Uracil herbicides

Urea and uracil herbicides tend to be persistent in soils and may carry over from one season to the next (299). However, there is significant variation between compounds. Bromacil is debrominated under anaerobic conditions but does not undergo further transformation (423), linuron is degraded in a field soil and does not accumulate or cause carryover problems (424), and terbacd [5902-51-2] is slowly degraded in a Russian soil by microbial means (425). The half-hves for this breakdown range from 76 to 2,475 days and are affected by several factors including moisture and temperature. Finally, tebuthiuron apphed to rangeland has been shown to be phytotoxic after 615 days, and the estimated time for total dissipation of the herbicide is from 2.9 to 7.2 years (426). 

Methyl acetoacetate (MAA) and ethyl acetoacetate (EAA) are the most widely used esters they are found ia the pharmaceutical, agricultural, and allied industries. Both esters are used extensively as amine protecting agents ia the manufacture of antibiotics and synthetic sweeteners (Dane Salts) (147). Principal outiets for MAA are the manufacture of the organophosphoms insecticide dia2inon [33341-5] (148,149) and the uracil herbicides bromacil [31440-9] and terbacil [5902-51-2] (150,151) (see Insect conztiol technology Herbicides). 

Use pattern Terbacil is a substituted uracil herbicide used primarily to control annual and perennial grass and broadleaved weeds in apple orchards, alfalfa, asparagus, mint and sugar cane. Other minor uses include blueberries, strawberries and seed grasses... 

Calverly and Denny [199] have described a rapid and sensitive procedure for the determination of residue levels of three uracil herbicides in soils. After addition of calcium hydroxide and Celite to the soil the herbicides are eluted from columns with water. After acidification of the eluate and partition into chloroform these herbicides are determined by gas chromatography using a nitrogen-selective detector. Recoveries from a range of soil types are better than 80%, with a sensitivity limit of 20pg kgy1. 

PAHS, organochlorine insecticides, benzene, uracil herbicides, carbamates, acid amides, phenoxy herbicides, pentachlorophenol, penzene, DCPB, organophosphate esters, PCBs, ethylene dibromide... 

Bromacil is a uracil herbicide introduced by du Pont in 1963 (Fig. 4). It controls a broad spectrum of weeds in citrus and pineapple crops. The chemical is also used for general vegetation control on noncrop areas such as railroads and industrial areas. 

Both underivatised and methylated samples are examined because uracil herbicides give a mixture of products on methylation and should be chromatographed in their underivatised forms, whereas carboxylic acid pesticides, e.g. chlorinated phenoxy acid herbicides, are best examined as methyl derivatives. 

Unisomnia, 815 Unitrim, 1049 Unitussin, 520 Uracil herbicides, 79 Uradal, 434... 

Researchers at FMC Corporation have synthesized 3-(4,6-substituted benzoheterocy-clyl)uracil herbicides. The uracil ring synthesis is based on conversion of aryl amines into isocyanates68. A useful fluorinated aniline herbicide is fluometuron. Chlorination of 2-methylpyridine (102), followed by animation at the 4-position, and hydrolysis, yields the hormone-weedkiller Picloram (103) (Scheme 22). 

Replacement of the triazolinone ring with a uracil ring results in a dramatic increase in biological activity. An example of a uracil herbicide is butafenacil developed by Novartis (now Syngenta AG). Its preparation is shown in Scheme 11.6. 

Uses Uracil herbicide applied to soil to control a wide variety of armiral and perennial grasses, broad-leaved weeds and general vegetation on uncropped land. It is also used for selective weed control in apple, asparagus, cane fruit, hops and citrus crops. 

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