Organophosphate

An organophosphate (sometimes abbreviated OP) is the general name for esters of phosphoric acid and is one of the organophosphorus compounds. They can be found as part of insecticides, herbicides, and nerve gases, amongst others. Some less-toxic organophosphates can be used as solvents, plasticizers, and EP additives. Early pioneers in the field include Lassaigne (early 1800s) and Philip de Clermount (1854).

The term organophosphate should strictly be reserved for an ester of phosphoric acid or one of its higher compounds (such as pyrophosphoric acid. But is used oftein to describe any organic phosphorus containing compound especially when dealing with neurotoxins. Many of the so called organophosphate contain C-P bonds which make them something else, for instance sarin is O-Isopropyl Methylphosphonofluoridate whose parent phosphorus acid is HP(O)(OH)_{2_{which is phosphorous acid not phosphoric acid. Also many compounds which are derivatives of phosphinic acid are used as organic phosphorus containing neurotoxin.}}

Overview of 'Organophosphate' neurotoxins

Basics

To act as an 'Organophosphate' neurotoxins the following structural requirements exist.

A terminal oxygen bonded by a double bond to the phosphorus
Two lipophilic groups bonded to the phosphorus
A leaving group bonded to the phosphorus

Terminal oxygen vs. terminal sulfur

Note that these compounds bearing terminal sulfur atoms on the phosphorus atom are much less toxic than the compounds (such as sarin, VX and tetraethyl pyrophosphate) which have an oxygen in place of this terminal sulfur. This is because the P=S compound is not active as an acetylcholinesterase inhibitor in either mammals or insects, in mammals the animals metabolism tends to remove lipophilic side groups from the phosphorus atom while an insect tends to oxidise the compound so removing the terminal sulfur and replacing it with a terminal oxygen which causes the compound to be more able to act as an acetylcholinesterase inhibitor.

Fine tuning

Within these requirements a large number of different lipophilic and leaving groups have been used. The variation of these groups is one means of fine tuning the toxicity of the compound.

Example

A good example of this chemistry are the P-thiocyanate compounds which use an aryl (or alkyl) group and an alkylamino group as the lipophilic groups. The thiocyanate is the leaving group.

It was claimed in a German patent that the reaction of 1,3,2,4-dithiadiphosphetane 2,4-disulfides with dialkyl cyanamides formed plant protection agents which contained six membered (P-N=C-N=C-S-) rings. It has been proven in recent times by the reaction of diferrocenyl 1,3,2,4-dithiadiphosphetane 2,4-disulfide (and Lawesson's reagent) with dimethyl cyanamide that in fact a mixture of several different phosphorus containing compounds is formed. Depending on the concentration of the dimethyl cyanamide in the reaction mixture either a different six membered ring compound (P-N=C-S-C=N-) or a nonheterocylic compound (FcP(S)(NR2)(NCS)) is formed as the major product, the other compound is formed as a minor product.

In addition small traces of other compounds are also formed in the reaction. It is unlikely that the ring compound (P-N=C-S-C=N-) {or its isomer} would act as a plant protection agent, but (FcP(S)(NR2)(NCS)) compounds can act as nerve poisons in insects.