Brønsted acids in ionic liquids: how acidity depends on the liquid structure

Area of Science:

• Physical Chemistry
• Materials Science
• Supramolecular Chemistry

Background:

• Ionic liquids offer tunable properties for chemical applications.
• Quantifying Brønsted acidity in complex mixtures is crucial for reaction design.
• The Gutmann Acceptor Number (AN) is a valuable parameter for assessing Lewis and Brønsted acidity.

Purpose of the Study:

• To determine Gutmann Acceptor Number (AN) values for Brønsted acid-ionic liquid mixtures.
• To explore the relationship between liquid speciation and measured acidity.
• To demonstrate a method for manipulating and quantifying Brønsted acidity in these systems.

Main Methods:

• Synthesis and characterization of four [C2mim][A]-HA systems (A(-) = [NTf2](-), [OTf](-), [OMs](-), [OAc](-)).
• Determination of Gutmann Acceptor Number (AN) values across a wide compositional range.
• Analysis of acid-anion interactions and speciation within the ionic liquid mixtures.

Main Results:

• AN values were successfully determined for Brønsted acid-ionic liquid mixtures.
• Brønsted acids (HOAc, HOMs, HOTf) formed hydrogen-bonded anionic clusters ([A(HA)x](-)) with ionic liquid anions.
• HNTf2 remained as a discrete molecule, while AN values reflected the presence of anionic clusters.
• Acidity could be buffered by controlling the formation of these anionic clusters.

Conclusions:

• The Gutmann Acceptor Number (AN) provides a convenient method for quantifying Brønsted acidity in acid-ionic liquid mixtures.
• Acid speciation, particularly the formation of anionic clusters, significantly influences the measured acidity.
• This work demonstrates a straightforward approach to tune and quantify acidity in ionic liquid systems.