65), a Michael addition.

S CHCO2CH2CH3 S CH2CO2CH2CH3 ii ii II i , N

(CH3O)2P—SH + CHCO2CH2CH3 -- (CH3O)2P—SCHCO2CH2CH3 (8-65)

Malathion

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