4-Methyl-2-Phenyl-1,3-Thiazole

The heterocyclic compound 4-Methyl-2-Phenyl-1,3-Thiazole (CAS 1826-12-6; molecular formula C₁₀H₉NS; molar mass 175.25 g·mol⁻¹) constitutes a substituted thiazole scaffold in which a methyl group occupies the 4‑position and a phenyl ring is attached at the 2‑position of the 1,3‑thiazole core. Commercial supply chains typically list this substance under product codes such as M2P‑THZ‑98 or TZ‑MP‑0421, with a declared purity floor of ≥98.0% via capillary gas chromatography (HP‑5 column, FID, area‑normalisation). The bulk material is a pale‑yellow to off‑white crystalline solid at ambient temperature; the melting range, determined by differential scanning calorimetry at a scan rate of 10 K·min⁻¹ and verified against USP 〈741〉 Class Ia, lies between 41 and 43 °C. Boiling‑point data recorded at 1013 hPa indicate a value of 284 °C (uncorrected), while the density at 25 °C is 1.113 g·cm⁻³. The product is offered in lot sizes from 1 g (analytical‑standard ampoules) to 25 kg (fibre‑drum with aluminium barrier liner), each accompanied by a batch‑specific certificate of analysis referencing the relevant monographs of the European Pharmacopoeia and the JECFA Combined Compendium of Food Additive Specifications.

What Distinguishes the 4‑Methyl‑2‑Phenyl Isomer from Its 2‑Methyl‑4‑Phenyl Counterpart?

Direct positional isomerism on the thiazole ring produces markedly different physicochemical and organoleptic profiles. In 2‑Methyl‑4‑Phenyl‑1,3‑Thiazole (CAS 1826-19-1) the phenyl substituent resides adjacent to the sulphur atom, altering the electron‑deficiency of the ring and thereby shifting the compound’s reactivity in electrophilic substitution and palladium‑catalysed cross‑coupling. Practically, the 4‑methyl‑2‑phenyl arrangement exhibits a higher melting point (by approximately 12 K versus the 28–30 °C reported for the 2‑methyl‑4‑phenyl isomer) and a distinctly different odour character. The former delivers a roasted, cocoa‑like, slightly nutty note with moderate tenacity, whereas the latter is often described as green, vegetative, and pyrazinic. These differences dictate formulation selection in compounded flavours: the 4‑methyl‑2‑phenyl variant is preferred for coffee, chocolate, and toasted‑bread profiles, where its lower volatility provides a longer‑lasting background note, while the 2‑methyl‑4‑phenyl isomer is more suited to early‑impact freshness in berry or mint compositions. A systematic comparison of the two regioisomers and a related flavour‑active thiazole is provided in the table below.

Specifications and Lot‑Release Criteria Under ISO 9001:2015 Batch Control

Each production lot is released against a minimum set of monograph‑level tests. The appearance is assessed visually against an internal colour‑reference panel and is reported as “pale‑yellow crystalline solid, free from foreign matter.” Identity confirmation employs both ¹H‑NMR (400 MHz, CDCl₃; characteristic singlets at δ 2.55 for the 4‑CH₃ and aromatic multiplets between δ 7.25–7.90) and FT‑IR spectroscopy (ATR, peak at 3105 cm⁻¹ aromatic C–H stretch, 1518 cm⁻¹ ring stretch). The assay, determined by capillary GC on a 5%‑phenyl‑methylpolysiloxane column using an FID detector and area‑normalisation following the principles of ASTM D3525, is specified at ≥98.0%. Water content is controlled by volumetric Karl Fischer titration (ASTM E203) with an upper acceptance limit of 0.5% w/w, because moisture above this threshold promotes gradual hydrolysis of the thiazole ring at elevated storage temperatures. Residual solvent limits are aligned with USP 〈467〉 Option 1, with specific attention to ethyl acetate (≤5000 ppm) and toluene (≤890 ppm) arising from the recrystallisation step. Heavy metals are screened by the sulphide precipitation method of Ph. Eur. 2.4.8 Method A, with a reporting limit of ≤10 ppm.

When 4‑Methyl‑2‑Phenyl‑1,3‑Thiazole Serves as a Key Intermediate in Heterocycle Synthesis

In pharmaceutical discovery chemistry, the compound functions as a robust building block for generating libraries of 2,4‑disubstituted thiazoles. The phenyl ring can be chemoselectively functionalised without disturbing the thiazole core. Electrophilic bromination using N‑bromosuccinimide in acetonitrile at 0–5 °C delivers the 4‑bromo‑phenyl derivative in a typical isolated yield of 78–85% after column chromatography (silica gel, hexane‑ethyl acetate 9:1), as monitored by TLC. This brominated intermediate is then engaged in Suzuki‑Miyaura cross‑coupling with arylboronic acids under standard conditions—Pd(PPh₃)₄ (3 mol%), aqueous Na₂CO₃, DME, 85 °C, 12 h—affording biaryl‑substituted thiazoles with coupling efficiencies frequently exceeding 90%. Published data for the scope of coupling partners indicates broad tolerance for electron‑rich and electron‑poor boronic acids, although ortho‑substituted substrates reduce yields by roughly 15–20% due to steric hindrance. Because the thiazole nitrogen possesses a lone pair that can coordinate palladium, an additional 5 mol% of ligand is advisable to maintain catalyst turnover; failure to do so has been observed in pilot‑scale campaigns (batch sizes ≥ 500 g) to slow conversion and increase palladium‑black precipitation. The compound’s utility is distinct from 2‑bromothiazole or 2‑phenylthiazole scaffolds, which often require far harsher lithiation or Grignard conditions for further elaboration.

In the flavour‑creation laboratory, the molecule is delivered as a 1% or 10% solution in triacetin or propylene glycol to facilitate precise dosing. Its recommended use level in finished consumer products spans 0.5 to 5 ppm, with higher concentrations in dry-blended seasonings and lower in liquid beverages. The compound’s threshold in water, reported via the ASTM E679-04 forced‑choice ascending concentration method, lies around 2.8 µg·L⁻¹. Its fundamental difference from 2‑acetylthiazole and 2‑acetyl‑2‑thiazoline is a markedly lower vapour pressure, which imparts a less volatile, more bass‑note profile. Thermogravimetric analysis (TGA, 10 K·min⁻¹, nitrogen purge) reveals an onset of mass loss at 112 °C, and a 5% mass loss temperature of 138 °C, confirming limited volatility under typical pasteurisation conditions (72–95 °C). Stability in propylene glycol at 40 °C/75% RH over 28 days shows UV absorbance variation below 0.02 AU, emphasising compatibility with shelf‑stable liquid flavour bases.

Performance Boundaries in Catalytic Cross‑Coupling and the Risk of Deactivation

When scaled to twin‑screw continuous processing for large‑volume intermediate production, the exothermicity of the Suzuki coupling demands precise thermal management. Adiabatic temperature rise calculations based on the measured reaction enthalpy (−168 kJ·mol⁻¹ by RC1e calorimetry) indicate that an uncooled batch exceeding 2 mol scale would exceed the solvent boiling point within 12 seconds of initiating the reaction. Therefore, process‑scale reactors are configured with jacket temperature control set to 70 ± 2 °C, coupled with a feed‑rate‑limiting interlock triggered when internal temperature exceeds 77 °C. In addition, the compound must be stored under inert gas (argon or nitrogen, residual oxygen ≤0.2% v/v) and at 2–8 °C to prevent discolouration and the formation of dimeric oxidation by‑products detectable by HPLC‑MS at m/z 349 [M+H]⁺. Combination with strong oxidisers such as concentrated nitric acid or peroxides results in rapid ring cleavage; a documented DSC scan of a 1:1 mixture with sodium peroxide gave an exotherm onset at 55 °C with a specific energy release of 820 J·g⁻¹, clearly prohibiting any compounding operation involving such agents without rigorous process hazard analysis. The product is REACH‑registered (EC number 217-356-3) and compliant with the TSCA inventory. It does not fall under the scope of the Rotterdam or Stockholm POP conventions, and its transport classification is non‑hazardous under ADG 7.7 when packed in accordance with P‑3 packaging instructions.