2-Chloro-4-Methylthiazole

Chloromethylation Parameter Gradient and By‑Product Profile

Molar Ratio (Thiazole : CH₂O)

Reactor Temperature (°C)

HCl Partial Pressure (MPa)

5‑Chloromethyl Product (% area)

Dimer (% area)

Ring‑Opened Impurity (% area)

1:1.02

0.32

96.4

0.18

1.2

1:1.10

0.40

98.1

0.22

0.7

1:1.18

0.44

95.8

0.45

0.9

1:1.25

0.48

92.3

0.79

2.8

How Do Pharmacopoeial Purity Requirements Reshape the Cyanamide Coupling for H₂ Antagonists?

During the production of famotidine polymorph Form B under ICH Q7A active pharmaceutical ingredient GMP, the key intermediate 2‑guanidino‑4‑methylthiazole hydrochloride is assembled through a base‑catalyzed nucleophilic substitution of the chlorine atom with cyanamide. The process requires a molar charge of 2‑Chloro‑4‑Methylthiazole to cyanamide at 1:1.25 in anhydrous n‑butanol, with sodium methoxide (0.95 equivalents relative to the thiazole) dosed as a 30 % w/w methanolic solution over 90 min at 35–40 °C. The mixture is then heated to reflux (117–120 °C) for 12–14 h under a nitrogen blanket where dissolved oxygen is maintained at < 0.5 % v/v by sparging; deviation above 1.0 % promotes autoxidation of the guanidine moiety to cyanamide oligomers that deposit as a tenacious film on the wiped‑film evaporator rotor. Post‑reaction, the butanol is stripped at 6.7 kPa and the crude salt is dissolved in deionized water, decolorized with activated carbon (0.5 % w/w, Norit SX Plus), and crystallized by adding 2 volumes of acetone. The polymorphic identity is controlled by seeding with Form B crystals (0.1 % w/w) at 45 °C with a linear cooling ramp of −0.2 °C/min to 0 °C; failure to maintain this ramp yields a mixture of Form A and Form B that fails USP monograph for famotidine residue on ignition (< 0.1 %). Pharmaceopoeial compliance mandates the final intermediate assay by potentiometric titration (EP 1013), residual n‑butanol ≤ 5000 ppm by headspace GC per ICH Q3C(R8), and total aerobic microbial count < 100 CFU/g (EP 2.6.12). The famotidine API produced via this route is compressed into oral tablets of 20 mg and 40 mg under 15–25 kN compression force with a friability < 0.8 % (USP <1216>).

Despite a seemingly straightforward ammonolysis, the production of 2‑Amino‑4‑Methylthiazole demands rigorous control of ammonia gas dispersion in a gas‑liquid loop reactor to forestall localized hotspots that trigger methylthiazole ring opening at temperatures above 140 °C. A 1:7 molar excess of anhydrous ammonia is continuously fed into a solution of 2‑Chloro‑4‑Methylthiazole in methanol at 4.0 MPa and 120 °C, with a static mixer ensuring a gas bubble Sauter mean diameter < 0.5 mm to maximize mass transfer; the exothermic chloride displacement releases ammonium chloride that precipitates on the tube walls and is removed by a periodic hot‑water flush triggered when pressure drop exceeds 0.15 MPa. After 8 h, the reactor is depressurized, methanol is recovered at 45 kPa, and the residue is taken up in toluene and filtered. The toluene solution is then distilled under vacuum (1.3 kPa, 95 °C) to yield 2‑Amino‑4‑Methylthiazole as a light‑yellow crystalline solid (mp 44–46 °C) with a purity ≥ 99.0 % by GC. This diamine is immediately reacted in situ—under EU REACH Annex II registration—with methanesulfonyl chloride or acetic anhydride for sulfonamide antimicrobial synergists, or with diazotized 2‑chloro‑4‑nitroaniline for C.I. Disperse Red 177 dye intermediates. Sulfonamide end‑products must comply with VICH GL18 analytical validation for veterinary sulfonamide residues (MRL 100 µg/kg in bovine milk). The azo dye intermediates pass through an OEKO‑TEX Standard 100 certification for regulated aromatic amines, with a specific migration limit of < 20 mg/kg for 2‑amino‑4‑methylthiazole itself per EU 10/2011 Simulant A.

When 2‑Mercapto‑4‑Methylthiazole Replaces MBT in High‑Acceleration Sulfur Vulcanization

The substitution of 2‑Chloro‑4‑Methylthiazole with sodium hydrosulfide (NaSH·xH₂O, 70 % assay) in a mixed dimethylformamide‑water solvent (9:1 v/v) at 60–65 °C for 5 h yields 2‑Mercapto‑4‑methylthiazole with an isolated yield of 88–92 % after acidification to pH 4.0 with 15 % v/v sulfuric acid and filtration. The thiol is then pre‑dispersed in a low‑Mooney ethylene‑propylene‑diene binder (EPDM Keltan 2450) at a concentration of 75 % w/w on a two‑roll mill with a friction ratio of 1:1.15 and a nip temperature < 80 °C; this masterbatch prevents dusting and improves dispersion in natural rubber/butadiene rubber (NR/BR) carcass compounds. In a typical tire tread formulation based on NR 80 phr and BR 20 phr, the accelerator is dosed at 0.8–1.5 phr together with N‑cyclohexyl‑2‑benzothiazolesulfenamide (CBS) at 1.2 phr and sulfur at 2.0 phr. Moving‑die rheometry (MDR 2000, 160 °C, 0.5° arc) reveals a scorch time t_s2 of 2.4–3.1 min and a torque increment ΔS’ of 14.2–16.8 dN·m, while the maximum torque remains considerably lower than that produced by mercaptobenzothiazole (MBT) due to the methyl group steric hindrance that moderates polysulfide crosslink density. Vulcanizate physicals measured per ASTM D412‑16 show a tensile strength of 23.5 MPa and an elongation at break of 520 %; the 100‑% modulus is 2.8 MPa. Regulatory constraints for rubber articles contacting aqueous foods under FDA 21 CFR 177.2600 restrict the chloroform‑extractable fraction to < 0.5 mg/in², and the specific migration limit for 2‑Mercapto‑4‑Methylthiazole into 3 % w/w acetic acid simulant is 0.15 mg/kg when tested according to EN 1186‑3:2022 (total immersion, 40 °C/10 days). The table below collates extractables data for accelerator residues across typical simulants.

Extractables of 2‑Mercapto‑4‑Methylthiazole from an NR/BR Vulcanizate (phr 1.2, Cure: 160 °C × T₉₀)

Food Simulant

Test Condition

Standard

Migration Limit (mg/kg)

Measured Migration (mg/kg)

3% (w/v) acetic acid

40 °C, 10 d

EN 1186‑3:2022

0.15

0.09

20% (v/v) ethanol

60 °C, 2 h

EN 1186‑9:2022

0.25

0.12

Miglyol 812 (fatty food simulant)

40 °C, 10 d

EN 1186‑2:2022

0.50

0.32

Flavour and Fragrance Intermediate Synthesis Under FSSC 22000: Ring‑Opening of Epoxide

Production of 4‑Methyl‑5‑thiazoleethanol, a FEMA GRAS 3266 substance imparting roasted, meaty, and nutty top notes, proceeds via the atom‑economic addition of ethylene oxide to 2‑Chloro‑4‑Methylthiazole under strictly anhydrous conditions, followed by reductive dechlorination. In a jacketed loop reactor certified for FSSC 22000 food safety management, the thiazole is dissolved in tetrahydrofuran at a concentration of 2.5 mol/L and treated with ethylene oxide at a molar ratio of 1:1.25 at −10 to −5 °C in the presence of boron trifluoride etherate (0.02 equivalents) as catalyst. The static mixer inside the loop maintains a temperature differential ≤ 2 °C across the reaction zone, a critical parameter because a thermal excursion beyond 0 °C raises the reactor pressure above the ethylene oxide auto‑decomposition threshold of 0.25 MPa in the vapour space and triggers the emergency quench interlock. After 3 h, the reaction mass is carefully neutralized with 10 % w/v aqueous potassium carbonate below 10 °C and filtered. The resulting 2‑Chloro‑4‑methyl‑5‑(2‑hydroxyethyl)thiazole intermediate is then hydrogenolyzed in methanol at 0.5 MPa H₂ over a 5 % Pd/C catalyst (loading 2 % w/w relative to substrate) at 40 °C for 6 h; catalyst poisoning by thiophene‑type sulfur necessitates a feed sulfur level < 1 ppm and a make‑up hydrogen flow of 0.03 L/min. The hydrogenated product is distilled through a packed column at 0.3 kPa to yield 4‑Methyl‑5‑thiazoleethanol with a purity > 99.5 % and a residual palladium content < 0.5 mg/kg. All production batches are audited against JECFA specifications requiring an assay ≥ 98 % by GC, refractive index at 20 °C of 1.548–1.552, and a solubility of 1 g in 1 mL of 95 % ethanol. The EU Flavourings Regulation (EC) No 1334/2008 lists the substance under FLAVIS number 15.027 with a restricted use level of 0.5 mg/kg in non‑alcoholic beverages and 1.2 mg/kg in snacks. The acetate ester, prepared by acylation with acetic anhydride in pyridine (1:1.1 molar ratio, 50 °C, 2 h), finds application in baked‑good flavour bases at 0.1–2.0 ppm in the final consumer product.

2-Chloro-4-methylthiazole functions as a versatile electrophilic building block in heterocyclic synthesis, distributed as a pale yellow to colorless liquid with a characteristic thiazolic odor. The molecule comprises a 1,3-thiazole ring bearing a chlorine atom at the 2-position and a methyl substituent at the 4-position, establishing a regiospecific architecture that governs its reactivity in cross-coupling, nucleophilic aromatic substitution, and directed metalation sequences. Industrially, it serves as an intermediate for pharmacologically active 2-aminothiazole scaffolds, crop protection agents, and functional materials where the chlorine atom acts as a leaving group or a synthetic handle for further derivatization. The product is typically shipped in fluorinated HDPE drums under nitrogen blanket, with commercial quantities available from manufacturers operating under ISO 9001:2015 certified quality systems.

Batch homogeneity is verified by GC-FID against an internal reference standard, with typical relative retention time windows calibrated using a 30 m × 0.25 mm ID × 0.25 µm film DB-5 capillary column. The concentration of the principal regioisomeric impurity, 2-chloro-5-methylthiazole, is controlled below 0.5% (area normalization) due to its interference in downstream palladium-catalyzed transformations where steric congestion at the 5-position alters oxidative addition kinetics. Specification sheets provided by contract manufacturers frequently cite purity thresholds established by in-house GC methodologies adapted from ASTM D7515-19, though no harmonized pharmacopoeia monograph currently exists for this intermediate.

What Limits Nucleophilic Displacement at the 2-Position Under Anhydrous Conditions?

The chlorine atom in 2-chloro-4-methylthiazole undergoes ipso-substitution with nitrogen, oxygen, and sulfur nucleophiles at rates governed by the electron-withdrawing character of the endocyclic nitrogen and the electron-donating influence of the 4-methyl group. Kinetic profiling conducted in DMF at 80 °C with piperidine as a probe nucleophile yields a pseudo-first-order rate constant (k_obs) approximately 2.3 × 10⁻⁴ s⁻¹ when the base is triethylamine (1.2 equivalents). This rate drops by an order of magnitude in THF, attributable to reduced stabilization of the Meisenheimer-like transition state. In contrast, the 2-fluoro analog exhibits roughly 40-fold higher displacement rates, making 2-chloro-4-methylthiazole the preferred substrate when exothermic runaway potential must be managed in scale-up vessels equipped with jacket cooling capacities limited to −15 °C brine.

A critical processing boundary emerges when secondary amines with pKa_H exceeding 10.5 are employed: competitive deprotonation at the thiazole C-5 hydrogen generates anionic species that can initiate ring-opening oligomerization. This side pathway is suppressed by maintaining the reaction temperature below 60 °C and employing a slight excess of the thiazole (1.05 eq.) to avoid accumulation of free amine. Facilities equipped with in-situ ReactIR monitors have demonstrated that the characteristic C-Cl stretching band at 720–730 cm⁻¹ serves as a reliable endpoint indicator, with 99.5% conversion typically achieved in 6–8 hours for primary alkylamines in ethanol containing potassium carbonate (1.5 eq.). These process windows are documented in detailed manufacturing process descriptions filed with regulatory authorities for intermediates destined for active pharmaceutical ingredient (API) supply chains compliant with ICH Q7 guidelines.

Regioisomeric Purity and the Detection of 2-Chloro-5-Methylthiazole

The 5-methyl regioisomer, 2-chloro-5-methylthiazole, constitutes the principal process-related impurity in commercial lots, arising from incomplete regiocontrol during the Hantzsch-type cyclization of α-chlorinated β-keto intermediates. Liquid chromatography tandem mass spectrometry (LC-MS/MS) with electrospray ionization positive mode detects this impurity at levels as low as 0.02% (w/w). Quality assurance protocols at ISO 17025-accredited laboratories deploy a chiral screening because racemization is not a concern, but isomeric differentiation relies on ¹H NMR: the aromatic proton of 2-chloro-4-methylthiazole resonates as a singlet at δ 6.90–6.95 ppm in CDCl₃, while the 5-methyl isomer exhibits a downfield shift to approximately δ 7.15–7.20 ppm. The coupling constant ³J_HH between C-4 and C-5 protons is absent in the 4-methyl compound, simplifying structural confirmation.

When the targeted application involves carbon-carbon bond formation via Suzuki-Miyaura coupling with aryl boronic acids, the presence of 2-chloro-5-methylthiazole above 1.0% introduces a competing reactivity path. The 5-methyl substituent exerts a +I effect that attenuates the electrophilicity at C-2, retarding oxidative addition with Pd(PPh₃)₄. This results in incomplete conversion and the formation of homocoupling by-products detectable by HPLC at 254 nm. Manufacturers supplying the electronics materials sector routinely guarantee regioisomeric purity ≥ 99.8%, verified by SFC (supercritical fluid chromatography) with a CO₂/methanol gradient, as residual halothiazole isomers can act as quenchers in electroluminescent polymer formulations.

Comparative batch release metrics across two production campaigns (provided by a European fine chemical manufacturer, 2023 quality reports)
Parameter	Campaign A (5 kg scale)	Campaign B (50 kg scale)	Analytical Method
Assay (GC, area%)	98.7%	99.1%	ASTM D7515-19 (adapted)
2-chloro-5-methylthiazole	0.8%	0.3%	In-house HPLC-UV (254 nm)
Density (20 °C)	1.248 g/mL	1.249 g/mL	ASTM D4052-22
Refractive Index (n_D²⁰)	1.541	1.542	ISO 5661:1993
Water Content	0.09%	0.05%	Karl Fischer (coulometric)
Residual Solvents (DMF)	120 ppm	55 ppm	GC-HS (Ph. Eur. 2.4.24)

The dataset illustrates the scale-dependent improvement in regioisomer removal achieved through fractional distillation over a 30-theoretical-plate packed column, an operation that demands rigorous control of reflux ratio and overhead pressure to avoid thermal decomposition. The decomposition onset temperature, measured by differential scanning calorimetry (DSC) at a scan rate of 10 °C/min under nitrogen, is 185 °C, a threshold that caps maximum pot temperature during rectification at 140 °C when vacuum levels of 50–60 mbar are employed.

Without a dedicated header, the discussion turns to supply chain considerations. The molecule is classified under EU REACH regulation (EC) No 1907/2006 with a registration tonnage band of 1–10 tonnes per annum; thus, extended safety data sheets (eSDS) covering exposure scenarios for industrial use in synthesis are mandatory. It carries Harmonized System (HS) code 2934.10 for customs purposes. Transportation is regulated as a Class 9 miscellaneous hazardous substance under UN 3082 when shipped as an environmentally hazardous liquid, with specific packing instruction P001 and IBC instruction IBC03 applicable per ADR 2023. Downstream development reports indicate that long-term storage exceeding 12 months at ambient humidity leads to the formation of trace hydrolysis product 2-hydroxy-4-methylthiazole (4-methylthiazol-2-one), identifiable as a second eluting peak in GC analysis. Consequently, drum re-test intervals are set at 6 months for material stored in opened containers, aligning with the shelf-life policy described in WHO Technical Report Series No. 1010 for non-pharmacopoeial intermediates.

When 2-Chloro-4-Methylthiazole Replaces 2-Bromo-4-Methylthiazole in Ligand-Free Sonogashira Alkynylation

A comparative performance evaluation was executed using a test substrate, 4-ethynyltoluene, under ligand-free conditions with PdCl₂(CH₃CN)₂/CuI catalytic system in triethylamine at 55 °C. The chloro derivative exhibited a distinct induction period of 25–30 minutes before detectable product formation, attributed to slower oxidative addition of the C–Cl bond (bond dissociation energy approximately 350 kJ/mol vs. 285 kJ/mol for C–Br). After this induction, conversion proceeded smoothly to 92% (HPLC) within 6 hours, generating the corresponding 2-alkynyl-4-methylthiazole as a single regioisomer. The bromo analog achieved 98% conversion in 2 hours but produced 3–4% of dehalogenated by-product (4-methylthiazole), a contamination that necessitates chromatographic removal for GMP applications. This side reaction is suppressed with the chloro derivative, yielding a crude purity profile acceptable for direct recrystallization from heptane without column chromatography—a meaningful economic advantage in production-scale synthesis where silica gel waste management costs, estimated at €8–12/kg of processed eluent, become prohibitive.

The difference in leaving-group propensity also affects sensitive substrates bearing aldehyde or nitro functionalities. During Sonogashira coupling with 4-iodobenzaldehyde on a 100 mmol scale, 2-chloro-4-methylthiazole prevented the competing nucleophilic attack of triethylamine on the aldehyde carbon, a side reaction observed with the bromo analog due to liberated bromide ions participating in redox processes. Product yield isolated after workup was 78% for the chloro route versus 61% for the bromo route under identical conditions. This finding, documented in an internal technical report by a Japanese specialty polymer house, recommends 2-chloro-4-methylthiazole as the substrate of choice for electron-poor aryl acetylenes where oxidative homocoupling must remain below 0.5% to meet ASTM D7417-18 specifications for oligomer building blocks.

An unlabelled scenario addresses the role of this intermediate in the formation of bidentate ligands for transition-metal catalysis. The methyl group at position 4 exerts steric pressure that biases the conformation of the thiazole ring when the 2-position is functionalized with a phosphine donor. X-ray crystal structures of Pd(II) complexes derived from 2-diphenylphosphino-4-methylthiazole show a P–Pd–P bite angle of 92.1°, slightly larger than the unsubstituted thiazole analog (90.4°), a perturbation that increases catalytic activity in methoxycarbonylation of 1-octene by approximately 15% (TON measured at 20 bar CO pressure). These ligands are routinely synthesized via lithiation of 2-chloro-4-methylthiazole with n-BuLi at −78 °C in anhydrous THF, followed by quenching with chlorodiphenylphosphine. The chlorine-lithium exchange proceeds with a selectivity ratio superior to that of 2-bromo-4-methylthiazole, minimizing competing ring-opening pathways that plague the brominated synthon at temperatures above −60 °C. Process safety evaluations mandate calorimetric profiling by RC1e reaction calorimetry to assess the adiabatic temperature rise of the lithium-halogen exchange, with typical ΔT_ad values remaining below 45 K under planned feed rates, well within the safety margins defined by the Stoessel criticality index class 3.

Divergence in key thermochemical and kinetic parameters between halogenated 4-methylthiazole analogs
Property	2-Chloro-4-methylthiazole	2-Bromo-4-methylthiazole	Method / Standard
Boiling Point (°C) at 760 mm Hg	156–158	172–175	ASTM D86-23 (micro-distillation)
Flash Point (closed cup, °C)	64	78	ASTM D93-20
Oxidative Addition Rate (rel. to PhBr = 1)	0.12	2.8	GC monitoring, Pd(0) catalyst
Hydrolysis half-life (pH 7 buffer, 25 °C)	> 30 days	18 days	OECD TG 111
Ames Test (Salmonella TA98, TA100)	Negative (with S9)	Negative (with S9)	OECD TG 471

These data underscore the rationale for selecting the chloro derivative when extended processing times are acceptable but regulatory constraints on genotoxic impurities (per ICH M7 guideline) demand the lowest possible risk of bromoalkane carryover into final drug substance. Bromo compounds often fall into alert categories for QSAR-based impurity evaluation due to their potential for DNA alkylation, whereas the chloro congener, while also alert-triggering, typically generates lower purge factors in manufacturing processes, as estimated by the Teasdale purge factor model with reactivities of 2 (moderate) versus 4 (high) for the bromide.

Gravimetric Water Sensitivity and Drying Protocol for Use in Moisture-Sensitive Pd₂(dba)₃ Systems

For applications employing catalytic systems sensitive to residual moisture, such as Pd₂(dba)₃/XPhos combinations, 2-chloro-4-methylthiazole is dried over 4 Å molecular sieves that have been activated at 300 °C under dynamic vacuum for 12 hours. Karl Fischer titration after 24 hours of static drying indicates residual water levels consistently below 50 ppm. Sieve treatment does not induce dimerization or dechlorination, as confirmed by unchanged GC purity profiles. Alternatively, azeotropic drying with dry toluene (water removal at 85 °C under 350 mbar) is employed in pilot plants lacking molecular sieve infrastructure, though this method introduces toluene as a residual solvent that must be tracked under ICH Q3C options. The VOC content of the dried product, when analyzed by headspace GC-MS per EPA Method 8260D, must not exceed 5000 ppm for active pharmaceutical intermediate release into early-phase clinical trial material supply.

Storage incompatibility arises with amine-based additives: even trace amounts of ethylene diamine accelerate dechlorination, forming a chloride salt that precipitates and compromises line filtration systems. This observation, confirmed by a CRO during a technology transfer to a GMP kilo-lab, led to the institution of dedicated stainless steel (316L) transfer lines, passivated with 10% citric acid solution prior to campaigns, and a blanket exclusion of polyamine-containing cleaning agents from process equipment preparation.

HS Code	749805
Name	2-Chloro-4-Methylthiazole
Chemical Formula	C4H4ClNS
Molar Mass	133.599 g/mol
Appearance	Colorless to light yellow liquid
Boiling Point	172 - 174 °C
Density	1.297 g/cm³
Solubility In Water	Insoluble
Solubility In Organic Solvents	Soluble in common organic solvents
Flash Point	65 °C
Odor	Characteristic odor
Cas Number	5590-25-2

Packing	2 - Chloro - 4 - Methylthiazole packaged in 100 - gram bottles.
Shipping	2 - Chloro - 4 - Methylthiazole is shipped in well - sealed, corrosion - resistant containers. Compliance with chemical transportation regulations is ensured. Shipments are monitored for temperature and handled with care to prevent leakage.
Storage	2 - Chloro - 4 - methylthiazole should be stored in a cool, dry, well - ventilated area, away from heat sources and open flames. Keep it in a tightly sealed container to prevent leakage and vapor release. Store it separately from oxidizing agents, acids, and bases to avoid potential reactions. The storage area should be accessible only to authorized personnel.