4-Methylpyridine CAS 108-89-4: Properties and Isonicotinic Acid Uses

What Is 4-Methylpyridine (CAS 108-89-4)?

4-Methylpyridine, also written as 4-picoline or gamma-picoline, is a heterocyclic nitrogen base in which a methyl substituent sits on the ring carbon para to the pyridine nitrogen. Its molecular formula is C6H7N and its molar mass is 93.13 g/mol, which makes it one of the simplest alkylpyridines still traded in tonne-scale drum and ISO tank quantities. The molecule carries two very different reactive sites: the ring nitrogen behaves as a weak base that is readily protonated by mineral acids, while the methyl side chain is activated towards oxidation and condensation because the electron-poor ring stabilises the carbanion and radical intermediates formed there. That second feature is the commercial reason the compound matters, because selective oxidation of the methyl group gives isonicotinic acid (pyridine-4-carboxylic acid), the entry point to a broad family of 4-substituted pyridine derivatives used in pharmaceuticals, crop protection and specialty chemistry.

For a purchasing laboratory, the identity of the material is defined by four items rather than one: the para substitution pattern, the assay expressed on a gas chromatography basis, the isomer distribution among 2-, 3- and 4-methylpyridine, and the residual water content. A specification that quotes only a purity figure is incomplete, because 2-methylpyridine and 3-methylpyridine are close-boiling companions of the 4-isomer. A drift of a few tenths of a percent in the isomer profile can shift downstream reaction selectivity without moving the headline assay number at all, so the certificate of analysis should always carry an isomer fingerprint in addition to the total alkylpyridine content.

Physical and Chemical Properties of 4-Picoline

The data below describe typical commercial material supplied at 99% minimum assay. These are the parameters normally reported on a technical data sheet and re-verified by the receiving laboratory on first article inspection. Values quoted as approximate are industry-typical figures rather than absolute constants; the exact number on a delivered lot should always be taken from the batch certificate.

Two practical consequences follow from this data set. First, complete miscibility with water means the material cannot be dried or separated by simple phase splitting; salting-out or extractive techniques are needed if an aqueous work-up is involved. Second, the flash point near 40 °C places the liquid close to the boundary between Packing Group II and Packing Group III for surface transport, so classification decisions must be taken from the measured closed-cup value of the actual lot rather than from a generic product family assumption.

As a weak base, 4-methylpyridine is easily converted into water-soluble 4-methylpyridinium salts by hydrochloric, sulfuric or carboxylic acids. This protonation behaviour is exploited during purification, in which the base is extracted into aqueous acid and released again by neutralisation, and it also underpins the preparation of acid-addition intermediates. The vapour is heavier than air and can travel along the floor towards an ignition source, which is why local exhaust ventilation and bonding of transfer equipment are treated as design requirements rather than optional good practice.

Commercial Grades and Specification Limits

Suppliers normally offer a single technical grade defined by a minimum assay, with tightened isomer and colour limits available on request for pharmaceutical and electronic applications. The limits below reflect the values that appear on typical specification sheets and certificates of analysis; they are stated as industry-typical ranges rather than as guarantees of any individual supplier.

Because the dominant downstream use is oxidation of the methyl group, an elevated water content is not merely a cosmetic defect. Water consumes oxidant, shifts the mass balance of the reaction and can promote ring degradation products that are difficult to remove from the isolated acid. Buyers running oxidation chemistry therefore tend to specify the lower end of the moisture window and to request isomer data explicitly.

Manufacturing Routes and Isomer Separation

Two industrial routes dominate supply. The first is recovery from the pyridine base fraction of coal tar, in which 4-methylpyridine is separated from pyridine and from the 2- and 3-isomers by a sequence of fractional distillation steps, sometimes assisted by azeotropic or extractive techniques to overcome the small relative volatility differences between the isomers. The second is the vapour-phase condensation of acetaldehyde with ammonia over an acid or metal-oxide catalyst, which produces a mixed picoline stream that must then be separated. In both cases isomer separation is the technically critical step, because the three methylpyridines differ in boiling point by only a few degrees and because each isomer has a distinct downstream reactivity profile.

From a supply-chain perspective, the consequence is that 4-methylpyridine availability is coupled to the wider alkylpyridine market rather than to a single dedicated plant. Purchasers assessing continuity of supply should therefore confirm the origin of the material, whether it is coal-tar derived or fully synthetic, and whether the producer controls the isomer separation step in house. Material from different origins can be chemically equivalent yet differ slightly in trace impurity fingerprints, which matters for regulated pharmaceutical intermediates where an established impurity profile has to be maintained.

Quality Control and Analytical Methods

Routine quality control for 4-methylpyridine rests on a small number of well-established techniques. Purity and isomer distribution are determined by capillary gas chromatography with flame ionisation detection against an external standard, with the identity of each peak confirmed either by retention time against certified reference materials or, where a laboratory maintains the capability, by mass spectrometry. Water is determined by Karl Fischer titration following the principles of GB/T 6283, which remains the reference approach for trace moisture in organic liquids. Colour is read against the platinum-cobalt scale, and non-volatile residue is obtained by controlled evaporation.

Sampling should follow the general principles of GB/T 6678 for chemical products, using clean, dry glass or stainless steel sample containers that are completely filled and tightly closed to limit moisture pickup and headspace oxidation. Each shipment should be accompanied by a certificate of analysis covering assay, isomer distribution, moisture, colour and residue, together with a safety data sheet prepared in accordance with ISO 11014 and the GHS classification applicable in the destination market. Laboratories receiving material for the first time are advised to retain a sealed reference sample under nitrogen so that later lots can be compared against a known baseline.

Applications Across Industries

The largest single use is as a pharmaceutical intermediate. Oxidation of the methyl group yields isonicotinic acid, which is converted to isonicotinohydrazide and related derivatives that form the backbone of several established anti-tuberculosis and pyridine-based active ingredients. Because these downstream products are regulated, the trace impurity profile and the isomer purity of the starting 4-methylpyridine feed have a direct influence on the impurity file of the finished active substance, and purchasers in this segment typically operate under formal supplier qualification with periodic re-audit.

In agrochemicals the 4-pyridyl building block appears in a range of crop-protection actives, where the nitrogen heterocycle contributes to target binding and to metabolic stability in the field. In the dye and pigment sector, 4-methylpyridine and its quaternary ammonium salts serve as intermediates for cationic and heterocyclic colourants. In rubber and polymer compounding, derivatives of the molecule act as accelerator intermediates, coupling agents and stabiliser precursors, and quaternised forms are used where a permanent cationic charge is required. Smaller volumes are consumed in the preparation of specialty ligands, organocatalysts and photographic or electronic intermediates, all of which again depend on a clean isomer profile.

Packaging, Storage and Logistics

Standard packaging is a 200 litre steel drum or a lined drum with a net fill in the region of 180 to 190 kilograms, with ISO tank containers used for larger volumes. Because 4-methylpyridine attacks some common metals in long-term contact and can discolour through iron pickup, stainless steel or lined carbon steel is preferred for tanks, pumps and transfer lines, and mild steel is generally avoided for extended storage. Drums should be fitted with gaskets resistant to aromatic nitrogen bases and should be blanketed with dry nitrogen wherever the material is held for long periods.

Warehousing should be cool, dry and well ventilated, with the material segregated from strong acids, oxidising agents, heat sources and ignition risks. Packages must be grounded and bonded during transfer, and pumps rather than pouring should be used wherever feasible to limit static generation and vapour release. Packaging marks and handling pictograms should follow the general requirements of GB/T 191 for pictorial marking of packages, in combination with the GHS labels required by the destination market.

Safety, GHS Classification and Handling

4-Methylpyridine is classified as a flammable liquid and is harmful if swallowed or in contact with the skin. It causes serious eye irritation, is irritating to the respiratory tract, and overexposure can affect the central nervous system, with headache, dizziness and nausea as the early indicators. In GHS terms the substance falls into the flammable liquid, acute toxicity, skin irritation, serious eye irritation and specific target organ toxicity categories applicable to the classification thresholds of the destination jurisdiction, and the label and safety data sheet must reflect the version of the classification actually registered there.

Engineering controls come first in any handling programme: closed transfer systems, local exhaust ventilation at filling and sampling points, bonding and grounding of drums and tanks, inert gas blanketing, and continuous or periodic monitoring where vapour could accumulate. Personal protective equipment normally comprises chemical-resistant gloves selected for aromatic nitrogen bases, safety goggles with a face shield for operations with splash potential, flame-retardant clothing and, in confined spaces or during emergency response, suitable respiratory protection. Smoking and open flames must be excluded from the storage and handling area, and spill response should use absorbent material that can be swept into a closed container, with the area ventilated before re-entry.

Purchasing and Acceptance Checklist

A practical acceptance routine begins with paperwork review: confirm that the certificate of analysis reports assay, isomer distribution, moisture, colour and residue; that the safety data sheet matches the destination regulatory framework; and that the packaging type and net weight match the purchase order. Physical inspection then covers drum condition, seal integrity, labelling and the absence of free liquid on the outside of packages. Laboratory verification normally confirms identity by refractive index or gas chromatography retention against a reference, checks the moisture content by Karl Fischer titration, and compares the isomer ratio with the contracted specification.

Three misconceptions recur in procurement discussions. The first is that a higher headline assay automatically means better quality, when in this product the isomer profile and moisture level often matter more to the downstream reaction than the last tenth of a percent of total purity. The second is that 4-methylpyridine can be treated as interchangeable with pyridine or with a mixed picoline cut; the reactivity of the side chain is what the downstream chemistry relies on, and mixed material will not behave the same way. The third is that mild steel is acceptable for storage because the liquid appears not to react with it; over months of contact, iron pickup and colour development are common and can disqualify the material for colour-sensitive applications.

FAQ

Q: What is the CAS number and molecular formula of 4-methylpyridine?
4-Methylpyridine is CAS 108-89-4, with the molecular formula C6H7N and a molar mass of 93.13 g/mol. It is also known as 4-picoline and gamma-picoline.

Q: How does 4-methylpyridine differ from pyridine?
It carries a methyl group at the 4-position of the pyridine ring, which makes it a slightly stronger base than pyridine and far more reactive at the side chain. That side-chain reactivity is what allows selective oxidation to isonicotinic acid.

Q: Is 4-methylpyridine miscible with water?
Yes. It is miscible with water in all proportions at ambient temperature and is equally soluble in ethanol, ether, acetone and aromatic solvents, so aqueous phase separation cannot be used to recover it from a water-containing stream.

Q: What purity is normally supplied?
Commercial grades are typically 99.0% minimum, with 99.5% available for pharmaceutical use, and certificates usually quote the isomer distribution, a moisture limit around 0.20% maximum and a colour figure on the Hazen scale.

Q: Why does moisture matter in this product?
Water consumes oxidant in the downstream oxidation step, disturbs the reaction mass balance and can encourage ring-degradation by-products that are difficult to remove, so regulated users tend to specify a low moisture window and verify it by Karl Fischer titration.

Q: How should 4-methylpyridine be stored and shipped?
Keep it in sealed, grounded containers under dry nitrogen in a cool, dry, well-ventilated area away from acids, oxidising agents and ignition sources. Stainless steel or lined carbon steel is preferred for tanks and lines, and it travels as a flammable liquid under the applicable Class 3 transport regulations.

Q: What analytical method is used to determine assay?
Capillary gas chromatography with flame ionisation detection against an external standard is the standard approach, and it also delivers the isomer distribution in the same run. Water is measured separately by Karl Fischer titration.