Commercially supplied as a colorless to pale yellow liquid with a characteristic amine odor, 1H-Pyrrole, 2,5-dihydro- (CAS RN 109-96-6) is a five-membered cyclic secondary amine containing a single endocyclic double bond. The material is routinely manufactured via reduction of pyrrole or by cyclization of 1,4-dihalobut-2-enes with primary amines, followed by fractional distillation under reduced pressure to achieve a typical assay of 97.0–99.5% (GC area%). Industrial batches are standardized to a water content of ≤0.5% (ISO 760:1978, Karl Fischer coulometric titration) and a peroxide value below 0.5 mmol/kg (iodometric titration, internal method adapted from ASTM E298-17a) due to the susceptibility of the allylic amine moiety to autoxidation. Refractive index (nD20) lies between 1.4655–1.4670 (ASTM D1218-21), density at 20°C is 0.909–0.915 g/mL (ASTM D4052-22), and the atmospheric boiling range is 90–92°C. The compound is miscible with common aprotic solvents (THF, DCM, toluene) and exhibits limited aqueous solubility of approximately 12 g/100 mL at 25°C, forming a mildly alkaline solution (pKa of the conjugate acid ≈ 10.0). Producers supply the product in 100 mL, 500 mL, and 2.5 L borosilicate glass bottles under nitrogen blanket, or in 200 L steel drums with internal phenolic lining for bulk users; all containers are fitted with PTFE-faced septa to minimize headspace oxygen ingress.
| Parameter | Specification | Test Method / Standard |
|---|---|---|
| Assay (GC) | ≥ 97.0% (area%) | In-house GC-FID, DB-5 column, 30 m × 0.25 mm × 0.25 µm, He carrier |
| Water Content | ≤ 0.5% w/w | ISO 760:1978 (K-F coulometric) |
| Peroxide Value | ≤ 0.5 mmol/kg | Iodometric titration, adapted from ASTM E298-17a |
| Refractive Index (nD20) | 1.4655 – 1.4670 | ASTM D1218-21 |
| Density (20°C) | 0.909 – 0.915 g/mL | ASTM D4052-22 |
| Boiling Range | 90 – 92°C (760 mmHg) | ASTM D86-23a (micro-distillation) |
| Color (APHA) | ≤ 100 | ASTM D1209-05(2019) |
| Stabilizer (BHT, optional) | 100 – 300 ppm | HPLC-UV, internal method |
Storage and Stability: The Role of Water and Oxygen
The allylic C–H bonds adjacent to the nitrogen atom render 1H-pyrrole, 2,5-dihydro- intrinsically sensitive to radical-mediated oxidation. On standing in air, hydroperoxide formation at the α-position proceeds with an induction period of roughly 6–12 hours at ambient temperature in the absence of stabilizer, followed by autocatalytic decomposition that yields ring-opened amino-aldehydes and discolored oligomeric species. To suppress this pathway, the product is routinely inhibited with 100–300 ppm of butylated hydroxytoluene (BHT). Even with BHT, headspace oxygen in partially emptied containers must be replaced with dry nitrogen (dew point ≤ −40°C) immediately after each withdrawal. Storage at 2–8°C extends shelf life to 12 months; storage at room temperature reduces the verified retest interval to 6 months. When the material is stored over molecular sieves (3A) for anhydrous applications, a steep exotherm has been observed during initial contact in adiabatic calorimetry, necessitating slow addition and external cooling if drying more than 500 mL per batch.
Addition of 2,5-dihydro-1H-pyrrole directly into reaction mixtures containing strong alkylating agents (methyl iodide, dimethyl sulfate) without controlled addition rate has led to thermal runaways on 10 L pilot-plant scale due to the amine’s exothermic quaternization (ΔH ≈ −85 to −110 kJ/mol estimated from analogous secondary amines). Production-scale reactors equipped with jacket cooling (ΔTjacket < 20°C), controlled addition via metering pump, and internal temperature probes with high-limit interlocks are standard when alkylation charges exceed 5 kg.
What Makes 2,5-Dihydro-1H-Pyrrole Distinct from Its Isomers?
Three partially hydrogenated pyrrole isomers exist, and confusion in procurement and synthetic planning often arises between 2,5-dihydro-1H-pyrrole (3-pyrroline), 2,3-dihydro-1H-pyrrole (2-pyrroline), and 3,4-dihydro-2H-pyrrole (1-pyrroline). In 2,5-dihydro-1H-pyrrole, the double bond is located at the 3,4-position, and the nitrogen atom exists as a secondary amine capable of N–H addition reactions, salt formation, and carbamate protection. The 2,3-dihydro isomer, by contrast, is a cyclic imine (C=N at the 2-position) that hydrolyzes rapidly in the presence of moisture to γ-aminobutyraldehyde, making it unsuitable for aqueous workups or long-term storage without anhydrous precautions. The 3,4-dihydro-2H-pyrrole (1-pyrroline) is also an imine, with the double bond at the 1,2-position, and is even more prone to oligomerization; its boiling point (87–89°C) overlaps with the title compound, so separation by simple distillation is not reliable. Industrial specifications for 1H-pyrrole, 2,5-dihydro- explicitly limit the 2,3-dihydro isomer content to ≤0.5% (GC) to avoid side reactions in electrophilic additions where the imine tautomer acts as a competitive nucleophile.
A direct comparison of the fully saturated analog pyrrolidine (CAS 123-75-1) reveals fundamentally different reactivity. Pyrrolidine (b.p. 87–88°C, pKa 11.3) lacks the alkene functionality and is favored when a more nucleophilic, fully sp³-hybridized amine is required. 2,5-Dihydro-1H-pyrrole retains the ring strain (ca. 5.2 kcal/mol above pyrrolidine, estimated from hydrogenation enthalpy) and the electron-rich double bond, allowing post-functionalization via hydroboration, epoxidation, or Diels–Alder cycloaddition with electron-deficient dienophiles. The fully aromatic pyrrole (pKa of protonated form ≈ −3.8) acts as a π-excessive heteroaromatic and undergoes electrophilic substitution; it is not a nucleophilic amine under neutral or basic conditions. Thus, selection among these four compounds pivots on whether the downstream chemistry demands an alicyclic secondary amine with a latent olefin handle (2,5-dihydro), a stable saturated amine (pyrrolidine), an imine susceptible to hydrolysis (2-pyrroline), or an aromatic ring (pyrrole).
| Property | 1H-Pyrrole, 2,5-dihydro- | Pyrrolidine | 2,3-Dihydro-1H-pyrrole (2-pyrroline) | Pyrrole |
|---|---|---|---|---|
| CAS RN | 109-96-6 | 123-75-1 | 5664-89-7 | 109-97-7 |
| B.p. (°C, 760 mmHg) | 90–92 | 87–88 | 89–91 | 129–131 |
| pKa (conjugate acid) | ~10.0 | 11.3 | ~7.5 (imine) | −3.8 |
| Ring unsaturation | 1 C=C, allylic amine | None | 1 C=N, endocyclic imine | Full aromatic |
| Stability to H2O | Stable, slow oxidation | Stable | Rapid hydrolysis | Stable |
| Preferred storage condition | 2–8°C, N2, BHT | RT, inert gas | −20°C, anhydrous | RT, dark |
When 3-Pyrroline Acts as a Dipolarophile in Cycloaddition Cascades
In medicinal chemistry programs targeting sp³-rich heterocyclic scaffolds, 2,5-dihydro-1H-pyrrole has been employed as a strained dipolarophile in 1,3-dipolar cycloadditions with nitrones and azomethine ylides. The electron-rich double bond, combined with the allylic strain imposed by the ring, accelerates cycloaddition rates relative to acyclic allylamines. In a typical process route documented in pilot-plant campaigns, 2,5-dihydro-1H-pyrrole (1.0 equiv) is combined with a nitrone generated in situ from N-benzylhydroxylamine and paraformaldehyde in toluene at 80°C for 16 hours, producing fused isoxazolidines with diastereomeric ratios exceeding 9:1 (trans:cis) when the nitrogen atom is unprotected. The secondary amine can then be elaborated via reductive cleavage of the N–O bond or retained for further functionalization. Process safety evaluations for these cycloadditions on 50 kg scale required differential scanning calorimetry (DSC) screening of the neat reaction mixture, which showed an onset of decomposition at 180°C with an energy release of −450 J/g. Consequently, the addition of the amine is performed as a controlled semibatch operation with the internal temperature maintained below the onset of the main exotherm by at least 70°C, providing a comfortable margin in the event of cooling failure.
Batch-to-batch variability in cycloaddition diastereoselectivity on 20 L scale was traced to residual peroxide content in the 2,5-dihydro-1H-pyrrole charge. When the peroxide value exceeded 0.7 mmol/kg, a 5–10% erosion in trans selectivity was observed, attributed to radical side-reactions that epimerize the α-stereocenter of the nitrone intermediate. The specification of ≤0.5 mmol/kg was tightened from an earlier limit of ≤1.0 mmol/kg after this investigation. Vendors now supply material pre-analysed for peroxides using an iodometric method adapted from ASTM E298-17a, and on-site quality-control laboratories re-verify the value upon receipt using a calibrated autotitrator (Metrohm 916 Ti-Touch or equivalent) with potentiometric endpoint detection.
Within the broad class of primary and secondary allylic amines, 2,5-dihydro-1H-pyrrole exhibits a boiling point sufficiently low to permit removal by distillation from high-boiling solvents, yet sufficiently high that it does not evaporate uncontrollably during ambient-pressure handling—a practical advantage over acyclic allylamine (b.p. 53°C) when conducting reactions at elevated temperatures. This volatility window has been leveraged in the synthesis of N-Boc-3-pyrroline (b.p. 65–67°C at 1.5 mmHg), where unreacted 2,5-dihydro-1H-pyrrole is recovered by atmospheric distillation and recycled. Reported recovery rates in kilo-lab campaigns exceed 92% when the distillation bottoms are not allowed to exceed 100°C, minimizing thermal oligomerization.
Polymer-Grade Monomer Purity and the Critical Threshold of Amine-Water Azeotrope Behavior
For applications in precision polymer synthesis—particularly in the preparation of poly(3-pyrroline)-based anion-exchange membranes—monomer purity above 99.5% is mandatory. 2,5-Dihydro-1H-pyrrole forms a minimum-boiling azeotrope with water at approximately 86°C (azeotropic composition ca. 30% water by weight), rendering simple distillation insufficient to achieve the ultralow water content (<50 ppm) demanded by living anionic polymerization. The compound must be dried by fractional distillation from calcium hydride (CaH2) under a static nitrogen atmosphere, discarding a 10% forerun, or by passage through a column of activated neutral alumina (Brockmann I) immediately prior to polymerization. In glovebox-based living polymerization setups with sec-butyllithium initiation at −78°C in THF, water content exceeding 80 ppm in the monomer feed has been correlated with broadened molecular weight distributions (Đ > 1.4) and premature termination, as observed by GPC (RI detection, polystyrene standards). The requirement for exhaustive drying represents a cost driver that has spurred evaluation of alternative purification protocols, including azeotropic drying with toluene using a Dean–Stark trap, but loss of monomer into the distillate remains a limitation, with typical recoveries of 75–80% after two cycles.
Free-radical polymerization of 2,5-dihydro-1H-pyrrole, though less rigorous with respect to water tolerance, is complicated by the amine’s chain-transfer activity. The N–H bond acts as a chain-transfer agent, reducing the achievable number-average molecular weight in bulk polymerization initiated by AIBN at 60°C to approximately 3,000–5,000 g/mol. This feature has been exploited deliberately in the synthesis of amine-terminated oligomers, where the degree of polymerization is controlled by the initial monomer-to-initiator ratio and the chain-transfer constant (Cs estimated at 0.012 for the N–H bond at 60°C). For production of high-molecular-weight polymers, the amine must be protected as the N-Boc or N-tosyl derivative prior to polymerization, and the deprotection step introduces additional process mass intensity, often necessitating trifluoroacetic acid in dichloromethane or hydrogenation over Pd/C, with associated EHS considerations.
Single-Phase vs. Biphasic N-Alkylation: Equipment Implications
An enduring operational challenge in fine chemical manufacture is the N-alkylation of 2,5-dihydro-1H-pyrrole with alkyl halides in the presence of aqueous base. Under the classic biphasic conditions (50 wt% aq. NaOH, tetrabutylammonium bromide as phase-transfer catalyst), the reaction mixture tends to emulsify on 500 L scale due to the surfactant-like properties of the quaternary ammonium salt and the partially water-soluble amine. Phase separations requiring more than 45 minutes have been documented, with rag layers stabilized by trace iron hydroxide introduced from carbon steel equipment. Transition to a homogeneous system using K2CO3 in anhydrous DMF at 60–65°C eliminated the phase-separation bottleneck, but necessitated rigorous removal of residual DMF from the product (final specification < 50 ppm by headspace GC) due to its reprotoxicity classification under REACH. The switch from biphasic to homogeneous conditions is thus contingent on the availability of thin-film evaporators (e.g., wiped-film, 0.1 m² surface area) capable of reducing high-boiling solvent carryover without thermally stressing the N-alkylated product, which undergoes retro-Michael elimination above 120°C.
Where the production asset does not include thin-film evaporation capability, solid-liquid phase-transfer catalysis using powdered KOH in toluene with Aliquat 336 has been implemented at 100 kg batch size, achieving conversion above 95% within 8 hours and simplifying workup to a straightforward filtration and distillation. However, the hygroscopic nature of powdered KOH under ambient humidity (>60% RH) demands enclosed transfer systems or nitrogen-purged glovebags to prevent deliquescence and subsequent hydroxide-mediated oligomerization of the double bond.
Published data on the long-term storage stability of N-alkyl-3-pyrrolines in plastic versus glass-lined containers is limited, but accelerated aging studies at 40°C/75% RH for 4 weeks indicate that iron contamination above 5 ppm catalyzes discoloration and viscosity increase, presumably via Fenton-type oxidation of the allylic position. For this reason, all product-contact surfaces in commercial-scale manufacturing and storage are specified as 316L stainless steel, glass-lined steel, or PTFE-lined, while carbon steel and copper alloys are strictly excluded.
Handling of 1H-Pyrrole, 2,5-dihydro- is governed by UN 2924 (flammable liquid, corrosive, n.o.s.), Packing Group II, with a flash point (closed cup) of approximately −5°C (ASTM D56-22). Ventilation must maintain vapor concentrations below the occupational exposure limit, typically set at an 8-hour TWA of 1 ppm by analogy with similar cyclic secondary amines, though no specific regulatory occupational exposure limit has been assigned. Process vents are routed through packed-bed scrubbers containing dilute sulfuric acid to capture amine vapors before release.
