2,4-Dimethyl-3-Ethylpyrrole

In commercial production batches spanning scales from 500 mL laboratory glassware to 2000 L glass-lined reactors, 2,4-dimethyl-3-ethylpyrrole (CAS RN 517-22-6) is isolated as a clear, pale-yellow to amber liquid with a characteristic heterocyclic amine odor. Molecular formula C₈H₁₃N and a relative molecular mass of 123.20 g·mol⁻¹ define the substance; typical assay by GC-FID, calibrated against a dual internal standard of n-dodecane and n-tetradecane, returns a purity floor of ≥ 97.0%. Bulk density at 20 °C lies in the range 0.905–0.915 g·cm⁻³, refractive index n20/D 1.485–1.495. Industrial synthesis routes involving sequential alkylation of pyrrole or ring-closure from appropriate 1,4-diketones generate exotherms exceeding 150 J·g⁻¹ during quench; temperature overshoot above 45 °C in the absence of jacket cooling promotes dimerization tars that lower isolated yield by 8–12%. Batch-to-batch color variation, commonly tracked via APHA scale in a 10 mm cell, is a sensitive proxy for oligomer content and directly affects performance in downstream macrocyclization reactions. When reactivity in porphyrin condensation requires steric control The mono-pyrrole unit supplies the precise β,β′-substitution pattern needed to construct etioporphyrin frameworks and related symmetric meso-substituted porphyrinoids. In a standard one-flask procedure adapted from Adler–Longo methodology, 1.0 mol of 2,4-dimethyl-3-ethylpyrrole is combined with 1.0 mol of the desired aldehyde in refluxing propionic acid (138–141 °C bath temperature, inert gas cap with 0.5 L·min⁻¹ nitrogen flow) at a total pyrrole concentration of 0.18 mol·L⁻¹. Under these conditions the macrocyclic tetramerization is kinetically favored, but the steric occupancy of the ethyl group at C-3 and the two methyl groups at C-2 and C-4 diminishes the rate of electrophilic attack at the free α-positions compared to unsubstituted pyrrole by a factor of approximately 0.3. The consequence is a narrow processing window: if headspace oxygen concentration exceeds 50 ppm during the 45–60 min reflux period, irreversible oxidative polymerization of the alkylpyrrole generates dark, insoluble precipitates that cannot be removed by Celite filtration without co-removing 5–10% of the target porphyrin. Post-condensation oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ, 1.15 equivalents relative to pyrrole, added at 25 °C after cooling) must be complete within 35 min; protracted stirring beyond 50 min leads to DDQ-adduct formation on the ethyl-substituted pyrrolic β‑positions, detectable as a shoulder at 615–618 nm in UV‑Vis spectra of the purified macrocycle. Production-scale synthesis of etioporphyrin I using this mono-pyrrole routinely achieves isolated yields of 12–18% after two recrystallizations from dichloromethane/methanol, limited primarily by scrambling equilibria that generate a statistical mixture of porphyrin isomers when the acid catalyst (commonly BF₃·OEt₂ at 2.0 mol%) is not rigorously controlled. Chromatographic separation (silica gel, hexane/dichloromethane gradient) of the undesired etioporphyrin II and III contaminants from the target isomer requires 4.2 L of solvent per gram of crude mixture, representing the primary cost-driver in cGMP manufacture. What distinguishes this alkylpyrrole from symmetrical isomers? When screens of substituted pyrroles are performed to identify building blocks with minimum by-product formation in dipyrromethane synthesis, the asymmetry of 2,4-dimethyl-3-ethylpyrrole produces regiochemical outcomes that differ markedly from both the 2,3,4-trimethyl and 2,5-dimethyl isomers. The presence of a single ethyl substituent adjacent to one α‑position unbalances the electron density in the pyrrolic ring to a greater extent than three methyl groups, as measured by the Hammett σₚ value derived from ¹³C NMR shifts of the α‑carbons: −0.18 versus −0.11 for 2,3,4-trimethylpyrrole. In condensation with p‑tolualdehyde under Lindsey conditions (BF₃·OEt₂, CH₂Cl₂, 25 °C, 2 h), the ethyl-bearing monomer directs 73% of the initial electrophilic attack to the sterically less hindered C-5 position, whereas the corresponding 2,4-dimethyl-3-isopropylpyrrole raises this selectivity to 87%, indicating a smooth steric tuning accessible by altering the C-3 alkyl chain. The symmetrical 2,5-dimethylpyrrole, lacking β-substitution, suffers simultaneous reaction at both α‑sites and produces intractable oligomeric mixtures with M_w > 5000 Da within 10 min under identical conditions. Table 1 compares key physical and reactivity parameters across the most commonly encountered alkylpyrroles in porphyrin chemistry.

Table 1. Comparative Properties of Alkylpyrrole Monomers Used in Macrocycle Synthesis

Monomer	Boiling Point (°C)	Relative Rate of α‑Attack (krel)	Dipyrromethane Selectivity (%)	Oxidative Onset Temperature (°C)a
Pyrrole (unsubstituted)	129–131	1.00	34	48
2,5-Dimethylpyrrole	165–167	0.82	41	52
2,4-Dimethyl-3-ethylpyrrole	194–197b	0.34	90	87
2,3,4-Trimethylpyrrole	200–204	0.29	88	91
3-Ethyl-2,4-dimethylpyrrole-5-carboxylic acid	—c	0.08	95	126

^a Determined by differential scanning calorimetry (DSC) under 20 mL·min⁻¹ O₂ flow, heating rate 10 K·min⁻¹.
^b Measured at atmospheric pressure; decomposition can begin above 180 °C in the presence of trace acid.
^c Decomposes before boiling; melting point 155–157 °C with decarboxylation. Storage stability and oxidative degradation thresholds Shipping and storage conditions directly govern the shelf life of low-molecular-weight alkylpyrroles susceptible to autoxidation. 2,4-Dimethyl-3-ethylpyrrole packaged under argon in amber Type III glass bottles (100 mL to 2.5 L) fitted with PTFE-lined phenolic caps retains assay values above 96.5% for 12 months when stored at 2–8 °C. At ambient warehouse temperatures (22–25 °C), headspace passive oxidation catalyzed by ambient light reduces the assay by approximately 0.8% per month, accompanied by the appearance of a 0.3% (area) GC peak at a relative retention time of 1.18 corresponding to the 2,2′-bipyrrolic coupling product. Accelerated aging at 40 °C and 75% relative humidity in vented containers degrades the material to 78% purity within 14 days, primarily through hydrolytic ring-opening reactions that require water activity above 0.6. Operators handling the liquid in open process vessels must ensure a pad of dry nitrogen at 50–100 mbar gauge and limit exposure to fluorescent lighting exceeding 500 lux for cumulative periods longer than 4 h. The compound reacts violently with strong oxidizing agents; contact with concentrated nitric acid or peroxides leads to instantaneous polymerization and gas evolution, mandating dedicated stainless steel 316L transfer lines and exclusion of brass or copper fittings that catalyze radical pathways. In terms of regulatory documentation, the substance is manufactured under ISO 9001:2015 quality management systems; laboratory-scale quantities supplied for R&D purposes are accompanied by certificates of analysis citing conformance to specifications derived from the supplier’s validated in-house method, which typically references USP ⟨621⟩ for chromatographic system suitability criteria and ISO 6353-1:1982 for general requirements of chemical reagents. Direct application of the mono-pyrrole as a ligand precursor in transition-metal catalysis exploits the strong σ‑donor character of the pyrrolide anion generated by deprotonation with potassium hydride or n‑butyllithium at −78 °C. When the resulting lithium or potassium salt is treated with titanium(IV) chloride in tetrahydrofuran at 0 °C, the corresponding mono(pyrrolyl)titanium trichloride complex crystallizes as a dark red solid; published data for this specific configuration is limited, but the mono-substituted species serves as a constrained-geometry catalyst component for ethylene/1‑octene copolymerization, where the ethyl and methyl substituents modulate the steric environment around the metal center sufficiently to alter comonomer incorporation by 1.3–1.8 mol% compared to the unsubstituted pyrrolyl analogue. Odor threshold characterization and fragrance intermediate handling In the synthesis route to certain alkylpyrazines used as nutty, roasted aroma chemicals, 2,4-dimethyl-3-ethylpyrrole undergoes oxidative ring expansion in the presence of hydrogen peroxide and a tungstate catalyst at 55–60 °C, forming a dihydropyrazine intermediate that decarboxylates to the target 2-ethyl-3,5-dimethylpyrazine. The air concentration at which the human olfactory detection probability reaches 0.5 for the pyrrole precursor has been measured at 0.17 ppb v/v; this necessitates closed-loop ventilation and packed-column scrubbing of reactor vents, as release of even 5 mL of the undiluted liquid into an unventilated 30 m³ room renders the space occupationally intolerable within minutes. The threshold limit value–time-weighted average (TLV-TWA) has not been established by ACGIH, so risk assessment relies on read-across from structural analogues and mandates supplied-air respirators for any open transfer exceeding 250 g. Analytical specifications and batch release criteria Each production batch is qualified against a master specification agreed between the manufacturer and the end-user in the context of the intended downstream transformation. Table 2 reproduces a representative release protocol for a research-grade product.

Table 2. Representative Batch Release Specification for 2,4-Dimethyl-3-ethylpyrrole (Research Grade)

Parameter	Acceptance Criterion	Test Method
Assay (GC)	≥ 97.0%	GC-FID, DB‑5HT column 30 m × 0.25 mm, temperature program 60 °C (2 min) → 10 °C·min⁻¹ → 280 °C (5 min)
Appearance	Clear pale-yellow to light amber liquid	Visual comparison against a 10 mm path-length reference standard
Water (Karl Fischer)	≤ 0.2%	ISO 760:1978, coulometric titration
Major Impurity	2,4-Dimethyl-3-ethyl-5-formylpyrrole ≤ 1.5%	Same GC method, area percent
Refractive Index n20/D	1.485–1.495	ISO 5661:1983
Residue on Ignition	≤ 0.05%	ISO 6353-1:1982, GM 21
Packaging Atmosphere	Argon, positive pressure ≥ 700 mbar absolute	Headspace oxygen analysis by polycarbonate paramagnetic probe

Elevated-temperature flash point measurement (ASTM D6450-16a, continuously closed cup) yields a value of 79 °C, placing the substance in combustible liquid Class IIIA under NFPA 30 criteria. Viscosity at 25 °C is 2.3 mPa·s, permitting turbulent flow in ½-inch stainless steel pipework at Reynolds numbers above 4000 with less than 0.15 bar·km⁻¹ friction loss, a non-critical parameter that simplifies plant design for multi-ton campaigns. Transition-metal-assisted homocoupling to 3,3′,5,5′-tetraalkyl-2,2′-bipyrrole frameworks has been examined as a route to extended π‑conjugated systems for organic field-effect transistor semiconductors. In an electrochemical cell fitted with a platinum working electrode and Ag/AgNO₃ reference, the onset oxidation potential of the monomer occurs at +0.74 V versus Fc/Fc⁺. Electropolymerization onto indium tin oxide substrates from 0.1 M acetonitrile/tetrabutylammonium hexafluorophosphate yields films with a bandgap of 2.6 eV, a value 0.3 eV higher than poly(3-hexylthiophene) and indicative of limited inter-chain delocalization attributable to the ethyl group twist angle of 22° out of the pyrrole plane, as computed by DFT at the B3LYP/6-311+G(d,p) level. Charge carrier mobility extracted from transfer curves of bottom-gate bottom-contact devices prepared on octadecyltrichlorosilane-treated SiO₂ dielectric reaches 4.2 × 10⁻³ cm²·V⁻¹·s⁻¹, positioning the material in the intermediate range for p‑type organic semiconductors; injection barrier lowering by the alkyl substitution pattern accounts for an improvement of one order of magnitude over unsubstituted polypyrrole prepared under identical conditions.