A case study the formulation of acids

Most commercial formulations of the phenoxyalkanoic acids or 'auxin-fype' herbicides (Chapter 7) contain the active ingredient (e.g. Figure 3.9) in the salt or ester form. Of the salts, the amine forms are commonly used, although the sodium, potassium and ammonium salts are also found. Some examples of the cationic amine salts are shown in Table 3.7. These salts are highly soluble in water and formulated as aqueous concentrates. This ensures that they can be applied at relatively high rates of active ingredient but at relatively low volume.

Phenoxyalkanoic acid esters may also be applied as emulsifiable concentrates and as such exhibit greater herbicidal efficacy than the parent acids. This is because the ester is more lipophilic and more rapidly absorbed by the target weed. The esters of low-molecular-weight alcohols (such as methyl, ethyl and propyl), however, are volatile and may cause unwanted phytotoxicity to non-target plants, including crops. This problem has been overcome with the use of low-volatile esters (Figure 3.10).

A further example has been the introduction of the butoxy-methyl ethyl ester of flurox-ypyr (Figure 3.11) (Snel et al- 1995), which can be formulated as an emulsion-in-water or a wettable powder without the aromatic solvents required by its EC predecessor, the methylheptyl ester.

The aryloxyphenoxypropanoates, such as diclofop (Figure 3.9- , are also formulated as esters, traditionally as emulsifiable concentrates that partition rapidly into the cuticle. In this case, the esters of the lower molecular weight alcohols are relatively stable, as



Cl Diclofop-acid

Cl Diclofop-acid

Figure 3.9 Structures of a typical phenoxyalkanoic acid (2,4-D) and an aryloxyphenoxypropanoate (diclofop-acid).

Table 3.7 Amine cations of phenoxyalkanoic acid amine salts (Loos, 1975).

Organic Group (R)

Cation structure and name

/ \ RH



















2-ethylhexyl CH3 -CH2 -CH2 -CH2 CH-CH2-


Iso-octyl CH3-CH-CH2-CH-CH2-CH2-Butoxyethyl CH3-CH2-CH2-CH2-O-CH2-CH2-Tetrahydrofurfuryl H2C-CH2


Figure 3.10 Examples of some low volatility esters used in herbicide formulation. NH2

Figure 3.11 Butoxy-methyl ethyl ester of fluroxpyr.

Figure 3.11 Butoxy-methyl ethyl ester of fluroxpyr.

evidenced by the commercialised examples diclofop- methyl, quizalofop- ethyl and flu-azifop-butyl. The ester itself is not the active moiety but requires conversion to the potent acid as a prerequisite for herbicidal activity.

De-esterification or hydrolysis in vivo was first demonstrated by the pioneering work of Crafts (1960) and an enzyme responsible was first partially purified and characterised from wild oat by Hill et al. in 1978. These authors found that this enzyme was non-specific and could de-esterify several ester types. Hence, the esters may be regarded as 'pro-herbicides', requiring enzymatic activation in the plant tissues.

The phenoxyacetic acids (including 2,4-D, MCPA and mecoprop) have log Kow values in the range of 2.9 to 3.6 and pKa values close to 3.0. Thus, they move effectively in the phloem but are strongly held in adjacent tissues by ion-trapping. On the other hand, the aryloxyphenoxypropanoates are more lipophilic (log Kow 3.4-4.4) and have pKa values in the region of 3.5. As may be predicted from such lipophilic molecules, their phloem mobility is very poor. However, the potency of these herbicides at the grass meristem is such that even though less than 1% of applied dose is mobile, it is sufficient to control grass weeds (see Chapter 8, and Carr et al., 1986).

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