Carborane acids are considered the strongest organic acids.
Carborane acids possess exceptional acidity due to the unique structure of the carborane cluster. This cluster, composed of boron, carbon, and hydrogen atoms, delocalizes the negative charge of the conjugate base very effectively. This delocalization stabilizes the anion, making it much easier to lose a proton (H+) and thus, increasing the acid strength.
In simpler terms, the carborane structure acts like a very large, stable sponge, spreading out the negative charge when the acid loses its proton. This widespread distribution minimizes the charge density, leading to a much more stable and less reactive anion compared to other organic acids like carboxylic acids or sulfonic acids. A more stable conjugate base means a stronger acid.
Key features contributing to the strength of carborane acids:
- Delocalized Charge: The carborane cage structure allows for significant delocalization of the negative charge after deprotonation.
- Stability of Conjugate Base: The conjugate base is exceptionally stable due to this charge delocalization.
- Weak Basicity of Conjugate Base: A stable conjugate base implies a weak base, which correlates with a strong acid.
While other acids such as triflic acid (trifluoromethanesulfonic acid) are also very strong organic acids, carborane acids generally surpass them in terms of acidity. The specific strength of a carborane acid depends on the substituents attached to the carborane cage, but they consistently exhibit remarkable acidity.
