Sciencemadness Discussion Board

Cetyltrimethylammonium tribromide - an easily prepared, powerful brominating reagent

Alchemica - 13-2-2026 at 01:23

Been experimenting with novel brominating reagent and found that cetyltrimethylammonium tribromide is a) easily prepared from readily available reagents b) easy to store and comparatively friendly as a reagent c) very effective for bromination d) often quite usefully selective for bromination.

It's said that amongst other quaternary ammonium tribromides including benzyltri(m)ethylammonium tribromide, CTABr3 is actually a more effective bromination reagent.

The literature suggests several methods of preparation including some solid state preparation with silica gel support, including KMnO4/NaBr/H2SO4, via persulfate, H2O2/H2SO4/V2O5 etc but I found these procedures not overly wonderful. One such procedure is below.
Quote:

Synthesis of Quaternary Ammonium Tribromides
Typical methodology of synthesis of QATBs involves the grinding of BTMAB
(1 mmol, 0.23g), KBr (2mmol, 0.24g), ammonium persulfate (1mmol, 0.23g), 4N sulfuric acid (0.1mL), and a little silica in a mortar for ca. 5–10min at room temperature (Scheme 1). The color of the mixture changed from white to orange-yellow, indicating the formation of the tribromide salt. The product was extracted with ethyl acetate (5mL) and concentrated under reduced pressure to get the tribromide in excellent yield (88%) with high purity. Analytical data for benzyltrimetylammonium tribromide: C10H16NBr3, mp: 99C, lit.[15] (99–101C), Calc: C, 30.8; H, 4.14; N, 3.59%. Found: C, 30.6; H, 4.19; N, 3.61%


Instead, I found simply adding 3 molar equiv. of a solid bromide salt (NaBr) to 50% cetrimide solution (cetyltrimethylammonium chloride - interestingly the chloride counterion seems not to pose any issues, allowing the cheap common OTC cosmetic quat. cetrimide to be used) and slowly adding 1 -2 molar equivalent Oxone [in ice bath] rapidly afforded excellent yields of the bright-orange tribromide, which could be easily extracted with DCM.

Some reactions

Bromination of phenols and anilines

Reaction of phenols/anilines with calculated amounts of Quat.Br3 in dichloromethane-methanol at room temperature gave the desirable mono-, di-, or tribromo-substituted phenols, selectively. For instance, reactions of phenol with 1 equiv, of Quat.Br3 gave 4-bromophenol, and with 2 equiv, of Quat.Br3 gave 2,4-dibromophenol; furthermore, reactions with 3 equiv, of Quat.Br3 gave 2,4,6-tribromophenol in good yields, respectively

Bromination of less reactive aromatics
Quat.Br3 in AcOH/ZnCl2

Benzylic bromination
Reaction of arenes eg PhMe with a calculated amount of Quat.Br3 in
refluxing benzene in the presence of AIBN gave the benzylic bromide

R-NH2 to nitriles
Reaction of primary amines and hydrazo compounds with Quat.Br3 in aq. sodium hydroxide or in water gave corresponding nitriles and azo compounds in satisfactory yields

Sulfides to sulfoxides

Halogenation of ketones to α-bromoketones
In MeOH or DCM/MeOH. 1eq. selectively gives monobromoketone, dibromoketones can be had using excess reagent

Haloform reaction of methyl ketones

N-bromination of amides and Hofmann degradation of amides

It seems to also facilitate a range of oxidation reactions, including easy and efficient oxidations of alcohols with H2O2 eg:

Substrate (1 mmol), Quat. tribromide (10 mol%), aqueous hydrogen peroxide (2 mmol, 30wt%), acetonitrile (5mL) at 60C.

Oxidation of alcohols and ethers

The reaction of primary alcohols or ethers with a calculated amount of
Quat.Br3 in carbon tetrachloride-water in the presence of Na2HPO4 at 60~ gave dimeric esters in good yields. In the case of benzyl alcohol, the only oxidation product was benzaldehyde. Cyclic ethers eg THF gave the lactone.


Hope this mention inspires some further mad science.



[Edited on 13-2-2026 by Alchemica]

Attachment: tbab quat tribromide.pdf (81kB)
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Attachment: Synthesis of Quaternary Ammonium Tribromides via persulfate kbr.pdf (274kB)
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Attachment: quat ammonium tribromides kmno4 prep.pdf (149kB)
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Attachment: BROMINATION AND OXIDATION Quat tribromide.pdf (803kB)
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Attachment: quat ammonium tribromides BTMABr3 ox, hoffman, bromination.pdf (803kB)
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Attachment: tbab quat tribromide.pdf (81kB)
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