Tricresyl phosphate

Tricresyl phosphate (TCP) is a mixture of organophosphate compounds most notably used as a flame retardant. Other uses include as a plasticizer in manufacturing for lacquers and varnishes and vinyl plastics and as an antiwear additive in lubricants. Pure tricresyl phosphate is a colourless, viscous liquid, although commercial samples are typically yellow. It is virtually insoluble in water, but easily soluble in organic solvents like toluene, hexane, and diethyl ether among others. It was synthesized by Alexander Williamson in 1854 upon reacting phosphorus pentachloride with cresol (a mixture of para-, ortho-, and meta- isomers of methylphenol), though today's manufacturers can prepare TCP by mixing cresol with phosphorus oxychloride or phosphoric acid as well. The ortho isomer of TCP is a rather toxic substance with a long history of causing mass poisonings.

Isomers

Many isomers of tricresyl phosphate exist and comprise typical samples of this compound. Neurotoxicity is believed to be restricted to isomers containing ortho-cresyl groups.

The most toxic isomer was initially thought to be the ooo-isomer, tri-ortho-cresyl phosphate (TOCP). The World Health Organization stated in 1990 that "Because of considerable variation among individuals in sensitivity to TOCP, it is not possible to establish a safe level of exposure" and "Both the pure ortho isomer and isomeric mixtures containing TOCP are, therefore, considered major hazards to human health." However, isomers with a lower proportion of ortho-cresyl groups are now known to be most dangerous.