Chemistry and classification of OP compounds

Abstract

The term organophosphorus (OP) compounds refers to any group of organic chemicals that contain phosphorus. The most predominant valances of phosphorus are 3 and 5. The majority of OP compounds (OPs) with environmental and industrial applications are of the pentavalent types. Organophosphates are a significant group of OPs which are essentially esters of phosphoric acids, in which the nature of the substituents attached to phosphorus plays a key role in determining the toxicity of the agents. The first OPs were prepared by alchemists in the mid ages. However, the systematic study of these chemicals began in 1820 with the esterification reaction of phosphoric acid and ethanol. Many reaction routes for the synthesis of OPs have been investigated and established. The majority of the OP pesticides lie under the category of phosphorothioate with thiophosphoryl bond (P=S) rather than a phosphoryl bond (P=O). Nerve agents (NAs) are mostly alkylphosphonic acid esters. OPs interact with acetylcholinesterase (AChE), resulting in inhibition of acetylcholine (ACh) hydrolysis and thus ACh accumulations in nerve endings. Some OPs with P=O bonding are direct AChE inhibitors, whereas those with P=S bonding, which are phosphothioates, do not possess anticholinesterase activities before biotransformation and thus are non-toxic per se. Apart from phosphates and phosphonates, most OPs only show anticholinesterase activities once they have undergone metabolic biotransformation.A new generation of OP NAs has been developed and named “Novichoks” (which means “new comer” in Russian). It has been claimed that their action mechanism may differ from the other OP NAs, and the toxicity of certain “Novichoks” may exceed that of VX. Research on the development of new OPs is ongoing, and the pesticides regulatory authorities and arm control organisations such as OPCW should prepare accordingly.