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Participants/Project Abstracts

Summer 2006 Bioanalytical Science REU Participants

UCR Student - Mark Nava

CAN A SUPERACID BE PROTONATEDM

Department of Chemistry, University of California - Riverside , CA 92521

Carborane acids, such as H(CHB₁₁Cl₁₁ ), appear to be the strongest Brønsted acids presently known. They protonate benzene whereas the superacid triflic acid (CF₃SO₃H) does not. (A superacid is defined as an acid stronger than 100% H₂SO₄ ). The acid strength of carborane acids have been qualitatively determined on an infrared-based scale using N-H frequencies of trioctylammonium salts of conjugate base anions. However, there is still no pK a type quantitative measure of the acidity of carborane acids.

The goal of this research project is (a) to find a solvent that is sufficiently weakly basic that carborane acidity is not leveled and in which a pK a type measurement can be made, and (b) can protonated mineral acids such as triflic acid be isolated as carborane salts?

The very weakly basic solvent sulfuryl chloride fluoride, SO₂ClF, was chosen as the most promising solvent for this project. It was successfully synthesized from SO₂Cl₂ by treatment with NH 4 F according to literature methods. Handling this volatile liquid (BPt7°C ) was demanding and considerable effort has been put into purification because errant nucleophiles (e.g. H₂O) will interfere seriously with the carborane superacid chemistry.

An attempt has been made to isolate protonated triflic acid. Instead of isolating a CF₃SO₃H 2 + carborane salt, an hydronium ion salt, [H₃O + ][CHB₁₁H 5 Cl 6 - ]·2CF₃SO₃H, was isolated. This has revealed that elimination of H₂O from triflic acid and presumed formation of triflic anhydride is a problem that must be surmounted if protonated triflic acid is to be isolated. We are approaching this problem by the use of low temperatures. SO₂ClF is a good solvent choice because the MPt is very low (-128°C).

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