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Dacthal

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(CAS#: 1861-32-1)

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Chemical Name: Dimethyl-2,3,5,6-tetrachlorobenzene-1,4-dicarboxylic acid

Structure:

Dacthal was previously marketed by ISK Biosciences, which discontinued production four years ago for economic reasons, causing a great deal of concern among growers accustomed to the many unique qualities of this widely used product. Amvac, whose corporate strategy is to acquire mature product lines from the large multinational companies, saw an opportunity to add a valuable herbicide to its growing portfolio of specialty products.

Toxicological Effects:

Acute toxicity: The compound has a very low toxicity to mammals. The LD50 values for DCPA in rats range from greater than 3000 mg/kg to 12,500 mg/kg. DCPA in rabbits and beagle dogs has an LD50 of greater than 10,000 mg/kg. The dermal LD50 in rabbits is greater than 2000 mg/kg. DCPA is not a skin sensitizer. It is a mild eye irritant. The inhalation LC50 (4-hour) is greater than 5.7 mg/L for rats [5,7].

Chronic toxicity: A 3 mg dose in a rabbit eye produced mild irritation, which disappeared in 24 hours. Dogs given high doses of 800 mg/kg/day for a month showed some adverse effects in the liver. In longer-term studies with rats (90 days), similar doses (about 750 mg/kg/day) caused no adverse effects [35]. In a 2-year study with rats, a dose of around 50 mg/kg/day was responsible for changes in the adrenal weights of the females and in the kidney weights of the males [35].

Reproductive effects: Rats fed high doses of DCPA (500 mg/kg/day) showed no changes in fertility, gestation, live births, or lactation [35]. The study was conducted over one full generation. These data suggest that the compound does not cause reproductive effects.
Teratogenic effects: Available data indicate that DCPA is not teratogenic. Pregnant rabbits fed moderate doses (up to 300 mg/kg) of DCPA on days 8 to 16 of gestation showed no skeletal or organ abnormalities in the offspring [35].

Mutagenic effects: No mutagenicity was seen in a number of tests, including mutation frequency and activity, cytogenetic tests, DNA repair, and dominant lethal tests [35]. This evidence indicates that DCPA is not mutagenic.

Carcinogenic effects: No carcinogenic effects were noted in rats in a 2-year study where diets contained up to 500 mg/kg/day of DCPA [35]. Thus, DCPA does not appear to be carcinogenic.
Organ toxicity: Long-term studies in test animals have indicated the liver and adrenal glands as target organs.
Fate in humans and animals: Much of the compound that is ingested is not absorbed. Cows excreted nearly all of a small dose of DCPA within 5 days, and dogs absorbed only small amounts (3%) of the compound. The remaining amount was eliminated within 4 days. When dairy cows were fed diets with up to 200 ppm of DCPA for 24 days, 0.26 ppm of the compound or its metabolites were found in milk, while 30 to 90 ppm for 9 or 23 days resulted in residues of 0.036 ppm and 0.066 ppm in milk. Residues in other tissues were generally less than 1 ppm [8].

Ecological Effects:

Effects on birds: DCPA appears to be moderately toxic to some young wildfowl, and practically nontoxic to the young of other species and to adult birds. The LD50 in young bobwhite quail is 5500 mg/kg [7]. Young mallards and young quail were more sensitive to the herbicide than adult birds. Diets containing about 250 mg/kg caused heavier mortality to young ducks in the first 5 days. Older birds had a higher survival rate [35].

Effects on aquatic organisms: DCPA is slightly toxic to practically nontoxic to fish depending on the species. It is practically nontoxic to bluegill sunfish and slightly toxic to rainbow trout. The compound is practically nontoxic to estuarine and marine organisms (invertebrates and some fish). The available data suggest that DCPA poses no hazard to endangered aquatic species [35].
Effects on other organisms: At high doses of DCPA, there was only 3% bee mortality. Thus, DCPA is only slightly toxic to bees [7].
Environmental Fate:

Breakdown in soil and groundwater: DCPA is moderately persistent. The half-life is from 14 to 100 days in most soils [21]. However, moisture is essential for degradation. In one study, there was no apparent buildup of pesticide residues in soil even after repeated application. The DCPA concentration declined slowly to 75 or 80% in 28 days. Later sampling showed a continued decline of DCPA and its breakdown products [7,34]. The DCPA metabolite, tetrachloro-terephthalic acid (TTA or diacid), is much more water soluble than the parent compound, and is subject to leaching in some soils. This metabolite has been detected in groundwater in the onion growing areas of eastern Oregon [23]. It has been detected in several other states in the U.S. as well [7].
Breakdown in water: There is virtually no degradation of DCPA in water ranging from moderately acidic to moderately alkaline (pH 5.0 to pH 9.0). Breakdown is due to the action of sunlight and the half-life is greater than 1 week [34].
Breakdown in vegetation: Plants may metabolize DCPA to the same two breakdown products that are seen in soils, with the proportion varying in different species. DCPA affects the seed and pre-emergence stage, but has little effect on crops or weeds after they have emerged. Limited information suggests that plants may remove the chlorine molecules of DCPA (8). In one study, pine trees took up nearly 1% of the soil applied chemical. The majority of the compound taken up by the trees remained in the root system, where it was rapidly diluted.

Physical Properties:

Appearance: DCPA consists of colorless crystals
Chemical Name: dimethyl-2,3,5,6-tetrachlorobenzene-1,4-dicarboxylic acid
CAS Number: 1861-32-1
Molecular Weight: 303.90
Water Solubility: 0.5 mg/L @ 25 C
Solubility in Other Solvents: benzene v.s.; toluene v.s.; acetone v.s.; carbon tetrachloride s.
Melting Point: 155-156 C
Vapor Pressure: 0.33 mPa @ 25 C
Partition Coefficient: Not Available
AdsorptionCoefficient: 5000 (chlorthal-dimethyl)