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2,4-dichlorophenoxyacetic acid

2,4,5-trichlorophenoxyacetic acid

2,4,5-trichlorophenoxyacetic acid

Note that the numbering scheme for the benzene ring begins at the carbon attached to the oxygen.

The 2,4-D compound (2,4-dichlorophenoxy acetic acid) is used to kill broad-leaf weeds in lawns, golf course fairways and greens, and agricultural fields. In contrast, 2,4,5-T (2,4,5-trichlorophenoxy acetic acid) is effective in clearing brush, for instance, on roadsides and powerline corridors. Like the P—O—C bonds in organophosphates, the O—C bond to the —CH,— group in 2,4-D and analogous phenoxy herbicides undergoes a hydrolysis reaction in the environment, degrading the compound to a phenol

R—O—CH — COOH + H—OH-* R—O—H + HOCH2—COOH

The herbicide MCPA is 2,4-D with the chlorine in the 2 position replaced by a methyl group, —CH3. The herbicides called dichbrprop, silvex, and mecoprop are identical to 2,4-D, to 2,4,5-T, and to MCPA, respectively, except that their molecules have a methyl group replacing one hydrogen atom of the —CH2— group in the acid chain; thus they are phenoxy herbicides that are based on propionic acid, CH3—CH2—COOH, rather than on acetic acid. The herbicide dicamba is the same as 2,4-D with a mcthoxy group at the 5 position of the benzene ring; it is often used as a weedkiller in corn fields.

Huge quantities of 2,4-D and its closely related analogs described above are used in developed countries for the control of weeds in both agricultural and domestic settings. In some communities, their continued use on lawns has become a controversial practice because of their suspected effects on human health. In particular, farmers in the midwestem United States who mix and apply large quantities of 2,4-D to their crops are found to have an increased incidence of the cancer known as non-Hodgkin's lymphoma.

Continue reading here: The Degradation of Pesticides

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