2,4-Diamino-6-chloropyrimidine (CAS 156-83-2): Properties and Uses

What Is 2,4-Diamino-6-chloropyrimidine?

2,4-Diamino-6-chloropyrimidine is a chlorinated heterocyclic building block that occupies a well-defined position in the pyrimidine branch of fine-chemical manufacturing. It is a small, highly functionalised molecule: a six-membered aromatic ring carrying two nitrogen atoms in the 1- and 3-positions, two amino substituents at the 2- and 4-positions, and a single chlorine atom at the 6-position. Because three of the four ring carbon atoms are substituted, the compound behaves less like a bulk solvent and more like a purpose-built intermediate that is converted into downstream actives and specialty chemicals.

Identity data used for documentation, customs declarations, and analytical method set-up are summarised below. Buyers should always confirm these values against the certificate of analysis of the specific lot, because different synthetic routes can leave different impurity profiles even when the assay value is identical.

The product is normally supplied as a dry, free-flowing solid rather than as a solution. That matters for logistics: solids must be protected from moisture pickup, and the package headspace should be kept free of acidic vapours, because the amino groups are basic and the ring system is sensitive to prolonged contact with strong acids.

Molecular Structure and the Reactivity of the C6 Chlorine

The chemistry of this intermediate is dominated by two structural features. The first is the pair of electron-donating amino groups at C2 and C4. They push electron density into the ring, raise electron density at the remaining positions, and provide two sites for acylation, alkylation, salt formation and hydrogen bonding. The second is the chlorine at C6, which sits next to ring nitrogen and is therefore activated towards nucleophilic aromatic substitution. In practical terms, the C6 chlorine can be displaced by a range of nucleophiles - amines, alkoxides, phenoxides, thiolates and other heteroatom donors - which is exactly why the material is valued as a scaffold rather than as an end product.

Two further consequences follow from the same structure. Amino-chloropyrimidines tend to be crystalline, high-melting solids with limited solubility in water and better solubility in polar aprotic solvents such as N,N-dimethylformamide or dimethyl sulfoxide. And because the 2- and 4-amino groups can interact with the ring nitrogen lone pairs, the ring can exist in tautomeric forms; this is relevant when interpreting nuclear magnetic resonance spectra and when designing purification by recrystallisation.

For buyers, the practical implication is that specification limits should be written in terms of the exact substitution pattern, not just the parent name. A lot that is 99% pure by area percentage can still contain a positional isomer or a dechlorinated analogue that will change the behaviour of the downstream reaction.

Typical Specification and Quality Control

Commercial material is usually offered in two broadly described grades: a technical or industrial grade intended for agrochemical and general organic synthesis, and a higher-purity grade for pharmaceutical and veterinary applications where the impurity profile must be documented in more detail. The table below reflects the parameters that are normally agreed between producer and buyer; the limits are representative of common commercial practice rather than a single mandatory standard, so the purchase contract governs.

Sampling should follow a written plan - GB/T 6678 for chemical products provides a workable framework for drawing representative samples from drums or fibre cartons. Where the material is used in a regulated pharmaceutical process, the analytical methods themselves are usually prepared and standardised in line with GB/T 601 for volumetric solutions and validated for the intended use. Moisture is measured by the Karl Fischer procedure of GB/T 6283, and this value should be read together with the assay: a lot with a high water content will give a lower corrected assay, and for hygroscopic aminopyrimidines this is one of the most common causes of a rejected shipment.

Because the compound is a solid powder, particle size and bulk density can also influence how easily it dissolves or disperses in the reaction medium. Neither parameter is universally specified, but a buyer running a fixed-volume reactor will often add a sieve fraction or an in-house dissolution test to the purchase specification.

Manufacturing Route and Industrial Positioning

Commercially, 2,4-diamino-6-chloropyrimidine is produced by assembling the pyrimidine ring from small aliphatic building blocks and then installing the chlorine at the last stage. The classical sequence starts with a condensation between guanidine and an ester of cyanoacetic acid; this builds a 2,4-diaminopyrimidine skeleton bearing an oxygen-containing substituent at C6. The resulting hydroxy or alkoxy intermediate is then converted with a chlorinating agent based on phosphorus oxychloride or phosphorus pentachloride to place chlorine at C6, and the crude product is isolated by quenching, filtration and purification (recrystallisation or slurrying). Detailed process parameters - molar ratios, temperature profiles, addition order and scale-up logic - are held by each producer and are outside the scope of this article; what matters commercially is that the chlorination step is the quality-critical operation, because it controls both the conversion and the level of over- or under-chlorinated by-products.

The industrial positioning of the molecule is therefore straightforward: it is an upstream, multi-purpose heterocyclic intermediate whose value depends on the reliability of two things - consistent substitution pattern and a low, well-characterised impurity profile. Producers that can hold a tight related-substances limit and supply a full analytical package generally serve the pharmaceutical and agrochemical segments; producers that supply on assay alone typically serve general organic synthesis.

Applications Across the Value Chain

The dominant use of this intermediate is as a scaffold in the synthesis of biologically active heterocycles. Two families of downstream chemistry stand out.

Agrochemical intermediates. Chloropyrimidine building blocks of this class are used in the manufacture of crop-protection actives, including organophosphate insecticide chemistry. Chlorpyrifos (CAS 2921-88-2), the well-known organophosphorus insecticide and acaricide, belongs to this wider segment of chlorinated heterocycle-derived crop-protection chemistry, and the substituted-pyrimidine route is one of the industrial pathways discussed for this family of products. The material is handled as an advanced intermediate: it is not applied to crops itself but is consumed inside the manufacturing chain of the finished active substance.

Pharmaceutical and veterinary scaffolds. The 2,4-diaminopyrimidine core is the pharmacophore of the antifolate class of medicines. Widely used antibacterial and antiprotozoal agents such as trimethoprim (CAS 738-70-5) and pyrimethamine (CAS 58-14-0) both contain a 2,4-diaminopyrimidine nucleus, and chlorinated versions of that nucleus are useful because the C6 chlorine can be replaced selectively by an aryl or alkylamino group in the final coupling step. The same reasoning applies to a range of veterinary anthelmintics and kinase-directed research compounds, where a defined substitution pattern at C6 is required for activity.

Dyes and specialty chemicals. Aminopyrimidines condense readily with carbonyl compounds and diazonium salts and are therefore used in the synthesis of colourants, dye intermediates and azo components, as well as in the preparation of ligands, agrochemical safeners and other fine chemicals. In these applications the buyer usually cares more about reactivity and freedom from tarry by-products than about pharmaceutical-grade documentation.

The common thread is that the same physical lot can serve several downstream industries. That is why the specification conversation, not the product description, determines which grade is actually needed.

Handling, Storage and Transport

As a fine organic powder, the material should be handled with engineering controls that keep airborne dust below occupational exposure limits: local exhaust ventilation at the point of charging, closed transfer where possible, and prompt cleaning of spills rather than dry sweeping, which re-suspends dust. Personal protective equipment for routine handling includes chemical safety goggles, nitrile or equivalent gloves, and a lab coat or coverall; a dust mask or respirator is appropriate where ventilation is not adequate.

Storage recommendations follow from the chemistry. Keep the product in well-closed containers in a cool, dry, ventilated area, protected from direct sunlight and from moisture. Segregate from strong acids and acid vapours, strong oxidising agents and strong bases, all of which can either protonate the amino groups, degrade the ring, or liberate heat. Drums and fibre cartons should be stacked according to the manufacturer's loading limits, and shipping marks should follow the conventional pictograms for packaging, handling and storage described in GB/T 191.

On hazard communication, amino-chloropyrimidine solids of this type should be treated as materials that may cause irritation of the eyes, skin and respiratory tract, and as harmful if swallowed. Safety data sheets are prepared in the sixteen-section format of ISO 11014, and the classification, precautionary statements and transport information actually applicable to a given grade are those stated in the current SDS issued with the shipment. Anyone receiving an unfamiliar lot should read that document before opening the container rather than relying on generic descriptions.

Procurement and Incoming Inspection

Incoming inspection of an intermediate such as this is best built around a short, decisive checklist. Confirm identity first - the infrared spectrum or the chromatographic retention time against a retained reference; a mass confirmation removes any doubt about the positional isomer. Then read the assay and the related-substances table together: a high main peak with an unusually large unspecified impurity usually indicates a different chlorination selectivity, which will show up as a changing yield downstream. Check water content by Karl Fischer and reconcile it with the assay. Finally, verify packaging integrity, batch number, manufacturing date and the supplier's retained sample arrangement, so that traceability is maintained if a deviation appears later in the process.

For development work, ask for a representative lot rather than a specially prepared sample. The purpose of a qualification batch is to prove that routine production material behaves reproducibly in your process, and a hand-picked sample defeats that purpose. Where the material will be used in a regulated application, request the impurity profile and, where applicable, the residual solvent data, and agree in advance on the analytical methods that will be used to settle any dispute.

Common Misconceptions

Several misunderstandings appear regularly in commercial descriptions of this product and are worth correcting before a specification is written.

The first is the idea that chlorpyrifos is a human antihypertensive medicine. It is not. Chlorpyrifos is an organophosphate insecticide and acaricide used in crop protection; any text that describes it as a blood-pressure medicine has confused it with an unrelated pharmaceutical. Buyers should treat such product pages as unreliable for technical data.

The second is the assumption that 2,4-diamino-6-chloropyrimidine and 2,4-diaminopyrimidine are interchangeable. They are different molecules with different molecular weights and different reactivity: the chlorinated compound offers a site for nucleophilic substitution that the unchlorinated parent does not. They must be specified and purchased as separate items.

The third is treating a single assay figure as a complete specification. For a reactive scaffold, the distribution of impurities is often more important than the headline purity number, because by-products derived from the same chlorination chemistry can compete in the downstream step and are difficult to remove once the reaction is complete.

Finally, the claim that one particular route is "the" synthetic route should be treated with caution. Multiple routes to the same ring system exist, and the choice depends on availability of raw materials, effluent load and the impurity profile the customer accepts. What can be compared objectively between suppliers is the certificate of analysis, the analytical methods behind it and the batch-to-batch consistency.

FAQ

Q: What is the CAS number, molecular formula and molecular weight of 2,4-diamino-6-chloropyrimidine?
It is CAS 156-83-2, with the molecular formula C4H5ClN4 and a molecular weight of about 144.56 g/mol. It is a white to off-white crystalline powder that is also listed as 6-chloro-2,4-diaminopyrimidine or 6-chloropyrimidine-2,4-diamine.

Q: Why is the chlorine atom at the 6-position chemically important?
The chlorine sits on a ring carbon adjacent to ring nitrogen, which activates it towards nucleophilic aromatic substitution. Amines, alkoxides, phenoxides and thiolates can displace it, so the molecule acts as a versatile scaffold for building substituted pyrimidines rather than as a finished chemical.

Q: Is 2,4-diamino-6-chloropyrimidine itself applied to crops?
No. It is an advanced intermediate consumed inside manufacturing chains. Crop-protection products based on chlorinated heterocycles, including organophosphate insecticides such as chlorpyrifos (CAS 2921-88-2), are manufactured downstream of intermediates of this type.

Q: Which quality parameters should be written into the purchase specification?
Assay by HPLC with an external standard, related substances with single and total limits, water content by Karl Fischer titration per GB/T 6283, residue on ignition, heavy metals and appearance. For regulated applications, add residual solvent data by headspace GC and agree the dispute-settlement methods in advance.

Q: How should the material be stored and transported?
Keep it in well-closed containers in a cool, dry, ventilated place away from moisture, strong acids, strong bases and oxidising agents. Packaging marks follow GB/T 191, and hazard classification, precautionary statements and transport data are those given in the current safety data sheet prepared in the ISO 11014 format.

Q: How can a buyer distinguish this compound from the unchlorinated 2,4-diaminopyrimidine?
By molecular weight and by the presence of the reactive C6 chlorine. Practically, confirm identity with infrared spectroscopy, chromatographic retention against a reference standard and a mass confirmation, and specify the two materials as separate purchase items with separate specifications.