I have had a long-standing interest in organolithium compounds, dating back to my graduate student days. Thus, I was excited to read Kass and Radom’s latest work on the computational and experimental evaluation of the acidity of lithium acetylide LiCCH.¹

The gas phase experimental acidity is accomplished by preparing the conjugate base of lithium acetylide through a procedure of collision-induced dissociation with loss of CO₂, as in Scheme 1. By reacting this anion with a variety of different acids, they were able to bracket the acidity and determine that ΔH_acid is 391.0 ± 1.3 kcal mol^-1. This is about 13 kcal mol^-1 less acidic than acetylene itself. The reduction is acidity understandable in terms of the C-Li being essentially ionic, and thereby loss of the proton builds up negative charge on a carbon adjacent to a carbon that already has a great deal of negative charge.

Scheme 1

Computations support this enthalpy for deprotonation. The G3, G4 and W1 values for the enthalpy deprotonation of lithium acetylide are 389.1, 388.9, and 390.4 kcal mol^-1, respectively. It should also be noted that the conjugate base of lithium acetylide posses a non-classical bridging geometry 1, which is well-known for organolithium species.²

References

(1) Meyer, M. M.; Chan, B.; Radom, L.; Kass, S., R., "Gas-Phase Synthesis and Reactivity of Lithium Acetylide Ion, LiCC^–," Angew. Chem. Int. Ed., 2010, 49, 5161-5164, DOI: 10.1002/anie.201001485

(2) Streitwieser, A.; Bachrach, S. M.; Dorigo, A.; Schleyer, P. v. R. In Lithium Chemistry: A Theoretical and Experimental Overview; Sapse, A.-M., Schleyer, P. v. R., Eds.; Wiley-Interscience: New York, 1995, p 1-45.