Designated chemically as 2,4-dichloro-1,3-thiazole-5-carbaldehyde and catalogued under CAS 95453-58-0, the heterocyclic aldehyde C4HCl2NOS (182.03 g·mol⁻¹) is a pale yellow to off-white crystalline solid with a faint, pungent odor. As supplied under nitrogen blanket in amber glass or fluorinated HDPE containers, the material typically exhibits a melting endotherm onset at 48–52 °C by differential scanning calorimetry, a boiling range of 235–240 °C at ambient pressure accompanied by partial sublimation, and a flash point of 94 °C (closed cup). Solubility at 25 °C exceeds 100 g·L⁻¹ in ethyl acetate, dichloromethane, and tetrahydrofuran, while aqueous solubility remains below 0.8 g·L⁻¹; the partition coefficient log P (octanol/water) is estimated at 1.35 from fragment-based calculations. The aldehyde functionality is activated by the electron-withdrawing effect of both chlorine substituents, which shifts the carbonyl stretching vibration to 1708 ± 3 cm⁻¹ (KBr) relative to the 1690 cm⁻¹ observed for the non-halogenated thiazole-5-carboxaldehyde. This electronic perturbation is exploited in the synthesis of pharmaceutical intermediates, agrochemical building blocks, and functional materials where the thiazole core must persist through downstream transformations without ring degradation.
Which Analytical Methods Are Employed to Verify Batch Integrity?
The core release panel uses high-performance liquid chromatography on a 150 × 4.6 mm octadecylsilane column (5 µm) with 0.1% v/v trifluoroacetic acid in water/acetonitrile gradient elution and UV detection at 254 nm, analogous to the system suitability criteria of Ph. Eur. 2.2.29. Under these conditions the main peak elutes at 6.2 ± 0.3 min, with resolution > 2.0 from the most abundant process impurity, identified as the symmetric dimer (bis-thiazole ether) formed via oxidative coupling during prolonged storage above 10 °C. Water content by coulometric Karl Fischer titration (USP <921>) is routinely maintained below 0.15% w/w, while heavy metals determined against USP <231> are controlled to ≤ 10 ppm. The following tabulation summarizes the release specifications for a typical commercial lot.
| Parameter | Specification | Test Method Reference |
|---|---|---|
| Assay (anhydrous, solvent-free) | 98.0–102.0% (anhydrous basis) | In-house HPLC-UV, per Ph. Eur. 2.2.29 |
| Melting range | 48–52 °C | Open capillary, heating rate 1 °C·min⁻¹ |
| Water (KF) | ≤ 0.15% | USP <921>, Method Ia |
| Residue on ignition | ≤ 0.10% | USP <281> |
| Heavy metals (as Pb) | ≤ 10 ppm | USP <231> |
| Purity by GC (mass balance) | ≥ 99.0% area | HP‑5 column, 30 m × 0.32 mm, FID |
| Chloride (ion chromatography) | ≤ 50 ppm | EPA 300.1 |
| Appearance | Pale yellow crystalline powder | Visual inspection vs. reference standard |
Batch-to-batch variability in the melting onset has been traced to trace residual solvents (ethyl formate or dimethylformamide from the Vilsmeier–Haack formylation step) that depress the crystal lattice energy by up to 1.5 °C at levels of 0.2–0.5%. Consequently, quantitation of residual solvents by headspace GC–MS per ICH Q3C is included in the extended certificate of analysis when the material is intended for GMP intermediate registrations.
Even at controlled temperatures of 2–8 °C, the aldehyde undergoes slow autoxidation when headspace oxygen exceeds 0.3% v/v. Head-to-head dimerization is catalysed by trace metal ions—especially iron and copper introduced during drum-filling operations—and yields a glassy orange trimer that cannot be re-dissolved in common aprotic solvents. Commercial lots are therefore filled under nitrogen with an overpressure of 0.2–0.5 bar and are fitted with PTFE-faced septa for syringe withdrawal. If the container has been opened for more than 45 minutes in a relative humidity exceeding 60%, pre-drying is essential: the powder is placed in a vacuum oven at 40 °C and <5 mbar for at least 8 hours, then used within the same working shift. During pilot-plant campaigns, operators observed that material stored under air for 72 hours at 22 °C and 55% RH developed a sticky surface crust that required mechanical delumping; the re-tested assay had dropped by 3.7 percentage points, primarily due to conversion to the carboxylic acid analogue.Reactivity Divergence from Monohalogenated and Non-Halogenated Thiazole Carboxaldehydes
The introduction of a second chlorine atom in the 2‑position fundamentally alters the electron density profile of the heterocycle. Hammett substituent constants (σm ≈ +0.37, σp ≈ +0.23) estimated from imidazole-thiazole surrogates place the 2,4-dichloro pattern among the most electron-deficient aldehyde platforms in the commodity thiazole catalogue. This translates into a measurable contraction of reaction times in nucleophile-initiated conjugate additions: in a model Knoevenagel condensation with ethyl cyanoacetate (1.05 eq.) catalysed by piperidine (1 mol%) in ethanol at 0 °C, the dichloro derivative reaches >90% HPLC conversion within 20 minutes, whereas the 2‑chloro‑5‑thiazolecarboxaldehyde (CAS 95453-57-9) requires approximately 65 minutes under identical conditions, and the non-halogenated 5‑thiazolecarboxaldehyde (CAS 1003-03-8) proceeds so sluggishly that 24 hours are needed to exceed 85% conversion. The difference is not merely kinetic; the di‑halogenated aldehyde also tolerates lower catalyst loadings, a feature that reduces base-catalysed ring-opening degradation to 3‑mercaptopropionamide by-products.
The physical property contrast is equally sharp. A comparison of key compound-specific constants is given below.
| Compound | CAS | Melting point (°C) | Physical state at 25 °C | Mol. weight (g·mol⁻¹) |
|---|---|---|---|---|
| 2,4-Dichloro-1,3-thiazole-5-carbaldehyde | 95453-58-0 | 48–52 | Crystalline solid | 182.03 |
| 2-Chloro-1,3-thiazole-5-carbaldehyde | 95453-57-9 | 40–44 | Waxy solid | 147.58 |
| 1,3-Thiazole-5-carbaldehyde | 1003-03-8 | −8 to −5 | Low-melting solid / viscous oil | 113.14 |
| 2,4-Dichlorothiazole-5-carboxylic acid | 167438-17-5 | 142–145 (dec.) | White crystalline powder | 198.03 |
The carboxylic acid analogue, often encountered as an oxidative degradation product or deliberate synthetic target, is conspicuously less electrophilic and cannot participate in carbonyl-amine condensations without prior activation to the acid chloride; its storage requirements are less stringent although decarboxylation occurs rapidly above 160 °C with gas evolution that can pressurise sealed containers. When an amide linkage is desired, the aldehyde route via reductive amination circumvents the genotoxic impurity burdens associated with chlorinating agent carryover into the acid chloride‑based workflow, and therefore aligns more readily with ICH M7 mutagenic impurity risk assessments.
In a typical kilogram-scale reductive amination documented in process development reports, 1.0 kg of the aldehyde was dissolved in 5.0 L of 1,2-dichloroethane at 0 °C, followed by portionwise addition of 1.05 eq. of 4‑fluorobenzylamine and 1.4 eq. of sodium triacetoxyborohydride. The internal temperature was maintained at 0–5 °C using a jacket thermostatted with a glycol chiller; a mild exotherm of ΔT = 4 °C was noted upon each addition. After 90 minutes, UPLC analysis indicated 94% conversion to the secondary amine, with the primary by-product being the over-reduced alcohol (2,4-dichlorothiazole-5-methanol) at 3.2% area. The crude extract was washed with 1.0 M aqueous citric acid to remove residual amine and then concentrated in a wiped-film evaporator operating at 45 °C jacket temperature and 20 mbar vacuum; the isolated yield of the hydrochloride salt was 86% with chemical purity 99.5% by 1H qNMR. This protocol is often selected over direct amidation of the corresponding acid because the aldehyde-amine condensation avoids the generation of corrosive hydrogen chloride gas and reduces the vessel material-of-construction burden from Hastelloy C276 to stainless steel 316L.When the Aldehyde Is Employed in Knoevenagel Condensations
Pilot-scale execution of the Knoevenagel reaction between 2,4-dichloro-5-thiazolecarboxaldehyde and malonitrile in ethanol revealed a threshold temperature of 10 °C, above which a sudden, non-linear acceleration in heat release was observed. In a 100 L glass-lined reactor equipped with a retreat-curve impeller and a jacket cooling capacity of –8 °C, the slow semi-batch addition of a pre-cooled (–5 °C) ethanolic solution of malonitrile (1.02 eq.) and piperidine (0.3 mol%) over 45 minutes maintained the reaction mass at –2 ± 1 °C. Process analytical technology (react-IR with a diamond ATR probe) tracked the disappearance of the aldehyde carbonyl peak at 1708 cm⁻¹ and the concurrent growth of the conjugated nitrile band at 2225 cm⁻¹. Once the addition was complete, the jacket was set to 5 °C; the product began to crystallize within 20 minutes and the slurry was isolated by centrifuge filtration through a cloth of 10 µm nominal retention. Yield: 92% of the dicyanovinyl intermediate as a bright yellow microcrystalline powder, residual aldehyde by HPLC <0.5%. Operators noted that when the bulk temperature exceeded 12 °C during a commissioning run, a deep amber colour developed within 3 minutes and the isolated yield fell to 61%, accompanied by a gelatinous side-fraction that required extended cleaning cycles with heated N‑methyl-2-pyrrolidone. This processing window is substantially narrower than that of the monochloro congener, which remains manageable up to 25 °C before discolouration sets in, and explains why the dichloro derivative was adopted only after modifying the standard Knoevenagel protocol to sub-ambient conditions.
Unlike 5‑thiazolecarboxaldehyde, which can be stored neat in ordinary polypropylene containers without measurable deterioration over 6 months at 25 °C, the 2,4-dichloro variant requires inerted aluminium laminate foil bags for multi-kilogram distribution. Contract manufacturing organizations that supply this intermediate to generic active pharmaceutical ingredient campaigns report that the sublimation tendency of the dichloro aldehyde—vapour pressure estimated at 0.08 Pa at 25 °C—causes gradual product loss through the closures of standard drum liners if storage exceeds 4 weeks at 20–25 °C; weigh-and-dispense operations are therefore carried out in a dry-room with a dew point of –40 °C or colder. A further critical incompatibility is with tertiary amine bases: mixing with triethylamine beyond trace residues initiates a rapid aldol-like self-condensation that evolves water, which, in the presence of HCl scavenged from the amine, catalyses hydrolysis to the carboxylic acid. For this reason, liquid reagent addition sequences in multi-step telescoping processes often introduce the aldehyde as the last component, immediately before the intended coupling partner, and never as a pre-mix with free-base amines.Electrophilic Aromatic Substitution Blocking by the 2,4-Dichloro Motif
The presence of chlorine at both the 2‑ and 4‑positions dramatically deactivates the thiazole ring toward electrophilic attack. Nitration under mixed acid conditions that functionalize the parent thiazole-5-carbaldehyde at the 2‑position (HNO3/H2SO4, 0–5 °C, 2 h) leaves the dichloro derivative unchanged at conversions below 2%. This deactivation is advantageous when downstream transformations require aggressive electrophilic conditions on a second aromatic ring tethered via the aldehyde handle; the thiazole core survives intact. In contrast, 2‑chloro-5‑thiazolecarboxaldehyde still undergoes partial bromination on the 4‑unsubstituted carbon with N‑bromosuccinimide in dimethylformamide at 20 °C, generating a regioisomeric mixture that frustrates preparative chromatography. The dichloro scaffold thus offers a unique degree of core stability, which is why it appears as a persistent substructure in several fungicidal lead series where the 2,4‑dichlorothiazole ring is required to withstand metabolic oxidation in soil degradation studies carried out under OECD 307 guidelines.
Practical handling in solid‑phase peptide synthesis laboratories has revealed a nuance in coupling efficiency: the aldehyde reacts with hydrazine-functionalized Wang resin at 0.8–1.2 mmol·g⁻¹ loading within 2 hours at 20 °C using 5 eq. of aldehyde in dichloromethane containing 1% acetic acid, while the mono‑chloro analogue requires 6 hours to reach equivalent loading. Yet, the dichloro hydrazone linkage exhibits sensitivity to repetitive trifluoroacetic acid cleavage cycles; more than four cycles of 95% TFA/5% water (v/v) cause 8–12% cleavage of the linker, measured by quantitation of 2,4-dichlorothiazole-5-carbaldehyde leached into the cleavage cocktail. This loss is not observed with the non-halogenated aldehyde and has been attributed to enhanced electrophilicity of the imine carbon under acidic conditions. Published data for this specific resin combination are limited, but the observation has led process groups to limit repeated cycles when the dichloro linker is used in fragment-based library construction.
