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Concept
Phenoxy herbicide
Summary
Phenoxy herbicides (or "phenoxies") are two families of chemicals that have been developed as commercially important herbicides, widely used in agriculture. They share the part structure of phenoxyacetic acid.
The first group to be discovered act by mimicking the auxin growth hormone indoleacetic acid (IAA). When sprayed on broad-leaf plants they induce rapid, uncontrolled growth ("growing to death"). Thus when applied to monocotyledonous crops such as wheat or maize (corn), they selectively kill broad-leaf weeds, leaving the crops relatively unaffected.
File:Indol-3-ylacetic acid.svg|IAA
File:2-(4-chloro-2-methylphenoxy)acetic acid 200.svg|MCPA
File:2,4-Dichlorophenoxyacetic acid structure.svg|2,4-D
File:2,4,5-T.svg|2,4,5-T
Introduced in 1946, these herbicides were in widespread use in agriculture by the middle of the 1950s. The best known phenoxy herbicides are (4-chloro-2-methylphenoxy)acetic acid (MCPA), 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). Analogues of each of these three compounds, with an extra methyl group attached next to the carboxylic acid, were subsequently commercialised as mecoprop, dichlorprop and fenoprop. The addition of the methyl group creates a chiral centre in these molecules and biological activity is found only in the (2R)-isomer (illustrated for dichlorprop).
File:Mecoprop structure.png|Mecoprop
File:(R)-Dichlorprop Structural Formula V.1a.svg|(2R)-Dichlorprop
File:Fenoprop.png|Fenoprop
File:4-(2,4-dichlorophenoxy)butanoic acid 200.svg|2,4-DB
File:MCPB.png|MCPB
Other members of this group include 4-(2,4-dichlorophenoxy)butyric acid (2,4-DB) and 4-(4-chloro-2-methylphenoxy)butyric acid (MCPB) which act as propesticides for 2,4-D and MCPA respectively: that is, they are converted in plants to these active ingredients. All the auxin herbicides retain activity when applied as salts and esters since these are also capable of producing the parent acid in situ.
The use of herbicides in US agriculture is mapped by the US Geological Survey.
Official source
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