Dacthal (1861-32-1) Physical and Chemical Properties
Dacthal
A tetrachlorinated aromatic diester supplied as a colorless crystalline material for formulation work, analytical standards and environmental testing.
| CAS Number | 1861-32-1 |
| Family | Chloroaromatic esters |
| Typical Form | Powder or crystalline solid |
| Common Grades | EP |
Dacthal is a tetra‑chlorinated dimethyl ester of terephthalic acid belonging to the class of chlorinated aromatic diesters. Structurally it is a 1,4‑disubstituted benzene core carrying four chlorine substituents at the 2,3,5,6 positions and two methyl carboxylate groups at the 1 and 4 positions; the IUPAC designation is dimethyl 2,3,5,6‑tetrachlorobenzene‑1,4‑dicarboxylate. The combination of four electron‑withdrawing chlorines and two ester carbonyls produces a molecule with pronounced lipophilicity, limited aqueous solubility, and a low tendency to donate hydrogen bonds (H‑bond donor count = 0) while offering multiple H‑bond acceptor sites (H‑bond acceptor count = 4).
Electronic and physical behavior is governed by the rigid, chlorinated aromatic core and polar ester groups. The high chlorination lowers polarity of the aromatic ring and raises the octanol–water partition coefficient (log Kow ≈ 4.4), favoring partitioning to organic phases, sediments, and biota. The ester linkages are susceptible to hydrolytic dealkylation under biological or catalytic conditions, yielding monomethyl and di‑acid derivatives; abiotic photolysis on surface films can also cleave esters and produce chlorinated benzenes. In the absence of hydrolytic or photochemical attack, the compound is thermally robust (decomposition onset reported near \(360!-!370\,^\circ\mathrm{C}\)) and stable in the pure state and to UV irradiation.
As a commercial agrochemical, Dacthal has been used as a selective pre‑emergence herbicide for control of annual grasses and certain broadleaf weeds in turf, ornamentals and a number of agronomic crops. Its physicochemical profile—low vapor pressure, strong soil adsorption, and moderate persistence under cool or dry conditions—determines formulation choices and environmental fate following application. Common commercial grades reported for this substance include: EP.
Basic Physical Properties
Density
Specific gravity (reported): 1.70 (listed as "SPECIFIC GRAVITY, C: 1.70"). This indicates a bulk density greater than water for the pure crystalline material and is consistent with heavy, chlorinated aromatic structures that are relatively dense.
Melting Point
Melting point (reported): 155-156 °C. Present as crystalline, colorless solid; typical crystallization reported from methanol.
Boiling Point
No experimentally established value for this property is available in the current data context.
Vapor Pressure
Experimental vapor pressure (reported): \(2.5\times10^{-6}\ \mathrm{mmHg}\) at \(25\,^\circ\mathrm{C}\) (also quoted as 0.0000025 mmHg). The very low vapor pressure at ambient temperature indicates negligible volatility from the pure solid under normal conditions, although measurable volatilization from moist soil surfaces can occur under field conditions and elevated temperatures.
Flash Point
No experimentally established value for this property is available in the current data context.
Chemical Properties
Solubility and Phase Behavior
Physical description: colorless crystalline solid, essentially odorless.
Aqueous solubility (reported): water solubility = \(0.5\ \mathrm{mg}\,\mathrm{L}^{-1}\) at \(25\,^\circ\mathrm{C}\); also described as "LESS THAN 5% IN WATER." Solubility in organic solvents (reported): 120 g in dioxan, 250 g in benzene, 170 g in toluene, 140 g in xylene, 100 g in acetone, 70 g in carbon tetrachloride; specific notes include "7% @ \(25\,^\circ\mathrm{C}\) IN CARBON TETRACHLORIDE" and "GREATER THAN 5% IN CYCLOHEXANONE, ACETONE, XYLENE." The combination of low water solubility and high solubility in nonpolar or moderately polar organics reflects the molecule's high log Kow (reported log Kow/logKoc relationship).
Phase behavior in environment: low vapor pressure and strong sorption to organic matter (reported estimated Koc ≈ 5900) favor partitioning to soil and sediment and binding to particulate matter in aquatic systems. In soil and surface films, volatilization and photodegradation are significant loss processes under certain conditions; volatilization from moist soils has been documented as a measurable pathway.
Reactivity and Stability
Stability (reported): "STABLE IN THE PURE STATE" and "STABLE TO UV IRRADIATION" are reported; however, thin surface films exposed to sunlight show comparatively rapid photodecomposition (reported half‑life on a glass plate of ca. 5 hours under direct irradiation, with formation of monomethyl and di‑acid products and some tetrachlorobenzene). Thermal decomposition is reported at \(360!-!370\,^\circ\mathrm{C}\). The ester functional groups are susceptible to hydrolysis (biological and chemical), producing sequential dealkylation products (monomethyl tetrachloroterephthalate and tetrachloroterephthalic acid) in environmental and biological matrices.
Reactivity considerations for handling and disposal: incineration is an accepted disposal option but requires acid gas (HCl) scrubbing because thermal decomposition/incineration will generate hydrogen chloride; standard precautions for chlorinated organics apply. Known impurity: hexachlorobenzene has been reported as an impurity at levels below 0.3% in some material batches.
Thermodynamic Data
Standard Enthalpies and Heat Capacity
Reported molar heat (vaporation): "AVG MOLAR VAPORIZATION HEAT IS CALCULATED TO BE 25 KCAL/MOL" — presented here as \(\approx25\ \mathrm{kcal}\,\mathrm{mol}^{-1}\) (calculated average molar enthalpy of vaporization). No experimental values for standard enthalpy of formation or heat capacity at constant pressure are available in the current data context.
Molecular Parameters
Molecular Weight and Formula
Molecular formula (reported): C10H6Cl4O4.
Molecular weight (reported): 332.0 — presented here as \(332.0\,\mathrm{g}\,\mathrm{mol}^{-1}\).
LogP and Polarity
Computed/experimental partition coefficient (reported): XLogP3 = 4.4; log Kow reported as 4.40 at \(25\,^\circ\mathrm{C}\). Topological polar surface area (reported): 52.6. Hydrogen bond donor count = 0; hydrogen bond acceptor count = 4. These parameters are consistent with a moderately high lipophilicity and a polar surface limited to the ester carbonyl and ester oxygen atoms.
Structural Features
IUPAC name (reported): dimethyl 2,3,5,6‑tetrachlorobenzene‑1,4‑dicarboxylate.
Key structural characteristics: a rigid 1,4‑disubstituted benzene (terephthalate) core bearing four chlorine atoms on the ring; two methyl ester groups provide sites for nucleophilic/enzymatic attack (hydrolysis) but do not confer significant aqueous solubility. The chlorination pattern reduces electron density on the aromatic ring and stabilizes the molecule toward some oxidative processes, while increasing molecular weight and density. Rotatable bond count (reported): 4; heavy atom count: 18; formal charge: 0.
Identifiers and Synonyms
Registry Numbers and Codes
- CAS: 1861-32-1
- Deprecated CAS (reported): 65862-98-8, 87209-56-1
- EC number / EINECS (reported): 217-464-7
- UNII (reported): ZU3X5G2QLR
- ChEBI (reported): CHEBI:34664
- ChEMBL (reported): CHEMBL1888079
- DSSTox Substance ID (reported): DTXSID0024000
- HMDB (reported): HMDB0250144
- KEGG ID (reported): C14744
- NSC Number (reported): 155745
- InChI: InChI=1S/C10H6Cl4O4/c1-17-9(15)3-5(11)7(13)4(10(16)18-2)8(14)6(3)12/h1-2H3
- InChIKey: NPOJQCVWMSKXDN-UHFFFAOYSA-N
- SMILES: COC(=O)C1=C(C(=C(C(=C1Cl)Cl)C(=O)OC)Cl)Cl
(Identifiers are reproduced as reported for unambiguous cross‑referencing in procurement, analytical method development, and inventory management.)
Synonyms and Structural Names
Reported synonyms include (selection): Dacthal; Chlorthal‑dimethyl; chlorthal dimethyl; DCPA; Dimethyl tetrachloroterephthalate; Dimethyl 2,3,5,6‑tetrachloroterephthalate; Tetrachloroterephthalic acid dimethyl ester; DAC 893; TCTP; Chlorthal‑dimethyl 10 microg/mL in Cyclohexane; Chlorthal dimethyl; Daktal; Tetral; 2,3,5,6‑Tetrachloroterephthalic acid dimethyl ester; Terephthalic acid, tetrachloro‑, dimethyl ester. These names reflect usage in regulatory, analytical and manufacturing contexts.
Industrial and Commercial Applications
Representative Uses and Industry Sectors
Dacthal has been used primarily as a selective pre‑emergence herbicide for control of annual grasses and certain annual broadleaf weeds. Reported crop/turf uses include turfgrass, ornamentals, strawberries, cotton, soybeans and field beans; it has been used to control species such as crabgrass, foxtails and panicum. The compound is non‑systemic (not translocated in plants) and acts on germinating seeds. Agricultural application and formulation activities (mixers, loaders, applicators) are primary occupational exposure settings.
Regulatory status and tolerances are managed regionally; acceptable daily intake and acute reference values have been established in certain jurisdictions and residue tolerances exist for many raw agricultural commodities.
Role in Synthesis or Formulations
Manufacturing route (reported): production by esterification of tetrachloroterephthalic acid (or by chlorination pathways from p‑xylene followed by conversion to the terephthaloyl chloride and methanolysis). Typical commercial formulations reported include wettable powders and granules (e.g., "DACTHAL W‑75" wettable powder, "DACTHAL G‑5" granules). Formulation practice addresses the low water solubility and dust potential of the crystalline powder, using carriers and wetting agents to deliver product to target soils.
Safety and Handling Overview
Acute and Occupational Toxicity
Toxicological summary (selected reported values and classifications): chronic oral reference dose (RfD) reported as \(1\times10^{-2}\ \mathrm{mg}\,\mathrm{kg}^{-1}\,\mathrm{day}^{-1}\). Cancer classification reported as "Group C — possible human carcinogen" in some assessments. Acute toxicity reported includes LD50 (rat, oral) \(>10000\,\mathrm{mg}\,\mathrm{kg}^{-1}\) and LD50 (rat, dermal) \(>10000\,\mathrm{mg}\,\mathrm{kg}^{-1}\). Inhalation LC50 values reported variably (e.g., LC50 (rat, 4 h) \(>5,700\,\mathrm{mg}\,\mathrm{m}^{-3}\); other sources note LC50 rat inhalation > 5 mg·L^-1 (4 h nominal)). Eye contact produced only mild, transient irritation in rabbit tests at small doses.
Occupational control measures: because of potential carcinogenic concern and particulate/dust exposure during handling of wettable powders or solids, respiratory protection (dust‑mist respirators), protective gloves, eye protection and measures to minimize dust generation are recommended for mixers, loaders, applicators and manufacturing personnel. Avoidance of release to aquatic systems is important given aquatic toxicity and potential for long‑term environmental effects.
Environmental hazards: reported aquatic toxicity and classifications include long‑term aquatic hazard (e.g., hazard statement H411 in some notifications). The compound has an estimated bioconcentration factor (BCF) ≈ 1300 and strong sorption to sediment and organic matter (high Koc), indicating potential for bioaccumulation and persistence in sediments.
Storage and Handling Considerations
Handling: store as a dry, tightly closed solid in a cool, well‑ventilated area; control dust and avoid formation of airborne particulates. Use local exhaust and enclosure where possible during transfer and formulation. Avoid release to drains and surface waters.
Storage stability: reported as stable in the pure state and to UV under bulk conditions; avoid prolonged exposure of thin surface films to sunlight because photodecomposition can be rapid under direct irradiation. Material decomposes at elevated temperature near \(360!-!370\,^\circ\mathrm{C}\).
Disposal: incineration with acid gas scrubbing for HCl is an accepted disposal option for concentrate wastes; follow product‑specific safety documentation and local environmental regulations. For detailed hazard, transport and regulatory information, users should refer to the product‑specific Safety Data Sheet (SDS) and local legislation.
