Phenoxy herbicides (or "phenoxies") are two families of chemicals that have been developed as commercially important

herbicide Herbicides (, ), also commonly known as weedkillers, are substances used to control undesired plants, also known as weeds.EPA. February 201Pesticides Industry. Sales and Usage 2006 and 2007: Market Estimates. Summary in press releasMain page fo ...

s, widely used in

agriculture Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people to ...

. They share the part structure of phenoxyacetic acid.

Auxins

The first group to be discovered act by mimicking the

growth hormone Growth hormone (GH) or somatotropin, also known as human growth hormone (hGH or HGH) in its human form, is a peptide hormone that stimulates growth, cell reproduction, and cell regeneration in humans and other animals. It is thus important in ...

indoleacetic acid Indole-3-acetic acid (IAA, 3-IAA) is the most common naturally occurring plant hormone of the auxin class. It is the best known of the auxins, and has been the subject of extensive studies by plant physiologists. IAA is a derivative of indole, con ...

(IAA). When sprayed on broad-leaf plants they induce rapid, uncontrolled growth ("growing to death"). Thus when applied to

monocotyledonous Monocotyledons (), commonly referred to as monocots, (Lilianae ''sensu'' Chase & Reveal) are grass and grass-like flowering plants (angiosperms), the seeds of which typically contain only one embryonic leaf, or cotyledon. They constitute one of t ...

crops such as

wheat Wheat is a grass widely cultivated for its seed, a cereal grain that is a worldwide staple food. The many species of wheat together make up the genus ''Triticum'' ; the most widely grown is common wheat (''T. aestivum''). The archaeological ...

corn Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American and Australian English), is a cereal grain first domesticated by indigenous peoples in southern Mexico about 10,000 years ago. The ...

, 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 Herbicides (, ), also commonly known as weedkillers, are substances used to control undesired plants, also known as weeds.EPA. February 201Pesticides Industry. Sales and Usage 2006 and 2007: Market Estimates. Summary in press releasMain page fo ...

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 MCPA (2-methyl-4-chlorophenoxyacetic acid) is a powerful, selective, widely used phenoxy herbicide. The pure compound is a brown-colored powder. MCPA has been extensively used in agriculture to control broad-leaf weeds as a growth regulator prim ...

), 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 Mecoprop (also known as methylchlorophenoxypropionic acid and MCPP) is a common general use herbicide found in many household weed killers and "weed-and-feed" type lawn fertilizers. It is primarily used to control broadleaf weeds.dichlorprop and

fenoprop Fenoprop, also called 2,4,5-TP, is the organic compound 2-(2,4,5-trichlorophenoxy)propionic acid. It is a phenoxy herbicide and a plant growth regulator, an analog of 2,4,5-T in which the latter's acetic acid sidechain is replaced with a propio ...

. The addition of the methyl group creates a

chiral Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from i ...

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 2,4-DB or 4-(2,4-dichlorophenoxy)butyric acid is a selective systemic phenoxy herbicide used to control many annual and perennial broad-leaf weeds in alfalfa, peanuts, soybeans, and other crops. Its active metabolite In biochemistry, a metabol ...

) and 4-(4-chloro-2-methylphenoxy)butyric acid (

MCPB MCPB, 2,4-MCPB, 4-(4-chloro-o-tolyloxy)butyric acid (IUPAC), or 4-(4-chloro-2-methylphenoxy)butanoic acid ( CAS) is a phenoxybutyric herbicide. In the United States it is registered for use on pea crops before flowering, for post-emergence contr ...

) 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 In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively cha ...

and

esters In chemistry, an ester is a chemical compound, compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an Alcohol ...

since these are also capable of producing the parent acid ''in situ''. 2,4-D usage USA

The use of herbicides in US agriculture is mapped by the US Geological Survey. In 2019, the latest date for which figures are available, 2,4-D was the most used of the auxins. were sprayed that year, compared to of the next most heavily applied, MCPA. The other auxin now used in comparable amounts to 2,4-D is

dicamba Dicamba (3,6-dichloro-2-methoxybenzoic acid) is a broad-spectrum herbicide first registered in 1967. Brand names for formulations of this herbicide include Dianat, Banvel, Diablo, Oracle and Vanquish. This chemical compound is a chlorinated ...

, where the 2019 figure was . It is a

benzoic acid Benzoic acid is a white (or colorless) solid organic compound with the formula , whose structure consists of a benzene ring () with a carboxyl () substituent. It is the simplest aromatic carboxylic acid. The name is derived from gum benzoin, wh ...

rather than a phenoxyacetic acid whose use has grown rapidly since 2016 as crops

genetically modified Genetic engineering, also called genetic modification or genetic manipulation, is the modification and manipulation of an organism's genes using technology. It is a set of technologies used to change the genetic makeup of cells, including ...

to be resistant to it have been cultivated.

ACCase inhibitors

In the 1970s,

agrochemical An agrochemical or agrichemical, a contraction of ''agricultural chemical'', is a chemical product used in industrial agriculture. Agrichemical refers to biocides (pesticides including insecticides, herbicides, fungicides and nematicides) and sy ...

companies were working to develop new herbicides to be complementary to the auxins. The aim was to find materials which would selectively control grass weeds in broad-leaf crops such as cotton and

soybean The soybean, soy bean, or soya bean (''Glycine max'') is a species of legume native to East Asia, widely grown for its edible bean, which has numerous uses. Traditional unfermented food uses of soybeans include soy milk, from which tofu an ...

In 1973,

Hoechst AG Hoechst AG () was a German chemicals then life-sciences company that became Aventis Deutschland after its merger with France's Rhône-Poulenc S.A. in 1999. With the new company's 2004 merger with Sanofi-Synthélabo, it became a subsidiary of th ...

filed patents on a new class of compound, the aryloxphenoxypropionates, which showed such selectivity and led to the commercialisation of diclofop. Then the Japanese company Ishihara Sangyo Kaisha (ISK) found improved biological activity in an analogue, chlorazifop, which replaced the aryloxy portion of diclofop with a

pyridine Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid with ...

ring containing the same two chlorine substituents. This area of research became very competitive and within three weeks of one another in 1977 ISK, Dow Chemicals and

Imperial Chemical Industries Imperial Chemical Industries (ICI) was a British chemical company. It was, for much of its history, the largest manufacturer in Britain. It was formed by the merger of four leading British chemical companies in 1926. Its headquarters were at Mi ...

(ICI) all filed patents covering another group of analogues, with a trifluoromethyl (CF₃) group in place of one of the chlorine atoms in the pyridine. Subsequently, ISK and ICI cross-licensed their

intellectual property Intellectual property (IP) is a category of property that includes intangible creations of the human intellect. There are many types of intellectual property, and some countries recognize more than others. The best-known types are patents, c ...

and first marketed

fluazifop Fluazifop is the ISO common name for an organic compound that is used as a selective herbicide. The active ingredient is the 2R enantiomer at its chiral centre and this material is known as fluazifop-P when used in that form. More commonly, it is ...

as its butyl ester in 1981 under the

brand name A brand is a name, term, design, symbol or any other feature that distinguishes one seller's good or service from those of other sellers. Brands are used in business, marketing, and advertising for recognition and, importantly, to create an ...

Fusilade while Dow marketed haloxyfop as its methyl ester. All these compounds have an additional oxygen-linked

aromatic In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satu ...

group in the para position of the

phenyl In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula C6 H5, and is often represented by the symbol Ph. Phenyl group is closely related to benzene and can be viewed as a benzene ring, minus a hydroge ...

ring with its OCH(CH₃)COOH group and as a class are called "fops", referring to their common fenoxy-phenoxy icfeature. This group of herbicides acts by inhibiting plant acetyl-CoA carboxylase (ACCase), a completely different

mechanism of action In pharmacology, the term mechanism of action (MOA) refers to the specific biochemical interaction through which a drug substance produces its pharmacological effect. A mechanism of action usually includes mention of the specific molecular targ ...

to that of the auxins. Their selectivity for grasses arises because they target the

plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cells of plants, algae, and some other eukaryotic organisms. They are considered to be intracellular endosymbiotic cyanoba ...

isoform A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene or gene family and are the result of genetic differences. While many perform the same or similar biological roles, some isof ...

of the

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. ...

present only in these species, making them ineffective on broad-leaf weeds and other organisms including mammals. When applied as an ester,

metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cel ...

in the target plant leads to the parent acid which is responsible for the herbicidal action. It is a coincidence that it is the (2R) stereoisomer which binds to plant ACCase, just as that isomer is responsible for the activity of dichlorprop as an auxin. Fenoxaprop-P-ethyl 200

Salts and esters of this class of herbicide are active owing to their ability to metabolise to the corresponding parent acid. For example, fenoxaprop-P ethyl was introduced by

Bayer Crop Science Bayer AG (, commonly pronounced ; ) is a German multinational pharmaceutical and biotechnology company and one of the largest pharmaceutical companies in the world. Headquartered in Leverkusen, Bayer's areas of business include pharmaceutica ...

and quizalofop-P ethyl by Nissan Chemical Corporation, both in 1989. In 1990, Dow introduced cyhalofop-P butyl for the control of weeds in rice. Fluazifop-P butyl still has significant use in the USA. were applied in 2018 — almost exclusively in soyabean. The "P" in the name of these materials refers to their use now as single

enantiomers In chemistry, an enantiomer ( /ɪˈnænti.əmər, ɛ-, -oʊ-/ ''ih-NAN-tee-ə-mər''; from Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''(méros)'' 'part') – also called optical isomer, antipode, or optical ant ...

References

{{DEFAULTSORT:Phenoxy Herbicide Herbicides