Tetra-n-butylammonium fluoride

Tetra-n-butylammonium fluoride or TBAF is a quaternary ammonium salt with the chemical formula (CH₃CH₂CH₂CH₂)₄N⁺F^-. It is commercially available as the trihydrate and as a solution in tetrahydrofuran.

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In the laboratory the compound is usually encountered in the latter form as a source of organic-soluble fluoride ion to remove silyl ether protecting groups. It is also used as a phase transfer catalyst and as a mild base.

As fluoride is such a strong hydrogen bond acceptor it is quite impossible to dry hydrated samples. Heating to 77 °C under vacuum causes decomposition to the hydrogen difluoride salt,^[1] and samples dried at 40 °C under high vacuum still contain 10-30 mol % of water and some 10 % of difluoride.^[2] Preparing anhydrous samples is of interest as the basicity of fluoride increases by more than 20 pK units on passing from aqueous to aprotic solvent. Recently, a preparation of anhydrous tetra-n-butylammonium fluoride from hexafluorobenzene and tetrabutylammonium cyanide has been reported. Despite the considerable basicity of "naked" fluoride ion, in the absence of water, solutions of the salt in acetonitrile and dimethyl sulfoxide are surprisingly stable towards elimination.^[3]

References

1. ^ Ramesh K. Sharma, James L. Fry (1983). "Instability of anhydrous tetra-n-alkylammonium fluorides". Journal of Organic Chemistry 48: 2112-4. doi:10.1021/jo00160a041.
2. ^ D. Phillip Cox, Jacek Terpinski, Witold Lawrynowicz (1984). "'Anhydrous' tetrabutylammonium fluoride: a mild but highly efficient source of nucleophilic fluoride ion". Journal of Organic Chemistry 49: 3216-9. doi:10.1021/jo00191a035.
3. ^ Haoran Sun and Stephen G. DiMagno (2005). "Anhydrous Tetrabutylammonium Fluoride". Journal of the American Chemical Society 127: 2050-1. doi:10.1021/ja0440497.