Triethyloxonium tetrafluoroborate

"Meerwein's salt" redirects here. For other uses, see Trimethyloxonium tetrafluoroborate.

Triethyloxonium tetrafluoroborate is the organic oxonium compound with the formula [(CH₃CH₂)₃O]⁺[BF₄]⁻. It is often called Meerwein's reagent or Meerwein's salt after its discoverer Hans Meerwein.^[2][3] 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.^[4]

Triethyloxonium tetrafluoroborate

Names
IUPAC name Triethyloxonium tetrafluoroborate
Identifiers
CAS Number	368-39-8 Y
3D model (JSmol)	Interactive image
Beilstein Reference	3598090
ChemSpider	2006158 N
ECHA InfoCard	100.006.096
PubChem CID	2723982
UNII	Z0B19DD36J Y
UN number	3261
CompTox Dashboard (EPA)	DTXSID20883372
InChI InChI=1S/C6H15O.BF4/c1-4-7(5-2)6-3;2-1(3,4)5/h4-6H2,1-3H3;/q+1;-1 N Key: IYDQMLLDOVRSJJ-UHFFFAOYSA-N N
SMILES F[B-](F)(F)F.CC[O+](CC)CC
Properties
Chemical formula	[(CH3CH2)3O]+[BF4]−
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[1]
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H314
Precautionary statements	P260, P280, P303+P361+P353, P304+P340+P310, P305+P351+P338, P310, P363
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Synthesis and reactivity

Triethyloxonium tetrafluoroborate is prepared from boron trifluoride, diethyl ether, and epichlorohydrin:^[5]

4 Et₂O·BF₃ + 2 Et₂O + 3 C₂H₃OCH₂Cl → 3 [Et₃O]⁺[BF₄]⁻ + B(OCH(CH₂Cl)CH₂OEt)₃

where the Et stands for ethyl. The trimethyloxonium salt is available from dimethyl ether via an analogous route.^[6] These salts do not have long shelf-lives at room temperature. They degrade by hydrolysis:

[Et₃O]⁺[BF₄]⁻ + H₂O → Et₂O + EtOH + H⁺[BF₄]⁻

The propensity of trialkyloxonium salts for alkyl-exchange can be advantageous. For example, trimethyloxonium tetrafluoroborate, which reacts sluggishly due to its low solubility in most compatible solvents, may be converted in situ to higher alkyl/more soluble oxoniums, thereby accelerating alkylation reactions.^[7]

This reagent is useful for esterification of carboxylic acids under conditions where acid-catalyzed reactions are infeasible: ^[8]

RCO₂H + (C₂H₅)₃OBF₄ → RCO₂C₂H₅ + (C₂H₅)₂O + HBF₄

Structure

The structure of triethyloxonium tetrafluoroborate has not been characterized by X-ray crystallography, but the structure of triethyloxonium hexafluorophosphate has been examined. The measurements confirm that the cation is pyramidal with C-O-C angles in the range 109.4°–115.5°. The average C–O distance is 1.49 Å.^[9]

Safety

Triethyloxonium tetrafluoroborate is a very 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. "Triethyloxonium tetrafluoroborate". Sigma Aldrich.
2. H. Meerwein; G. Hinz; P. Hofmann; E. Kroning & E. Pfeil (1937). "Über Tertiäre Oxoniumsalze, I". Journal für Praktische Chemie. 147 (10–12): 257. doi:10.1002/prac.19371471001.
3. H. Meerwein; E. Bettenberg; H. Gold; E. Pfeil & G. Willfang (1940). "Über Tertiäre Oxoniumsalze, II". Journal für Praktische Chemie. 154 (3–5): 83. doi:10.1002/prac.19391540305.
4. 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
5. H. Meerwein (1966). "Triethyloxonium fluoroborate". Organic Syntheses. 46: 113. doi:10.15227/orgsyn.046.0113.
6. T. J. Curphey (1988). "Trimethyloxonium tetrafluoroborate". Organic Syntheses; Collected Volumes, vol. 6, p. 1019.
7. Vartak A.P. & 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.
8. Raber, Douglas J.; Gariano, Jr, Patrick; Brod, Albert O.; Gariano, Anne L.; Guida, Wayne C. (1977). "Esterification of Carboxylic Acids with Trialkyloxonium Salts: Ethyl and Methyl 4-Acetoxybenzoates". Organic Syntheses. 56: 59. doi:10.15227/orgsyn.056.0059.
9. Watkins, Michael I.; Ip, Wai Man; Olah, George A.; Bau, Robert (1982). "Structure of Oxonium Ions: An X-Ray Crystallographic Study of Triethyloxonium Hexafluorophosphate and Triphenyloxonium Tetraphenylborate". Journal of the American Chemical Society. 104 (9): 2365–2372. doi:10.1021/ja00373a006.