Experimental and computational determination of the effect of the cyano group on carbon acidity in water

Rate constants k(DO) (M^-1 s^-1) for the deprotonation of cyanoalkanes by deuterioxide ion in D₂O at 25 °C were determined by following the appearance of the deuterium-labeled cyanoalkanes by ¹H NMR. These data were evaluated to give the following pK(a)'s in water: CH₃CN, 28.9; CH₃CH₂CN, 30.9; NCCH₂CH₂CN, 26.6. High level ab initio calculations on cyanoalkanes and α-cyano carbanions and combined QM/Monte Carlo calculations of their free energies of solvation were carried out. The interaction between a carbanionic center and an α-cyano substituent is concluded to be largely polar. The 5.1-fold difference in α-cyano and β-cyano substituent effects on carbon acidity in water which, nominally, is consistent with significant resonance stabilization of α-cyano carbanions is attributed to the differential solvation of cyanoalkanes and cyanocarbanions. The free energy change for the highly unfavorable tautomerization of acetonitrile to ketenimine in water was computed as ΔG(T) = 30.7 kcal/mol. We propose that the large instability of the ketenimine cumulative double bond favors the valence bond resonance form of the α-cyanocarbanion in which there is a formal carbon-nitrogen triple bond and the negative charge is localized at the α-carbon.