1-Butyl-3-methylimidazolium bromide (BMIM Br, CAS 85100-77-2) is the most widely used entry point into the imidazolium ionic liquid family. It is a white crystalline solid that melts in the low 70s, dissolves freely in water, and costs little enough to be used by the kilogram rather than the gram. Almost every laboratory that works with ionic liquids starts here, and a large share of the published work on “imidazolium ionic liquids” is work on this salt or its chloride analogue.
It is also one of the most frequently misdescribed products in the catalogue. Two questions come up again and again, and both have precise answers: is BMIM Br a base, and can it replace sodium hydroxide, and why does the molecular weight in a database not match the certificate. This guide answers both, then covers the chemistry, manufacture, specification and application.
1. The molecule
| Property | Value | Practical meaning |
|---|---|---|
| Chemical name | 1-Butyl-3-methyl-1H-imidazolium bromide | A quaternised imidazole ring carrying a butyl chain on one nitrogen and a methyl on the other |
| Formula / MW | C8H15BrN2 / 219.12 | Cation C8H15N2+ (139.22) + Br- (79.90) |
| Melting point | Reported around 70-73 deg C | Solid at ambient – it is warmed to melt, or dissolved in a co-solvent |
| Appearance | White crystal; colourless to pale yellow melt | Colour is the first visible sign of thermal history or residual starting material |
| Solubility | Miscible with water; soluble in methanol, ethanol, acetone, dichloromethane | Water-miscible means aqueous workup does not remove it – separation needs a different strategy |
| PubChem CID | 25153273 (see caution below) | The record at this CID does not describe the same molecular formula |
The molecular weight caution
Look up CAS 85100-77-2 in PubChem and the returned record gives C8H17BrN2, MW 221.14, described as a 2H-imidazole hydrobromide. The salt on this page is C8H15BrN2, MW 219.12 – two hydrogens lighter. The imidazolium cation resolves separately to CID 2734162 (C8H15N2+, 139.22), and 139.22 + 79.90 = 219.12 confirms the figure.
This matters because molecular weight is used to convert between mass and moles. A specification or a dosing calculation built on 221.14 is about 1% wrong. Verify ionic liquid molecular weights by adding the cation and the counter-ion, not by copying a database entry – the databases index neutral and protonated forms inconsistently for salts.
2. Is BMIM Br a base? Can it replace sodium hydroxide?
This question is asked often enough that it deserves a direct answer.
BMIM Br itself is a neutral salt, not a base. The imidazolium cation is a very weak acid (its C2 proton has a reported pKa in the low 20s), and bromide is the conjugate base of a strong acid. A solution of BMIM Br in water sits near neutrality. It does not neutralise acids, and it cannot substitute for sodium hydroxide in a stoichiometric sense.
What is true – and what the question is really about – is that imidazolium ionic liquids with basic anions are excellent basic media, and BMIM Br is their precursor. Swap the bromide for hydroxide, acetate, carbonate or imidazolate and the salt becomes genuinely basic:
| Ionic liquid | Anion | Basic? Do not confuse with BMIM Br |
|---|---|---|
| BMIM Br | Bromide | Neutral salt. A reaction medium and phase-transfer agent, not a base |
| BMIM OH | Hydroxide | Strongly basic; the anion is the base, and the cation is essentially a spectator |
| BMIM OAc | Acetate | Moderately basic; used in condensations where a milder base is wanted |
| BMIM imidazolate | Deprotonated imidazole | Basic and nucleophilic – a genuine organocatalyst rather than a solvent |
In aldol and Knoevenagel condensations reported with “BMIM Br”, the salt is almost always doing one of two jobs that are not base catalysis: it is acting as a polar, thermally stable reaction medium that stabilises the enolate intermediate, or it is acting as a phase-transfer catalyst carrying hydroxide or alkoxide from an aqueous phase into the organic phase. The base in those systems is sodium hydroxide or potassium carbonate, added separately.
A practical consequence for anyone trying to replace NaOH with BMIM Br: it will not work stoichiometrically. What works is a two-part system – a hydroxide or carbonate as the base, and BMIM Br as the phase-transfer agent or medium that lets a biphasic reaction proceed without a volatile organic solvent.
3. How it is made, and why the impurity profile looks the way it does
BMIM Br is made in one step: 1-methylimidazole is alkylated with 1-bromobutane. The reaction is exothermic, needs no catalyst, and proceeds in the melt or in a solvent such as acetonitrile, ethyl acetate or toluene under reflux:
1-methylimidazole + 1-bromobutane -> 1-butyl-3-methylimidazolium bromide
The crude product is a two-phase or slurry system containing the salt, unreacted starting materials and coloured by-products. Purification is where the quality is made, and it follows a standard sequence: partitioning between water and an organic solvent to remove the excess alkyl halide, treatment with activated carbon or alumina to strip colour, vacuum drying to the water specification, and for higher grades recrystallisation from ethyl acetate or acetonitrile.
Three impurities survive an incomplete purification, and each maps to a specification line:
| Impurity | Where it comes from | Specification line |
|---|---|---|
| 1-Methylimidazole (MIm) | Unreacted alkylation substrate; also a hydrolysis product if the salt is heated wet | Max 100 ppm |
| Water | The salt is hygroscopic and the workup is aqueous | Max 2000 ppm by Karl Fischer |
| Colour bodies | Thermal degradation and oxidation during drying | Appearance: white crystal |
Why residual MIm matters more than the assay
The assay figure for an ionic liquid is nearly always 99% or better, which leaves up to 1% unaccounted. In BMIM Br, that 1% could be almost entirely 1-methylimidazole and still pass the assay line. The difference between a 100 ppm and a 5000 ppm batch is not cosmetic:
- MIm is a much stronger base than the imidazolium cation. It will deprotonate acidic substrates and bind protons that the salt itself would not touch, changing the outcome of acid-sensitive reactions.
- MIm is a ligand. It coordinates transition metals, so in a metal-catalysed reaction it competes with the intended ligand and changes both rate and selectivity.
- MIm is volatile and has a distinct amine odour. A batch that smells of it will be noticed before it fails any instrument.
This is why a 100 ppm ceiling is a functional specification, not a marketing one, and why the two numbers – assay and MIm – must be read as a pair on any certificate.
4. Physical behaviour that surprises first-time users
Ionic liquids do not behave like molecular solvents, and three of their properties trip up people who are used to dichloromethane or THF:
Viscosity depends on water. Dry BMIM Br is noticeably viscous above its melting point; a few thousand ppm of water can cut the viscosity substantially. A reaction that “will not stir” is often a reaction whose solvent has been dried more than intended (or less than intended).
Water is not an inert impurity here. For electrochemistry the water content narrows the electrochemical window at both ends. For cellulose dissolution it changes the hydrogen-bond basicity that does the dissolving. For reactions run in the salt as a medium, it shifts the polarity. The 2000 ppm specification exists because 200 ppm and 2000 ppm are different solvents, not because water is “dirty”.
Extraction does not remove it. Because the salt is water-miscible, a standard aqueous workup does not separate product from ionic liquid – the ionic liquid goes into the water phase. Recovery is by evaporation, by precipitation with an anti-solvent, or by extraction of the product into a solvent that does not dissolve the salt. This is the single biggest practical difference from a conventional solvent and the one that most often makes an ionic liquid process uneconomic at scale.
5. Applications
| Sector | Use | What the salt is doing |
|---|---|---|
| Reaction medium | Condensations, substitutions, metal-catalysed couplings | Polar, non-volatile, thermally stable medium; enables solvent-free or high-temperature operation |
| Phase-transfer catalysis | Biphasic reactions with aqueous hydroxide or carbonate | Carries the anion into the organic phase; replaces a volatile PTC solvent |
| Biomass and cellulose | Dissolution and fractionation of cellulose and lignocellulose | Bromide is a strong hydrogen-bond acceptor and dissolves cellulose readily at moderate temperature |
| Electrolytes | Electrodeposition, batteries, supercapacitors | Wide electrochemical window and negligible vapour pressure, subject to the water spec |
| Gas separation | CO2 absorption and separation media | Physical and chemical absorption depending on the anion |
| Lubrication research | Base fluid and anti-wear additive studies | Thermal stability and low volatility at high contact temperature |
6. The anion family, and why bromide is the starting point
Bromide is chosen first because it is cheap, readily available, and the alkylation reaction to make it goes cleanly. It is not always the anion the final application wants. The common exchanges:
| Anion | Effect of the swap | Typical reason to move |
|---|---|---|
| Br- | The starting point | Cheapest, easy to make, water-miscible, good cellulose solvent |
| Cl- | Similar chemistry, slightly more hydrophilic, cheaper | Cost, and where a smaller anion is wanted |
| BF4-, PF6- | Hydrophobic; water-immiscible | Biphasic catalysis and extraction, where the ionic liquid must stay in the organic phase |
| OTf-, NTf2- | Hydrophobic, low viscosity, wide electrochemical window | Electrolytes; PF6- and NTf2- are the standard for battery work |
| OH-, OAc- | Basic | Where the basicity discussed in section 2 is the point |
The exchange is normally done by metathesis – the bromide is reacted with a sodium or potassium salt of the target anion in water or an alcohol, and the product is separated according to whether it is now water-miscible or not.
7. Buying guide
- Ask for the MIm figure, not just the assay. A certificate with 99% assay and no MIm line is not describing the product you need.
- Ask for water by Karl Fischer with the test date. The salt picks up water in storage; a figure measured at packing tells you what you received, not what is in the drum now.
- Confirm the molecular weight basis. 219.12, not 221.14 – and check that a quoted specification is internally consistent with it.
- Specify the halide. A certificate that says “halide” without saying bromide is ambiguous; ion chromatography gives the bromide and chloride separately.
- Decide on pack size against your drying capability. If you cannot vacuum-dry, buy smaller packs and keep them sealed.
- Ask whether the grade is recrystallised. Recrystallised material is white and low in MIm; non-recrystallised material is off-white and carries more of everything.
8. Safety, handling and storage
BMIM Br is not classified as a dangerous good for transport in normal pack sizes, and its toxicity profile – while less well established than that of common industrial solvents – is generally low for an organic salt of this type. The practical hazards are physical:
- Hot melt. It must be melted before use, and the melt is hot enough to cause contact burns.
- Hygroscopicity. Opened containers absorb water; store sealed and dry, and do not use a container that has been open in a humid laboratory for a reaction that needs 200 ppm water.
- Thermal degradation. The salt is thermally stable well above its melting point, but prolonged heating above 200 deg C darkens it and generates the same colour bodies that recrystallisation exists to remove.
Halide-containing ionic liquids are also worth handling with the anion in mind: bromide at high temperature and in the presence of acid can release hydrogen bromide. Follow the SDS for the specific grade.
9. FAQ
Is 1-butyl-3-methylimidazolium bromide a base?
No. It is a neutral salt. Ionic liquids with basic anions (hydroxide, acetate, carbonate, imidazolate) are basic; BMIM Br is their precursor and a reaction medium, not a base.
Can BMIM Br replace sodium hydroxide in an aldol condensation?
Not stoichiometrically. It is not a base. What works is BMIM Br as a phase-transfer agent or reaction medium alongside a hydroxide or carbonate base – a two-component system, not a one-for-one substitution.
What is the molecular weight of BMIM Br?
219.12 g/mol (C8H15BrN2). Some databases return 221.14 for CAS 85100-77-2 because they index a protonated form; that figure is about 1% high and should not be used for mole calculations.
Why is the 1-methylimidazole limit so tight?
Because MIm is a stronger base and a stronger nucleophile than the imidazolium cation, so a few hundred ppm changes the behaviour of the solvent. It also smells, which is how most people notice it first.
Can I dry it in an oven?
Vacuum drying with stirring at moderate temperature is the standard method, with water confirmed by Karl Fischer. Oven drying at atmospheric pressure will not reach the low-ppm figures and risks colour formation.
Is it soluble in water?
Fully miscible. That is why aqueous workup does not remove it, and it is the main practical obstacle to recovering the ionic liquid after a reaction.
References and notes
- CAS 85100-77-2; formula and molecular weight derived as C8H15N2+ (cation, PubChem CID 2734162) plus Br- (79.90) = 219.12. The PubChem record returned for the CAS itself (CID 25153273) is a protonated 2H-imidazole hydrobromide of formula C8H17BrN2 and MW 221.14, which is a different species.
- Melting point quoted as a reported range (low 70s deg C); values in the literature vary slightly with water content and measurement method.
- Specification limits (assay, MIm, water) are from our current product certificate.
- pKa statements about the imidazolium C2 proton are qualitative; reported values vary with the measurement method and solvent.
Disclaimer. This article describes the product, its chemistry and its specification. It is not a process recommendation: the right medium and the right base for a given transformation are established experimentally. Data on toxicity and ecotoxicity of individual ionic liquids varies by grade and is not summarised here.
For a certificate showing MIm and Karl Fischer figures, a 100 g sample, or pricing on drum quantities, contact PolyBlueChem, or start from the BMIM Br product page.