Triethyloxonium tetrafluoroborate

Triethyloxonium tetrafluoroborate

Names
IUPAC name Triethyloxonium tetrafluoroborate
Identifiers
CAS Number	368-39-8
Beilstein Reference	3598090
ChemSpider	2006158
Jmol 3D image	Interactive graph
PubChem	2723982
UN number	3261
InChI InChI=1S/C6H15O.BF4/c1-4-7(5-2)6-3;2-1(3,4)5/h4-6H2,1-3H3;/q+1;-1 Key: IYDQMLLDOVRSJJ-UHFFFAOYSA-N
SMILES F[B-](F)(F)F.CC[O+](CC)CC
Properties
Chemical formula	C6H15BF4O
Molar mass	189.99 g·mol^−1
Melting point	91 to 92 °C (196 to 198 °F; 364 to 365 K)
Solubility in water	Reacts
Hazards
EU classification (DSD)	C
R-phrases	R14, R34
S-phrases	S22, S26, S36/37/39
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Triethyloxonium tetrafluoroborate is the organic oxonium compound with the formula [(CH₃CH₂)₃O]BF₄. It is often called Meerwein's Reagent after its discoverer Hans Meerwein.^[1] Also well known and commercially available is the related trimethyloxonium tetrafluoroborate. The compounds are white solids that dissolve in polar organic solvents. They are strong alkylating agents. Aside from the BF−
4 salt, many related derivatives are available.^[2]

Synthesis and reactivity

Triethyloxonium tetrafluoroborate is prepared from boron trifluoride, diethyl ether, and epichlorohydrin:^[3]

4 Et₂O·BF₃ + 2 Et₂O + 3 C₂H₃(O)CH₂Cl → 3 Et₃O⁺BF−
4 + B[(OCH(CH₂Cl)CH₂OEt]₃

The trimethyloxonium salt is available from dimethyl ether via an analogous route.^[4] These salts do not have long shelf-lives at room temperature. They degrade by hydrolysis:

[(CH₃CH₂)₃O]⁺BF−
4 + H₂O → (CH₃CH₂)₂O + CH₃CH₂OH + HBF₄

The propensity of trialkyloxonium salts for alkyl-exchange can be advantageous. For example, trimethyloxonium tetrafluoroborate, which reacts sluggishly due to it low solubility in most compatible solvents may be converted in situ to higher alkyl/more soluble oxoniums, thereby accelerating alkylation reactions.^[5]

Structure

The compound features a pyramidal oxonium cation, isoelectronic with triethylamine, and a tetrahedral fluoroborate anion. Reflecting its ionic character, the salt dissolves in polar but inert solvents such as dichloromethane, sulfur dioxide, and nitromethane.

Safety

Triethyloxonium tetrafluoroborate is a strong alkylating agent, although the hazards are diminished because it is non-volatile. It releases strong acid upon contact with water. The properties of the methyl derivative are similar.

References

1. H. Meerwein, G. Hinz, P. Hofmann, E. Kroning, and E. Pfeil (1937). "Über Tertiäre Oxoniumsalze, I". Journal für Praktische Chemie 147 (10-12): 257. doi:10.1002/prac.19371471001.H. Meerwein, E. Bettenberg, H. Gold, E. Pfeil, and G. Willfang (1940). "Über Tertiäre Oxoniumsalze, II". Journal für Praktische Chemie 154 (3-5): 83. doi:10.1002/prac.19391540305.
2. Hartwig Perst, Dave G. Seapy "Triethyloxonium Tetrafluoroborate" in Encyclopedia of Reagents for Organic Synthesis John Wiley & Sons, New York, 2008. doi:10.1002/047084289X.rt223.pub2. Article Online Posting Date: March 14, 2008
3. H. Meerwein (1973). "Triethyloxonium fluoroborate". Org. Synth.; Coll. Vol. 5, p. 1080
4. T. J. Curphey (1988). "Trimethyloxonium tetrafluoroborate". Org. Synth.; Coll. Vol. 6, p. 1019
5. Vartak A.P. and Crooks P.A. (2009). "A Scalable Enantioselective synthesis of the alpha2-adrenergic Agonist, Lofexidine". Org. Process Res. Dev. 13 (3): 415–419. doi:10.1021/op8002689.