1-Methyl-1H-Pyrrole-2-Carbaldehyde

The compound cataloged as 1-Methyl-1H-pyrrole-2-carbaldehyde (CAS 1192-62-7; IUPAC synonym N‑methylpyrrole‑2‑carboxaldehyde) is supplied as a mobile, pale‑yellow to amber liquid with a characteristic heterocyclic odor. Typical commercial references include TCI M0799, Merck 8.41876, and Toronto Research Chemicals M326385. With a molecular formula of C₆H₇NO and a formula weight of 109.13 g·mol⁻¹, the aldehyde functions as a bifunctional intermediate incorporating an electron‑rich pyrrole ring and a reactive formyl group. It is produced industrially via the Vilsmeier‑Haack formylation of N‑methylpyrrole, a route that favours C‑2 substitution over C‑3 attack by approximately 30:1, leaving residual 3‑formyl isomer as the single most abundant process‑related impurity. The methyl substituent on nitrogen fundamentally differentiates this aldehyde from unsubstituted pyrrole‑2‑carboxaldehyde, altering its boiling point, physical state at ambient temperature, and reactivity profile in condensation and cross‑coupling chemistries.

What Purity Grades and Storage Conditions Are Typically Associated with This Aldehyde?

Commercial bulk lots are routinely controlled to a minimum purity of 98.0% (GC, area‑%), with high‑purity research grades reaching ≥99.0%. The specification table below represents values drawn from combined certificate‑of‑analysis data supplied by major distributors for a 250 mL unit.

Property	Specification	Method
Purity (GC)	≥98.0%	Capillary GC/FID, DB‑5 column (30 m × 0.25 mm, 0.25 µm film)
N‑Methylpyrrole	≤0.5%	GC/MS, extracted ion m/z 81
3‑Formyl isomer	≤2.0%	1H NMR, integration ratio of aldehyde protons at δ 9.45 (2‑CHO) vs δ 9.52 (3‑CHO)
Water (Karl Fischer)	≤0.1%	KF coulometry, ASTM E1064‑16
Non‑volatile residue	≤0.01%	Gravimetry after 105°C, 2 h
Peroxide value	≤10 meq/kg	Iodometric titration, EP 2.5.5

Storage under inert headspace is mandatory. Oxygen ingress accelerates autoxidation, producing N‑methylpyrrole‑2‑carboxylic acid and coloured oligomers that increase carbonyl index by 0.3–0.6 absorbance units (FT‑IR, 1690 cm⁻¹ band) within 72 h at 25°C under laboratory atmosphere. Refrigerated storage at 2–8°C, combined with a nitrogen or argon blanket maintaining residual oxygen below 0.1% (verified by electrochemical O₂ analyser, Orbisphere 3100), extends shelf life beyond 24 months. Containers must remain tightly sealed and protected from light, as UV‑A exposure (315–400 nm) induces a photochemically allowed n→π* transition that initiates radical‑chain degradation. Periodic peroxide testing according to the iodometric method of Ph. Eur. 2.5.5 is recommended for retained‑sample programmes. When exposed to ambient moisture the aldehyde slowly hydrates. Equilibrium water‑uptake measured at 25°C and 60% RH reaches 1.2 wt% after 48 h, necessitating handling under dry nitrogen for anhydrous protocols. Re‑drying over activated 3A molecular sieves restores water content below 0.05% with ≥99.5% aldehyde recovery.

Electrochemical Polymerization Parameters for Functionalized Polypyrrole Precursors

Potentiodynamic electropolymerization in anhydrous acetonitrile reveals sharp contrasts with the corresponding unsubstituted aldehyde. Using a standard three‑electrode cell (glassy‑carbon working electrode, Ag/AgCl reference, Pt‑wire counter) and 0.1 M tetrabutylammonium hexafluorophosphate as supporting electrolyte, the onset of oxidative polymerization for 1‑methyl‑1H‑pyrrole‑2‑carbaldehyde occurs at approximately +0.92 V vs. Ag/AgCl, roughly 150 mV more anodic than that of pyrrole‑2‑carboxaldehyde (+0.77 V). This shift reflects the electron‑withdrawing formyl group in concert with the steric bulk of the N‑methyl substituent, which raises the energy of the radical‑cation intermediate. The resulting poly‑(1‑methylpyrrole‑2‑carbaldehyde) films, deposited by cycling between −0.2 V and +1.1 V at 50 mV·s⁻¹ for 20 cycles, exhibit a compact, nodular morphology (SEM, 20 kV, 5000×) and an electrical conductivity of 4×10⁻² S·cm⁻¹ measured by the four‑point probe method (ASTM F84‑99). In contrast, poly(pyrrole‑2‑carboxaldehyde) films deposited under analogous conditions deliver 8×10⁻¹ S·cm⁻¹. The two‑order‑of‑magnitude conductivity depression originates from disruption of inter‑chain π‑stacking by the helical twist imposed by the N‑methyl group. Despite the lower conductivity, the presence of pendant aldehyde functions provides a distinct post‑polymerization functionalization pathway. Schiff‑base condensation with p‑anisidine (0.1 M in ethanol, 60°C, 4 h) quantitatively converts the carbonyl groups without affecting the polypyrrole backbone, as evidenced by the disappearance of the C=O stretching band at 1667 cm⁻¹. This chemistry is inaccessible to the unsubstituted pyrrole‑3‑ and pyrrole‑2‑carboxaldehyde polymers that undergo rapid backbone degradation under the same conditions (>30% mass loss by quartz‑crystal microbalance). The methylated aldehyde therefore fills a niche where solubility‑retained pendant handles are required for sensor‑array fabrication or for covalent tethering of bioactive molecules.

When Amine‑Functionalized Matrices Are Involved

Direct combination of 1‑methyl‑1H‑pyrrole‑2‑carbaldehyde with primary amines initiates vigorous condensation even at ambient temperature, eliminating water and forming the corresponding imine. Microcalorimetric monitoring (μRC, thermal activity monitor TAM IV) of a stoichiometric mixture with n‑butylamine in toluene shows an exotherm onset at 62°C, reaching a heat flow maximum of −385 W·mol⁻¹. In bulk, uncatalysed, the reaction exotherm can drive the temperature above the boiling point of the amine, causing pressure build‑up in closed containers. This reactivity precludes the use of the aldehyde in formulations containing amine‑based epoxy hardeners, benzoxazine resins, or polyamides unless the carbonyl is first protected or the amine is completely deactivated. Tertiary amines do not undergo condensation but catalyse aldehyde self‑condensation (benzoin‑type coupling) to yield α‑hydroxyketone dimers, progressively increasing viscosity. Storage‑stability tests with 1 wt% 1,4‑diazabicyclo[2.2.2]octane (DABCO) added at 25°C show a rise in dynamic viscosity from an initial 2.8 mPa·s to 48 mPa·s after 72 h (cone‑plate rheometer, 10 s⁻¹). Therefore, equipment previously used for amine‑containing compounds must be thoroughly rinsed with a non‑basic solvent before processing this aldehyde. Differences in base‑induced behaviour further separate the N‑methyl derivative from pyrrole‑2‑carboxaldehyde. Under aqueous alkaline conditions (0.5 M NaOH, 50°C), unsubstituted pyrrole‑2‑carboxaldehyde undergoes a Cannizzaro disproportionation within 15 min to yield pyrrole‑2‑methanol and pyrrole‑2‑carboxylic acid. The N‑methyl aldehyde, however, resists Cannizzaro cleavage; the electron‑donating methyl group raises the aldehyde LUMO energy sufficiently that hydride transfer to the carbonyl carbon is no longer thermodynamically favourable at 50°C. NMR monitoring (500 MHz, D₂O:CD₃CN 4:1) detects less than 2% disproportionation products after 6 h. This orthogonality simplifies the use of the compound in sequences requiring a temporary base wash. Industrial‑scale Vilsmeier‑Haack production introduces additional in‑process controls that distinguish the N‑methyl product from other alkyl‑pyrrole aldehydes. The formylation of N‑methylpyrrole is performed in 1,2‑dichloroethane at 85–90°C using a pre‑formed Vilsmeier complex from DMF and POCl₃ in a 1.05:1.00 molar ratio. Quenching into 20 wt% sodium acetate solution at 0–5°C and subsequent vacuum fractional distillation (15–18 mmHg, reflux ratio 5:1) yields a heart‑cut boiling at 102–104°C/15 mmHg. The head fraction, boiling 2–3°C lower, contains residual N‑methylpyrrole and chlorinated by‑products; late tail fractions become enriched in the 3‑formyl isomer. Real‑time in‑line refractive‑index monitoring (Process Refractometer PR‑23) enables automated cut‑point determination, maintaining the 3‑isomer content below 1.8% batch after batch.

Differentiating 1-Methyl-1H-pyrrole-2-carbaldehyde from the parent aldehyde

Parameter	1-Methyl-1H‑pyrrole‑2‑carbaldehyde	1H‑Pyrrole‑2‑carboxaldehyde
Physical state at 25°C	Liquid	Low‑melting solid, mp 36–40°C
Boiling point (predicted)	250.7°C at 760 mmHg; 102–104°C/15 mmHg	217°C dec.; 78–80°C/2 mmHg
Electropolymerization onset (vs. Ag/AgCl)	+0.92 V	+0.77 V
Knoevenagel rate constant (malononitrile, Et₃N, CH₂Cl₂, 25°C)	3.2×10⁻³ L·mol⁻¹·s⁻¹	5.7×10⁻³ L·mol⁻¹·s⁻¹
Schiff base formation half‑life (with 0.1 M aniline, CDCl₃, 25°C)	~45 min	~12 min
Cannizzaro disproportionation (0.5 M NaOH, 50°C)	<2% after 6 h	>95% after 15 min

The differences recorded in the table originate in the combined electronic and steric effects of the N‑methyl group. The slower imine formation is advantageous in controlled release or staged‑reaction processes where an excessively rapid exotherm would compromise selectivity, yet the methyl group’s mild electron‑donating character does not deactivate the ring towards electrophilic aromatic substitution to the same extent as an acetyl protecting group. Thus, bromination of 1‑methyl‑1H‑pyrrole‑2‑carbaldehyde with N‑bromosuccinimide in THF at 0°C cleanly furnishes the 4‑bromo derivative in 89% isolated yield, whereas the parent aldehyde requires careful temperature control to avoid dibromination. These reactivity contrasts define the product’s positioning among heterocyclic aldehydes: it occupies a reactivity window where the aldehyde group retains sufficient electrophilicity for standard carbonyl chemistry while the methyl‑shielded nitrogen mitigates unwanted side reactions that plague the unsubstituted analogue.