Pyrrolidine CAS 123-75-1: Properties, Grades and Industrial Use

Pyrrolidine at a Glance: Identity, Nomenclature and Role in the Fine Chemical Chain

Pyrrolidine is the fully saturated five-membered nitrogen heterocycle with the molecular formula C4H9N and CAS number 123-75-1. Because one hydrogen remains on the ring nitrogen, the compound behaves as a secondary amine, and its ring sits between azetidine and piperidine in size. That intermediate geometry produces a distinctive combination of high basicity, complete water miscibility and a boiling point that allows straightforward atmospheric distillation. Technical and commercial documents also refer to the material as tetrahydropyrrole, azolidine, azacyclopentane and perhydropyrrole. Because several of those names are used loosely in the market, a sound purchase specification is written against the CAS number first and the chemical name second, so that a delivery cannot be substituted with a related heterocycle such as pyrrole, 2-pyrrolidinone or N-methylpyrrolidine.

Under ambient conditions pyrrolidine is a colourless, low-viscosity liquid with the pungent, ammoniacal odour that characterises short-chain aliphatic amines. It mixes in all proportions with water, ethanol, diethyl ether, chloroform and most polar organic solvents. That solubility profile separates it clearly from the longer-chain alkylamines and from aromatic amines, and it is the property most frequently exploited during work-up and extraction in downstream synthesis. Exposed to air and light the liquid darkens towards pale yellow, and in the presence of atmospheric carbon dioxide part of the free base is converted into the corresponding carbamate. Both effects are manageable with nitrogen blanketing, opaque packaging and short storage cycles.

In value-chain terms pyrrolidine is an intermediate rather than a finished article. Essentially the whole volume is consumed through reaction: N-alkylation, N-acylation, reductive amination, Mannich-type condensation and ring functionalisation are the standard transformation families. The pyrrolidine ring, or the aminobutyl fragment derived from it, then appears in the finished active ingredient, agrochemical or specialty additive. This is why commercial contracts for the material are normally written around assay, water content, colour and trace amine profile rather than around application performance figures.

Physicochemical Property Profile and the Basis of Specification

The table below collects the parameters that appear most often on certificates of analysis and on technical data sheets. The values are typical industry ranges for commercial liquid product; they are not a substitute for a batch certificate, and acceptance limits should always be agreed against a validated analytical method.

Two entries in that table deserve emphasis in a purchasing context. The first is basicity: the conjugate acid of pyrrolidine has a pKa near 11.3 in water, so the free base is a genuinely strong amine and reacts vigorously with acids, phenols and acidic impurities. Wetted surfaces, gaskets and instrument wetted parts must therefore be selected for alkaline service. The second is volatility: with a boiling point in the high eighties Celsius and a flash point near ambient temperature, pyrrolidine generates flammable vapour at ordinary working temperatures and its vapour pressure rises quickly with modest heating. Both features are reflected in the hazard classification and in the engineering controls described later in this article.

Viscosity is low and density is below that of water, so phase behaviour in aqueous extraction is straightforward: neat amine floats. Refractive index is a convenient and inexpensive identity check during incoming inspection, and a value that deviates appreciably from the agreed range is a strong signal of either water pick-up or contamination with a heavier amine such as piperidine.

Commercial Grades and Purity Specifications

The market is served in three practical quality tiers. Standard industrial grade is normally specified at 99.0 percent minimum by gas chromatography, with water at or below about 0.3 percent by Karl Fischer titration and colour not darker than an agreed platinum-cobalt standard. Pharmaceutical or fine chemical grade tightens assay to 99.5 percent or higher, reduces water to the 0.1 to 0.2 percent range, controls individual unspecified impurities and imposes limits on residual solvents where the material feeds a registered manufacturing process. A third, smaller tier is anhydrous or low-water material packed under nitrogen for moisture-sensitive chemistry.

The impurities that matter are predictable from the manufacturing route: unreacted ammonia and low-boiling amines, oxidation products related to pyrrole and pyrrolidinone, higher homologues such as piperidine, and water. In pharmaceutical supply the impurity discussion is normally framed around the requirements of the applicable pharmacopoeial monograph and general chapter, together with the residual solvent expectations of the ICH quality guidelines, rather than around an arbitrary internal number, and the analytical procedure used to demonstrate compliance must be validated for specificity and limit of quantitation.

Packaging follows the same logic. Research and pilot quantities are supplied in glass or fluoropolymer-lined bottles; bulk moves in steel drums, intermediate bulk containers or road tankers, all under a nitrogen pad. Because the material is hygroscopic and sensitive to carbon dioxide, an opened container should be re-blanketed before resealing, and drums should not be kept where they are repeatedly warmed and cooled, since thermal cycling draws moist air in through imperfect closures and accelerates discolouration.

Industrial Manufacture: Route Families and Process Considerations

Commercial pyrrolidine is produced by a small number of well-established route families, and the choice between them is driven by feedstock availability and by the need to minimise separation duty rather than by any single dominant technology.

The most common industrial family is reductive amination of a four-carbon oxygenated feedstock. 1,4-Butanediol, gamma-butyrolactone or tetrahydrofuran can be reacted with ammonia over a metal or metal-oxide catalyst at elevated temperature in the vapour or liquid phase, giving pyrrolidine with water as the principal co-product. Because the same chemistry carried out in the presence of an alcohol or a ketone delivers the corresponding N-alkylpyrrolidines, plants configured for this family often run a small product slate and switch between the free base and its N-substituted derivatives.

A second family starts from nitrogen-containing four-carbon building blocks. Succinonitrile or a succinate derivative can be cyclised and then hydrogenated, and dicarboxylic acid derivatives can be routed through the corresponding diamine before ring closure. A third, more specialised family hydrogenates pyrrole or a pyrrole derivative catalytically; this is attractive where pyrrole is already available economically, but it demands careful control of partial hydrogenation products.

Whatever the route, the downstream train is dominated by removal of water and ammonia, recovery of unreacted feedstock, and fractional distillation to separate pyrrolidine from piperidine, pyrrolidinone and higher amines. Columns are therefore built in corrosion-resistant material and product is drawn under nitrogen to prevent discolouration. A purchaser assessing a supplier should look for evidence of a validated distillation control strategy and of routine colour and trace amine monitoring, because those two factors are the practical indicators of a stable, reproducible product.

Analytical Methods and Acceptance Testing

Assay is almost always determined by gas chromatography with flame ionisation detection, using an external standard or an internal standard calibration on a capillary column suited to volatile amines. Because the analyte is strongly basic, column selection and deactivation matter greatly; a poorly deactivated column gives tailing peaks and biased results. A companion Karl Fischer titration, performed according to the general method for water determination such as GB/T 6283, provides the moisture value. Where a wet chemical cross-check is preferred, non-aqueous titration of the base against a standard acid solution prepared and standardised in accordance with GB/T 601 gives an independent assay confirmation.

Refractive index and relative density at the stated temperature serve as physical identity tests, and colour is assessed against a platinum-cobalt scale with appearance judged visually against a reference. For fine chemical and pharmaceutical grades, residual solvents are screened by headspace gas chromatography and the impurity profile is evaluated against the limits agreed in the specification, using a validated stability-indicating method where the material is held for extended periods.

Sampling should follow a written plan consistent with the general provisions of GB/T 6678 for chemical products, with the sample drawn under an inert blanket and transferred promptly to a sealed container to limit carbon dioxide uptake. Assay and moisture results obtained from a sample that has stood open to air for an extended period should be treated with caution, because both drift quickly once the seal is broken. Packaging marks for the drums and intermediate bulk containers should follow GB/T 191, and the safety data sheet should be issued in the sixteen-section ISO 11014 layout.

Pharmaceutical and Fine Chemical Applications

The dominant use of pyrrolidine is as a pharmaceutical intermediate. The ring is a privileged scaffold in medicinal chemistry: it is small, strongly basic, metabolically well-behaved in many contexts, and it can be N-alkylated or N-acylated to tune the basicity and lipophilicity of a candidate molecule. The free base is used directly in amide and sulfonamide formation, in reductive amination to build N-alkylpyrrolidines, and as a base to scavenge acid released during coupling reactions.

Downstream products that incorporate the pyrrolidine ring or its derivatives span a wide therapeutic range, from cardiovascular agents and central nervous system medicines that rely on a basic amine centre for receptor binding, through to the large family of N-substituted pyrrolidine building blocks used in modern synthetic routes to antiviral and oncology candidates. The established pharmacology of those finished drugs is a separate matter from the function of pyrrolidine itself, which is purely that of a reactive amine feedstock supplied to a registered or developmental manufacturing process.

Outside drug manufacturing, pyrrolidine is used to prepare pyrrolidine dithiocarbamates, agrochemical and rubber additives, and functionalised heterocycles for electronic and photochemical materials. It is also a common reagent in the synthesis of N-alkylpyrrolidinium salts and ionic liquids, where the quaternised nitrogen contributes thermal stability and a wide electrochemical window.

Because pyrrolidine reacts readily with carbon dioxide and other acidic gases, cyclic amines of this family are also studied and applied as absorbents or absorbent components for acid gas removal in gas treating, with derivatives such as N-methylpyrrolidine evaluated in the same role. That application depends on reversible carbamate formation and on the regeneration behaviour of the amine rather than on the compound as an end product.

Agrochemical, Rubber, Coating and Other Industrial Applications

In crop protection chemistry the pyrrolidine ring appears in fungicidal and insecticidal structures, and the free base is used both as an intermediate and as an acid scavenger. As in pharmaceuticals, the platform value comes from the basic nitrogen: it improves the water solubility of a salt form, modulates the octanol-water partition coefficient of the active substance, and provides a convenient handle for salt and co-crystal formation. Only the commercial positioning of the material is relevant here; process parameters for any specific active substance are the responsibility of the registered manufacturer.

In rubber and polymer processing, pyrrolidine derivatives serve as accelerators, coupling agents and stabilisers, and the parent amine is used to prepare functional monomers. In coatings, adhesives and printing inks the material is used to synthesise amine-functional additives, curing agents and pigment dispersants, where the strong base neutralises acidic groups and improves pigment wetting and dispersion stability.

Other established uses include textile and paper treatment auxiliaries, photographic and photosensitive chemicals, and templating agents in the hydrothermal synthesis of certain zeolite frameworks, where a cyclic amine directs the pore structure during crystallisation. Animal nutrition is a further outlet, for instance in the manufacture of rumen-protected amino acid products, with the amine acting as a building block rather than as a nutritional ingredient in itself.

In every one of those applications the requirement is the same: a low-water, low-colour intermediate with a consistent trace amine profile. By-products carried into a reaction appear as colour bodies, odour problems or catalyst poisons in the finished material, so the specification discipline described above translates directly into yield and quality at the customer site.

Safe Handling, Storage, Packaging and Regulatory Status

Pyrrolidine is a flammable, strongly alkaline, corrosive liquid. It is classified as a flammable liquid and as a substance that causes severe skin burns and serious eye damage, and it is harmful if swallowed or inhaled. The vapour is heavier than air and can travel to a remote ignition source, so vapour management, rather than liquid containment alone, is the central safety issue in any facility that stores or handles the material. Work should take place in a fume hood or under local exhaust ventilation, with chemical splash goggles, a face shield where splashing is credible, chemically resistant gloves and arm protection; bulk operations should use closed transfer systems with static bonding and grounding.

First aid follows standard practice for corrosive amines: immediate and prolonged flushing of affected skin or eyes with water while removing contaminated clothing, removal to fresh air for inhalation exposure, and medical attention in every significant case. Eyewash stations and safety showers should be within unobstructed reach of the operating area, and spill kits should contain an acid-neutralising absorbent rather than a combustible material.

Storage is in a cool, dry, well-ventilated area away from acids, oxidising agents, ignition sources and direct sunlight. Tanks and drums should be nitrogen padded and the material kept below its flash point at every point in the supply chain, including during transfer and sampling. For transport, the substance is handled as a flammable liquid with subsidiary corrosive properties under the applicable model regulations for road, rail and sea freight, and the segregation, packaging and labelling requirements for such a dual-hazard material must be checked against the current modal regulations before each shipment.

Regulatory obligations vary by jurisdiction. In the European Union, pyrrolidine is registered under REACH and supplied with a classification and labelling consistent with the CLP Regulation. In China, classification follows the GB 30000 series of standards that implement the GHS. In the United States the substance is listed on the TSCA inventory, and where it is used in food-contact or cosmetic contexts the permitted use is governed by the relevant positive lists rather than by its general industrial classification. Downstream users should confirm inventory status in each destination country before placing orders.

Frequently Asked Questions

Q: What is the CAS number and molecular formula of pyrrolidine?
Pyrrolidine is CAS 123-75-1 with the molecular formula C4H9N and a molecular weight of 71.12 g/mol. It is a saturated secondary amine, also called tetrahydropyrrole or azolidine.

Q: What does a typical certificate of analysis report for pyrrolidine?
A standard certificate states assay by gas chromatography, water content by Karl Fischer titration, colour against a platinum-cobalt reference, refractive index and relative density. Fine chemical grades add residual solvent data and an individual impurity profile with agreed limits.

Q: Why does pyrrolidine turn yellow during storage?
Two mechanisms dominate. Slow air oxidation generates coloured nitrogenous by-products, and reaction with atmospheric carbon dioxide converts part of the free base into carbamate salts. Nitrogen blanketing, opaque packaging and short storage cycles keep colour within specification.

Q: How should pyrrolidine be stored and handled?
Store in a cool, dry, ventilated area away from acids, oxidising agents and ignition sources, with tanks under a nitrogen pad. Transfer under closed conditions with bonding and grounding, and use splash goggles, face shield, chemically resistant gloves and local exhaust ventilation.

Q: What is the difference between pyrrolidine and pyrrole?
Pyrrole is the unsaturated, aromatic five-membered heterocycle with the formula C4H5N and CAS number 109-97-7, and its lone pair participates in the aromatic system, so it is only weakly basic. Pyrrolidine is the fully hydrogenated analogue, C4H9N, CAS 123-75-1, and is a strongly basic secondary amine.

Q: Is pyrrolidine miscible with water?
Yes. Pyrrolidine is miscible with water in all proportions and also dissolves freely in ethanol, diethyl ether and chloroform. This complete water miscibility distinguishes it from the longer-chain alkylamines and simplifies aqueous extraction steps in downstream processing.