A Publication
of Reliable Methods
for the Preparation
of Organic Compounds
Annual Volume
Org. Synth. 1966, 46, 113
DOI: 10.15227/orgsyn.046.0113
[Oxonium compounds, triethyloxonium tetrafluoroborate]
Submitted by H. Meerwein1
Checked by B. C. Anderson, O. H. Vogl, and B. C. McKusick.
1. Procedure
A 2-l. three-necked flask, a stirrer, a dropping funnel, and a condenser provided with a drying tube are dried in an oven at 110°, assembled while hot, and cooled in a stream of dry nitrogen. Sodium-dried ether (500 ml.) and 284 g. (252 ml., 2.00 moles) of freshly distilled boron fluoride etherate (Note 1) and (Note 2) are placed in the flask. Epichlorohydrin (140 g., 119 ml., 1.51 moles) is added dropwise to the stirred solution at a rate sufficient to maintain vigorous boiling (about 1 hour is needed). The mixture is refluxed an additional hour and allowed to stand at room temperature overnight. The stirrer is replaced by a filter stick, and the supernatant ether is withdrawn from the crystalline mass of triethyloxonium fluoborate; nitrogen is admitted through a bubbler during this operation to prevent atmospheric moisture from entering the flask. The crystals are washed with three 500-ml. portions of sodium-dried ether. The flask is transferred to a dry box, and triethyloxonium fluoborate is collected on a sintered-glass filter and bottled in a stream of dry nitrogen. The fluoborate is colorless; m.p. 91–92° (dec.), yield 244–272 g. (85–95%) (Note 3).
2. Notes
1. The checkers obtained boron fluoride etherate and epichlorohydrin from Eastman Organic Chemicals and redistilled each through a 23-cm. Vigreux column immediately before use.
2. It is convenient to measure the liquids with syringes using the densities: epichlorohydrin d425 1.179; boron fluoride etherate, d425 1.125.
3. Triethyloxonium fluoborate is very hygroscopic. It should be stored in a tightly closed screw-cap bottle at 0–5° and should be used within a few days of the time it is made. It should be weighed and transferred in a dry box. It can be stored indefinitely under ether or at −80°.
3. Discussion
The procedure used is essentially that described by Meerwein and co-workers.2,3 The salt also has been prepared from ethyl fluoride and boron fluoride etherate, and from silver fluoborate, ethyl bromide, and ether.4
4. Merits of the Preparation
This simple procedure easily provides large amounts of triethyloxonium fluoborate. Triethyloxonium fluoborate readily ethylates such compounds as ethers, sulfides, nitriles, ketones, esters, and amides on oxygen, nitrogen, or sulfur to give onium fluoborates (often isolable) that can react with nucleophilic reagents to give useful products.5 For example, dimethylformamide gives the imino ether fluoborate [(CH3)2NCH-OC2H5]+ BF4, which is converted to (CH3)2NCH(OC2H5)2 by sodium ethoxide.5 Since an imino ether fluoborate is easily hydrolyzed to the corresponding amine and ester, triethyloxonium fluoborate is a useful reagent for converting amides to amines under mild conditions.6 Curphey provides a fuller discussion of the alkylating properties of trialkyloxonium fluoborates.7
If there is no advantage in ethylation over methylation, trimethyloxonium fluoborate8 or trimethyloxonium 2,4,6-trinitrobenzenesulfonate9 may be preferable alkylating agents; their preparation is more laborious, but they may be stored for a longer period of time.

References and Notes
  1. Deceased October 24, 1965; formerly at University of Marburg, Marburg, Germany.
  2. H. Meerwein, E. Bettenberg, H. Gold, E. Pfeil, and G. Willfang, J. Prakt. Chem., [2] 154, 83 (1940).
  3. H. Meerwein, G. Hinz, P. Hofmann, E. Kroning, and E. Pfeil, J. Prakt. Chem., [2] 147, 257 (1937).
  4. H. Meerwein, V. Hederich, and K. Wunderlich, Arch. Pharm., 291, 552 (1958).
  5. H. Meerwein, P. Borner, O. Fuchs, H. J. Sasse, H. Schrodt, and J. Spille, Ber., 89, 2060 (1956).
  6. H. Muxfeldt and W. Rogalski, J. Am. Chem. Soc., 87, 933 (1965).
  7. T. J. Curphey, Org. Syntheses, 51, 142 (1971).
  8. H. Meerwein, this volume, p. 1096.
  9. G. K. Helmkamp and D. J. Pettitt, this volume, p. 1099.

Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

Sodium-dried ether

epichlorohydrin d425


ether (60-29-7)

Epichlorohydrin (106-89-8)

Ethyl bromide (74-96-4)

oxygen (7782-44-7)

nitrogen (7727-37-9)

sulfur (7704-34-9)

sodium ethoxide (141-52-6)

dimethylformamide (68-12-2)

silver fluoborate (14104-20-2)

boron fluoride etherate (109-63-7)

Triethyloxonium fluoborate (368-39-8)

ethyl fluoride (353-36-6)

trimethyloxonium fluoborate (420-37-1)

Trimethyloxonium 2,4,6-trinitrobenzenesulfonate (13700-00-0)

Oxonium compounds, triethyloxonium tetrafluoroborate