 |  |  |
| 2,4-D | MCPA | 2,4,5-T |
| The differences from 2,4-D are shown in red |
2,4,5-T (2,4,5-trichlorophenoxyacetic acid) is the chemical which received notoriety after it was extensively used as a defoliant in the Vietnam war. It is closely related to a number of other herbicides, such as 2,4-D (2,4-dichlorophenoxyacetic acid) and MCPA (2-methyl-4-chlorophenoxyacetic acid), which are plant growth stimulants which cannot be metabolised by plants. Therefore if they are applied in high concentrations they cause lethal, uncontrollable and grossly distorted growth. 2,4-D and MCPA were the first hormone herbicides and the first really selective weedkillers, which killed weeds but did not harm other plants or animals. 2,4-D is still extensively used in the USA, but in the UK MCPA is more commonly used, especially in the form of a combined fertiliser/weedkiller for controlling dandelions and daisies in lawns.
| | |
| 2,4-D | MCPA | 2,4,5-T |
| The differences from 2,4-D are shown in red |
Whereas 2,4-D and MCPA have been used extensively all over the world for the past 50 years without any problems, their chemically similar derivative, 2,4,5-T has been surrounded with controversy. This is because it was widely used as a defoliant by the USA in the Vietnam war in the 1960s, and combined with 2,4-D, this spray was named Agent Orange. 2,4,5-T is extremely effective as a defoliant, and has even been used in the UK to control brambles encroaching on footpaths or railway lines by the Forestry Commission and British Rail, respectively. It has great advantages in that is has low toxicity to animals and is cheap to manufacture.
It is the second step in the reaction that has caused all the problems, since the temperature has to be carefully controlled. If it rises above 160°C a side-reaction between two of the sodium intermediates occurs producing the deadly tetrachlorodioxin.
Although the 2,4,5-T molecule itself was non-toxic, the small quantities of the dioxin impurities caused many problems. These included a greater incidence than normal of malformed babies in villages close to zones that had been sprayed with Agent Orange. The US National Cancer Institute established that the dioxin, which was present in 2,4,5-T at concentrations of only 10-30 ppm, was responsible for producing malformations in mammalian babies. Various court cases are still pending in the US for compensation for Vietnam veterans who claim exposure to Agent Orange has been responsible for birth defects in their children and developments of cancer in themselves. The arguments for and against banning of 2,4,5-T have therefore focused upon the concentration of dioxin impurity it should be allowed to contain. In Germany and the USA is has now been banned completely, but in the UK it can still be used for some agricultural applications - although alternative herbicides are now increasingly being used instead.
2,4,5-T and dioxin hit the headlines again in 1976, when an explosion occurred at the Givaudan plant near the town of Seveso in Northern Italy. The plant was manufacturing 2,4,5-T, and for some reason there was a rapid, unexpected rise in temperature which caused a rapid increase in pressure, resulting in a safety valve blowing and an explosion. A cloud of chemicals was released over the surrounding area and town, including mostly the harmless 2,4,5-T, but also the potentially deadly impurity, dioxin. It is estimated that 2 kg of the dioxin were in the cloud, and the area was immediately sealed off and the population was evacuated. Locally grown food was banned for several months, and several inches of topsoil were removed and incinerated. However, despite all these worries, only one person, to date, appears to have died from liver cancer, although there were a number of cases of skin disease (chloracne). No employees of the company suffered damage to their organs, but some of the town's women had abortions. The incidence of malformed children was comparable to that found elsewhere, so it appears as though the inhabitants of Seveso escaped relatively lightly. Nevertheless, the Seveso incident has made people a lot more aware of the dangers of having chemical industry located close to populated areas.